https://openjdk.java.net/ 
/*
 * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.util;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.IntFunction;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import java.util.stream.IntStream;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
import jdk.internal.misc.SharedSecrets;


This class consists exclusively of static methods that operate on or return
collections.  It contains polymorphic algorithms that operate on
collections, "wrappers", which return a new collection backed by a
specified collection, and a few other odds and ends.

The methods of this class all throw a NullPointerException if the collections or class objects provided to them are null. 
The documentation for the polymorphic algorithms contained in this class
generally includes a brief description of the implementation.  Such
descriptions should be regarded as implementation notes, rather than
parts of the specification. Implementors should feel free to substitute other algorithms, so long as the specification itself is adhered to. (For example, the algorithm used by sort does not have to be a mergesort, but it does have to be stable.)

The "destructive" algorithms contained in this class, that is, the algorithms that modify the collection on which they operate, are specified to throw UnsupportedOperationException if the collection does not support the appropriate mutation primitive(s), such as the set method. These algorithms may, but are not required to, throw this exception if an invocation would have no effect on the collection. For example, invoking the sort method on an unmodifiable list that is already sorted may or may not throw UnsupportedOperationException. 
This class is a member of the

Java Collections Framework.

Author: Josh Bloch,  Neal Gafter
See Also:
Since:  1.2
/**
 * This class consists exclusively of static methods that operate on or return
 * collections.  It contains polymorphic algorithms that operate on
 * collections, "wrappers", which return a new collection backed by a
 * specified collection, and a few other odds and ends.
 *
 * <p>The methods of this class all throw a {@code NullPointerException}
 * if the collections or class objects provided to them are null.
 *
 * <p>The documentation for the polymorphic algorithms contained in this class
 * generally includes a brief description of the <i>implementation</i>.  Such
 * descriptions should be regarded as <i>implementation notes</i>, rather than
 * parts of the <i>specification</i>.  Implementors should feel free to
 * substitute other algorithms, so long as the specification itself is adhered
 * to.  (For example, the algorithm used by {@code sort} does not have to be
 * a mergesort, but it does have to be <i>stable</i>.)
 *
 * <p>The "destructive" algorithms contained in this class, that is, the
 * algorithms that modify the collection on which they operate, are specified
 * to throw {@code UnsupportedOperationException} if the collection does not
 * support the appropriate mutation primitive(s), such as the {@code set}
 * method.  These algorithms may, but are not required to, throw this
 * exception if an invocation would have no effect on the collection.  For
 * example, invoking the {@code sort} method on an unmodifiable list that is
 * already sorted may or may not throw {@code UnsupportedOperationException}.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Collection
 * @see     Set
 * @see     List
 * @see     Map
 * @since   1.2
 */

public class Collections {
    // Suppresses default constructor, ensuring non-instantiability.
    private Collections() {
    }

    // Algorithms

    /*
     * Tuning parameters for algorithms - Many of the List algorithms have
     * two implementations, one of which is appropriate for RandomAccess
     * lists, the other for "sequential."  Often, the random access variant
     * yields better performance on small sequential access lists.  The
     * tuning parameters below determine the cutoff point for what constitutes
     * a "small" sequential access list for each algorithm.  The values below
     * were empirically determined to work well for LinkedList. Hopefully
     * they should be reasonable for other sequential access List
     * implementations.  Those doing performance work on this code would
     * do well to validate the values of these parameters from time to time.
     * (The first word of each tuning parameter name is the algorithm to which
     * it applies.)
     */
    private static final int BINARYSEARCH_THRESHOLD   = 5000;
    private static final int REVERSE_THRESHOLD        =   18;
    private static final int SHUFFLE_THRESHOLD        =    5;
    private static final int FILL_THRESHOLD           =   25;
    private static final int ROTATE_THRESHOLD         =  100;
    private static final int COPY_THRESHOLD           =   10;
    private static final int REPLACEALL_THRESHOLD     =   11;
    private static final int INDEXOFSUBLIST_THRESHOLD =   35;

    
Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list). 
This sort is guaranteed to be stable:  equal elements will
not be reordered as a result of the sort.

The specified list must be modifiable, but need not be resizable.

Params:
  • list – the list to be sorted.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
See Also:
Implementation Note: This implementation defers to the List.sort(Comparator) method using the specified list and a null comparator.
/**
     * Sorts the specified list into ascending order, according to the
     * {@linkplain Comparable natural ordering} of its elements.
     * All elements in the list must implement the {@link Comparable}
     * interface.  Furthermore, all elements in the list must be
     * <i>mutually comparable</i> (that is, {@code e1.compareTo(e2)}
     * must not throw a {@code ClassCastException} for any elements
     * {@code e1} and {@code e2} in the list).
     *
     * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
     * not be reordered as a result of the sort.
     *
     * <p>The specified list must be modifiable, but need not be resizable.
     *
     * @implNote
     * This implementation defers to the {@link List#sort(Comparator)}
     * method using the specified list and a {@code null} comparator.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be sorted.
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> (for example, strings and integers).
     * @throws UnsupportedOperationException if the specified list's
     *         list-iterator does not support the {@code set} operation.
     * @throws IllegalArgumentException (optional) if the implementation
     *         detects that the natural ordering of the list elements is
     *         found to violate the {@link Comparable} contract
     * @see List#sort(Comparator)
     */
    @SuppressWarnings("unchecked")
    public static <T extends Comparable<? super T>> void sort(List<T> list) {
        list.sort(null);
    }

    
Sorts the specified list according to the order induced by the
specified comparator.  All elements in the list must be mutually
comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list). 
This sort is guaranteed to be stable:  equal elements will
not be reordered as a result of the sort.

The specified list must be modifiable, but need not be resizable.

Params:
  • list – the list to be sorted.
  • c – the comparator to determine the order of the list. A null value indicates that the elements' natural
       ordering should be used.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
See Also:
Implementation Note: This implementation defers to the List.sort(Comparator) method using the specified list and comparator.
/**
     * Sorts the specified list according to the order induced by the
     * specified comparator.  All elements in the list must be <i>mutually
     * comparable</i> using the specified comparator (that is,
     * {@code c.compare(e1, e2)} must not throw a {@code ClassCastException}
     * for any elements {@code e1} and {@code e2} in the list).
     *
     * <p>This sort is guaranteed to be <i>stable</i>:  equal elements will
     * not be reordered as a result of the sort.
     *
     * <p>The specified list must be modifiable, but need not be resizable.
     *
     * @implNote
     * This implementation defers to the {@link List#sort(Comparator)}
     * method using the specified list and comparator.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be sorted.
     * @param  c the comparator to determine the order of the list.  A
     *        {@code null} value indicates that the elements' <i>natural
     *        ordering</i> should be used.
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator.
     * @throws UnsupportedOperationException if the specified list's
     *         list-iterator does not support the {@code set} operation.
     * @throws IllegalArgumentException (optional) if the comparator is
     *         found to violate the {@link Comparator} contract
     * @see List#sort(Comparator)
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    public static <T> void sort(List<T> list, Comparator<? super T> c) {
        list.sort(c);
    }


    
Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the natural ordering of its elements (as by the sort(List<Z#0-T#370>) method) prior to making this call. If it is not sorted, the results are undefined. If the list contains multiple elements equal to the specified object, there is no guarantee which one will be found. 
This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). If the specified list does not implement the RandomAccess interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons. 
Params:
  • list – the list to be searched.
  • key – the key to be searched for.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
  • ClassCastException – if the list contains elements that are not
        mutually comparable (for example, strings and
        integers), or the search key is not mutually comparable
        with the elements of the list.
Returns:the index of the search key, if it is contained in the list;
        otherwise, (-(insertion point) - 1).  The
        insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.
/**
     * Searches the specified list for the specified object using the binary
     * search algorithm.  The list must be sorted into ascending order
     * according to the {@linkplain Comparable natural ordering} of its
     * elements (as by the {@link #sort(List)} method) prior to making this
     * call.  If it is not sorted, the results are undefined.  If the list
     * contains multiple elements equal to the specified object, there is no
     * guarantee which one will be found.
     *
     * <p>This method runs in log(n) time for a "random access" list (which
     * provides near-constant-time positional access).  If the specified list
     * does not implement the {@link RandomAccess} interface and is large,
     * this method will do an iterator-based binary search that performs
     * O(n) link traversals and O(log n) element comparisons.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be searched.
     * @param  key the key to be searched for.
     * @return the index of the search key, if it is contained in the list;
     *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The
     *         <i>insertion point</i> is defined as the point at which the
     *         key would be inserted into the list: the index of the first
     *         element greater than the key, or {@code list.size()} if all
     *         elements in the list are less than the specified key.  Note
     *         that this guarantees that the return value will be &gt;= 0 if
     *         and only if the key is found.
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> (for example, strings and
     *         integers), or the search key is not mutually comparable
     *         with the elements of the list.
     */
    public static <T>
    int binarySearch(List<? extends Comparable<? super T>> list, T key) {
        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key);
        else
            return Collections.iteratorBinarySearch(list, key);
    }

    private static <T>
    int indexedBinarySearch(List<? extends Comparable<? super T>> list, T key) {
        int low = 0;
        int high = list.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            Comparable<? super T> midVal = list.get(mid);
            int cmp = midVal.compareTo(key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    private static <T>
    int iteratorBinarySearch(List<? extends Comparable<? super T>> list, T key)
    {
        int low = 0;
        int high = list.size()-1;
        ListIterator<? extends Comparable<? super T>> i = list.listIterator();

        while (low <= high) {
            int mid = (low + high) >>> 1;
            Comparable<? super T> midVal = get(i, mid);
            int cmp = midVal.compareTo(key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    
Gets the ith element from the given list by repositioning the specified
list listIterator.

/**
     * Gets the ith element from the given list by repositioning the specified
     * list listIterator.
     */
    private static <T> T get(ListIterator<? extends T> i, int index) {
        T obj = null;
        int pos = i.nextIndex();
        if (pos <= index) {
            do {
                obj = i.next();
            } while (pos++ < index);
        } else {
            do {
                obj = i.previous();
            } while (--pos > index);
        }
        return obj;
    }

    
Searches the specified list for the specified object using the binary search algorithm. The list must be sorted into ascending order according to the specified comparator (as by the sort(List, Comparator) method), prior to making this call. If it is not sorted, the results are undefined. If the list contains multiple elements equal to the specified object, there is no guarantee which one will be found. 
This method runs in log(n) time for a "random access" list (which provides near-constant-time positional access). If the specified list does not implement the RandomAccess interface and is large, this method will do an iterator-based binary search that performs O(n) link traversals and O(log n) element comparisons. 
Params:
  • list – the list to be searched.
  • key – the key to be searched for.
  • c – the comparator by which the list is ordered. A null value indicates that the elements' natural ordering should be used.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
  • ClassCastException – if the list contains elements that are not
        mutually comparable using the specified comparator,
        or the search key is not mutually comparable with the
        elements of the list using this comparator.
Returns:the index of the search key, if it is contained in the list;
        otherwise, (-(insertion point) - 1).  The
        insertion point is defined as the point at which the key would be inserted into the list: the index of the first element greater than the key, or list.size() if all elements in the list are less than the specified key. Note that this guarantees that the return value will be >= 0 if and only if the key is found.
/**
     * Searches the specified list for the specified object using the binary
     * search algorithm.  The list must be sorted into ascending order
     * according to the specified comparator (as by the
     * {@link #sort(List, Comparator) sort(List, Comparator)}
     * method), prior to making this call.  If it is
     * not sorted, the results are undefined.  If the list contains multiple
     * elements equal to the specified object, there is no guarantee which one
     * will be found.
     *
     * <p>This method runs in log(n) time for a "random access" list (which
     * provides near-constant-time positional access).  If the specified list
     * does not implement the {@link RandomAccess} interface and is large,
     * this method will do an iterator-based binary search that performs
     * O(n) link traversals and O(log n) element comparisons.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be searched.
     * @param  key the key to be searched for.
     * @param  c the comparator by which the list is ordered.
     *         A {@code null} value indicates that the elements'
     *         {@linkplain Comparable natural ordering} should be used.
     * @return the index of the search key, if it is contained in the list;
     *         otherwise, <code>(-(<i>insertion point</i>) - 1)</code>.  The
     *         <i>insertion point</i> is defined as the point at which the
     *         key would be inserted into the list: the index of the first
     *         element greater than the key, or {@code list.size()} if all
     *         elements in the list are less than the specified key.  Note
     *         that this guarantees that the return value will be &gt;= 0 if
     *         and only if the key is found.
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator,
     *         or the search key is not mutually comparable with the
     *         elements of the list using this comparator.
     */
    @SuppressWarnings("unchecked")
    public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c) {
        if (c==null)
            return binarySearch((List<? extends Comparable<? super T>>) list, key);

        if (list instanceof RandomAccess || list.size()<BINARYSEARCH_THRESHOLD)
            return Collections.indexedBinarySearch(list, key, c);
        else
            return Collections.iteratorBinarySearch(list, key, c);
    }

    private static <T> int indexedBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
        int low = 0;
        int high = l.size()-1;

        while (low <= high) {
            int mid = (low + high) >>> 1;
            T midVal = l.get(mid);
            int cmp = c.compare(midVal, key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    private static <T> int iteratorBinarySearch(List<? extends T> l, T key, Comparator<? super T> c) {
        int low = 0;
        int high = l.size()-1;
        ListIterator<? extends T> i = l.listIterator();

        while (low <= high) {
            int mid = (low + high) >>> 1;
            T midVal = get(i, mid);
            int cmp = c.compare(midVal, key);

            if (cmp < 0)
                low = mid + 1;
            else if (cmp > 0)
                high = mid - 1;
            else
                return mid; // key found
        }
        return -(low + 1);  // key not found
    }

    
Reverses the order of the elements in the specified list.

This method runs in linear time.

Params:
  • list – the list whose elements are to be reversed.
Throws:
/**
     * Reverses the order of the elements in the specified list.<p>
     *
     * This method runs in linear time.
     *
     * @param  list the list whose elements are to be reversed.
     * @throws UnsupportedOperationException if the specified list or
     *         its list-iterator does not support the {@code set} operation.
     */
    @SuppressWarnings({"rawtypes", "unchecked"})
    public static void reverse(List<?> list) {
        int size = list.size();
        if (size < REVERSE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=0, mid=size>>1, j=size-1; i<mid; i++, j--)
                swap(list, i, j);
        } else {
            // instead of using a raw type here, it's possible to capture
            // the wildcard but it will require a call to a supplementary
            // private method
            ListIterator fwd = list.listIterator();
            ListIterator rev = list.listIterator(size);
            for (int i=0, mid=list.size()>>1; i<mid; i++) {
                Object tmp = fwd.next();
                fwd.set(rev.previous());
                rev.set(tmp);
            }
        }
    }

    
Randomly permutes the specified list using a default source of
randomness.  All permutations occur with approximately equal
likelihood.

The hedge "approximately" is used in the foregoing description because
default source of randomness is only approximately an unbiased source
of independently chosen bits. If it were a perfect source of randomly
chosen bits, then the algorithm would choose permutations with perfect
uniformity.

This implementation traverses the list backwards, from the last
element up to the second, repeatedly swapping a randomly selected element
into the "current position".  Elements are randomly selected from the
portion of the list that runs from the first element to the current
position, inclusive.

This method runs in linear time. If the specified list does not implement the RandomAccess interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place. 
Params:
  • list – the list to be shuffled.
Throws:
/**
     * Randomly permutes the specified list using a default source of
     * randomness.  All permutations occur with approximately equal
     * likelihood.
     *
     * <p>The hedge "approximately" is used in the foregoing description because
     * default source of randomness is only approximately an unbiased source
     * of independently chosen bits. If it were a perfect source of randomly
     * chosen bits, then the algorithm would choose permutations with perfect
     * uniformity.
     *
     * <p>This implementation traverses the list backwards, from the last
     * element up to the second, repeatedly swapping a randomly selected element
     * into the "current position".  Elements are randomly selected from the
     * portion of the list that runs from the first element to the current
     * position, inclusive.
     *
     * <p>This method runs in linear time.  If the specified list does not
     * implement the {@link RandomAccess} interface and is large, this
     * implementation dumps the specified list into an array before shuffling
     * it, and dumps the shuffled array back into the list.  This avoids the
     * quadratic behavior that would result from shuffling a "sequential
     * access" list in place.
     *
     * @param  list the list to be shuffled.
     * @throws UnsupportedOperationException if the specified list or
     *         its list-iterator does not support the {@code set} operation.
     */
    public static void shuffle(List<?> list) {
        Random rnd = r;
        if (rnd == null)
            r = rnd = new Random(); // harmless race.
        shuffle(list, rnd);
    }

    private static Random r;

    
Randomly permute the specified list using the specified source of
randomness.  All permutations occur with equal likelihood
assuming that the source of randomness is fair.

This implementation traverses the list backwards, from the last element
up to the second, repeatedly swapping a randomly selected element into
the "current position".  Elements are randomly selected from the
portion of the list that runs from the first element to the current
position, inclusive.
 This method runs in linear time. If the specified list does not implement the RandomAccess interface and is large, this implementation dumps the specified list into an array before shuffling it, and dumps the shuffled array back into the list. This avoids the quadratic behavior that would result from shuffling a "sequential access" list in place. 
Params:
  • list – the list to be shuffled.
  • rnd – the source of randomness to use to shuffle the list.
Throws:
/**
     * Randomly permute the specified list using the specified source of
     * randomness.  All permutations occur with equal likelihood
     * assuming that the source of randomness is fair.<p>
     *
     * This implementation traverses the list backwards, from the last element
     * up to the second, repeatedly swapping a randomly selected element into
     * the "current position".  Elements are randomly selected from the
     * portion of the list that runs from the first element to the current
     * position, inclusive.<p>
     *
     * This method runs in linear time.  If the specified list does not
     * implement the {@link RandomAccess} interface and is large, this
     * implementation dumps the specified list into an array before shuffling
     * it, and dumps the shuffled array back into the list.  This avoids the
     * quadratic behavior that would result from shuffling a "sequential
     * access" list in place.
     *
     * @param  list the list to be shuffled.
     * @param  rnd the source of randomness to use to shuffle the list.
     * @throws UnsupportedOperationException if the specified list or its
     *         list-iterator does not support the {@code set} operation.
     */
    @SuppressWarnings({"rawtypes", "unchecked"})
    public static void shuffle(List<?> list, Random rnd) {
        int size = list.size();
        if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
            for (int i=size; i>1; i--)
                swap(list, i-1, rnd.nextInt(i));
        } else {
            Object[] arr = list.toArray();

            // Shuffle array
            for (int i=size; i>1; i--)
                swap(arr, i-1, rnd.nextInt(i));

            // Dump array back into list
            // instead of using a raw type here, it's possible to capture
            // the wildcard but it will require a call to a supplementary
            // private method
            ListIterator it = list.listIterator();
            for (Object e : arr) {
                it.next();
                it.set(e);
            }
        }
    }

    
Swaps the elements at the specified positions in the specified list.
(If the specified positions are equal, invoking this method leaves
the list unchanged.)

Params:
  • list – The list in which to swap elements.
  • i – the index of one element to be swapped.
  • j – the index of the other element to be swapped.
Throws:
Since:1.4
/**
     * Swaps the elements at the specified positions in the specified list.
     * (If the specified positions are equal, invoking this method leaves
     * the list unchanged.)
     *
     * @param list The list in which to swap elements.
     * @param i the index of one element to be swapped.
     * @param j the index of the other element to be swapped.
     * @throws IndexOutOfBoundsException if either {@code i} or {@code j}
     *         is out of range (i &lt; 0 || i &gt;= list.size()
     *         || j &lt; 0 || j &gt;= list.size()).
     * @since 1.4
     */
    @SuppressWarnings({"rawtypes", "unchecked"})
    public static void swap(List<?> list, int i, int j) {
        // instead of using a raw type here, it's possible to capture
        // the wildcard but it will require a call to a supplementary
        // private method
        final List l = list;
        l.set(i, l.set(j, l.get(i)));
    }

    
Swaps the two specified elements in the specified array.

/**
     * Swaps the two specified elements in the specified array.
     */
    private static void swap(Object[] arr, int i, int j) {
        Object tmp = arr[i];
        arr[i] = arr[j];
        arr[j] = tmp;
    }

    
Replaces all of the elements of the specified list with the specified
element. 

This method runs in linear time.

Params:
  • list – the list to be filled with the specified element.
  • obj – The element with which to fill the specified list.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
/**
     * Replaces all of the elements of the specified list with the specified
     * element. <p>
     *
     * This method runs in linear time.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be filled with the specified element.
     * @param  obj The element with which to fill the specified list.
     * @throws UnsupportedOperationException if the specified list or its
     *         list-iterator does not support the {@code set} operation.
     */
    public static <T> void fill(List<? super T> list, T obj) {
        int size = list.size();

        if (size < FILL_THRESHOLD || list instanceof RandomAccess) {
            for (int i=0; i<size; i++)
                list.set(i, obj);
        } else {
            ListIterator<? super T> itr = list.listIterator();
            for (int i=0; i<size; i++) {
                itr.next();
                itr.set(obj);
            }
        }
    }

    
Copies all of the elements from one list into another.  After the
operation, the index of each copied element in the destination list
will be identical to its index in the source list.  The destination
list's size must be greater than or equal to the source list's size.
If it is greater, the remaining elements in the destination list are
unaffected. 

This method runs in linear time.

Params:
  • dest – The destination list.
  • src – The source list.
Type parameters:
  • <T> – the class of the objects in the lists
Throws:
/**
     * Copies all of the elements from one list into another.  After the
     * operation, the index of each copied element in the destination list
     * will be identical to its index in the source list.  The destination
     * list's size must be greater than or equal to the source list's size.
     * If it is greater, the remaining elements in the destination list are
     * unaffected. <p>
     *
     * This method runs in linear time.
     *
     * @param  <T> the class of the objects in the lists
     * @param  dest The destination list.
     * @param  src The source list.
     * @throws IndexOutOfBoundsException if the destination list is too small
     *         to contain the entire source List.
     * @throws UnsupportedOperationException if the destination list's
     *         list-iterator does not support the {@code set} operation.
     */
    public static <T> void copy(List<? super T> dest, List<? extends T> src) {
        int srcSize = src.size();
        if (srcSize > dest.size())
            throw new IndexOutOfBoundsException("Source does not fit in dest");

        if (srcSize < COPY_THRESHOLD ||
            (src instanceof RandomAccess && dest instanceof RandomAccess)) {
            for (int i=0; i<srcSize; i++)
                dest.set(i, src.get(i));
        } else {
            ListIterator<? super T> di=dest.listIterator();
            ListIterator<? extends T> si=src.listIterator();
            for (int i=0; i<srcSize; i++) {
                di.next();
                di.set(si.next());
            }
        }
    }

    
Returns the minimum element of the given collection, according to the
natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.

Params:
  • coll – the collection whose minimum element is to be determined.
Type parameters:
  • <T> – the class of the objects in the collection
Throws:
See Also:
Returns:the minimum element of the given collection, according
        to the natural ordering of its elements.
/**
     * Returns the minimum element of the given collection, according to the
     * <i>natural ordering</i> of its elements.  All elements in the
     * collection must implement the {@code Comparable} interface.
     * Furthermore, all elements in the collection must be <i>mutually
     * comparable</i> (that is, {@code e1.compareTo(e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the collection).<p>
     *
     * This method iterates over the entire collection, hence it requires
     * time proportional to the size of the collection.
     *
     * @param  <T> the class of the objects in the collection
     * @param  coll the collection whose minimum element is to be determined.
     * @return the minimum element of the given collection, according
     *         to the <i>natural ordering</i> of its elements.
     * @throws ClassCastException if the collection contains elements that are
     *         not <i>mutually comparable</i> (for example, strings and
     *         integers).
     * @throws NoSuchElementException if the collection is empty.
     * @see Comparable
     */
    public static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll) {
        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (next.compareTo(candidate) < 0)
                candidate = next;
        }
        return candidate;
    }

    
Returns the minimum element of the given collection, according to the
order induced by the specified comparator.  All elements in the
collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.

Params:
  • coll – the collection whose minimum element is to be determined.
  • comp – the comparator with which to determine the minimum element. A null value indicates that the elements' natural
        ordering should be used.
Type parameters:
  • <T> – the class of the objects in the collection
Throws:
See Also:
Returns:the minimum element of the given collection, according
        to the specified comparator.
/**
     * Returns the minimum element of the given collection, according to the
     * order induced by the specified comparator.  All elements in the
     * collection must be <i>mutually comparable</i> by the specified
     * comparator (that is, {@code comp.compare(e1, e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the collection).<p>
     *
     * This method iterates over the entire collection, hence it requires
     * time proportional to the size of the collection.
     *
     * @param  <T> the class of the objects in the collection
     * @param  coll the collection whose minimum element is to be determined.
     * @param  comp the comparator with which to determine the minimum element.
     *         A {@code null} value indicates that the elements' <i>natural
     *         ordering</i> should be used.
     * @return the minimum element of the given collection, according
     *         to the specified comparator.
     * @throws ClassCastException if the collection contains elements that are
     *         not <i>mutually comparable</i> using the specified comparator.
     * @throws NoSuchElementException if the collection is empty.
     * @see Comparable
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    public static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp) {
        if (comp==null)
            return (T)min((Collection) coll);

        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (comp.compare(next, candidate) < 0)
                candidate = next;
        }
        return candidate;
    }

    
Returns the maximum element of the given collection, according to the
natural ordering of its elements. All elements in the collection must implement the Comparable interface. Furthermore, all elements in the collection must be mutually
comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.

Params:
  • coll – the collection whose maximum element is to be determined.
Type parameters:
  • <T> – the class of the objects in the collection
Throws:
See Also:
Returns:the maximum element of the given collection, according
        to the natural ordering of its elements.
/**
     * Returns the maximum element of the given collection, according to the
     * <i>natural ordering</i> of its elements.  All elements in the
     * collection must implement the {@code Comparable} interface.
     * Furthermore, all elements in the collection must be <i>mutually
     * comparable</i> (that is, {@code e1.compareTo(e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the collection).<p>
     *
     * This method iterates over the entire collection, hence it requires
     * time proportional to the size of the collection.
     *
     * @param  <T> the class of the objects in the collection
     * @param  coll the collection whose maximum element is to be determined.
     * @return the maximum element of the given collection, according
     *         to the <i>natural ordering</i> of its elements.
     * @throws ClassCastException if the collection contains elements that are
     *         not <i>mutually comparable</i> (for example, strings and
     *         integers).
     * @throws NoSuchElementException if the collection is empty.
     * @see Comparable
     */
    public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll) {
        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (next.compareTo(candidate) > 0)
                candidate = next;
        }
        return candidate;
    }

    
Returns the maximum element of the given collection, according to the
order induced by the specified comparator.  All elements in the
collection must be mutually comparable by the specified comparator (that is, comp.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the collection).

This method iterates over the entire collection, hence it requires
time proportional to the size of the collection.

Params:
  • coll – the collection whose maximum element is to be determined.
  • comp – the comparator with which to determine the maximum element. A null value indicates that the elements' natural
       ordering should be used.
Type parameters:
  • <T> – the class of the objects in the collection
Throws:
See Also:
Returns:the maximum element of the given collection, according
        to the specified comparator.
/**
     * Returns the maximum element of the given collection, according to the
     * order induced by the specified comparator.  All elements in the
     * collection must be <i>mutually comparable</i> by the specified
     * comparator (that is, {@code comp.compare(e1, e2)} must not throw a
     * {@code ClassCastException} for any elements {@code e1} and
     * {@code e2} in the collection).<p>
     *
     * This method iterates over the entire collection, hence it requires
     * time proportional to the size of the collection.
     *
     * @param  <T> the class of the objects in the collection
     * @param  coll the collection whose maximum element is to be determined.
     * @param  comp the comparator with which to determine the maximum element.
     *         A {@code null} value indicates that the elements' <i>natural
     *        ordering</i> should be used.
     * @return the maximum element of the given collection, according
     *         to the specified comparator.
     * @throws ClassCastException if the collection contains elements that are
     *         not <i>mutually comparable</i> using the specified comparator.
     * @throws NoSuchElementException if the collection is empty.
     * @see Comparable
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    public static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp) {
        if (comp==null)
            return (T)max((Collection) coll);

        Iterator<? extends T> i = coll.iterator();
        T candidate = i.next();

        while (i.hasNext()) {
            T next = i.next();
            if (comp.compare(next, candidate) > 0)
                candidate = next;
        }
        return candidate;
    }

    
Rotates the elements in the specified list by the specified distance. After calling this method, the element at index i will be the element previously at index (i - distance) mod list.size(), for all values of i between 0 and list.size()-1, inclusive. (This method has no effect on the size of the list.) 
For example, suppose list comprises [t, a, n, k, s]. After invoking Collections.rotate(list, 1) (or Collections.rotate(list, -4)), list will comprise [s, t, a, n, k]. 
Note that this method can usefully be applied to sublists to move one or more elements within a list while preserving the order of the remaining elements. For example, the following idiom moves the element at index j forward to position k (which must be greater than or equal to j): 
    Collections.rotate(list.subList(j, k+1), -1);

 To make this concrete, suppose list comprises [a, b, c, d, e]. To move the element at index 1 (b) forward two positions, perform the following invocation: 
    Collections.rotate(l.subList(1, 4), -1);

 The resulting list is [a, c, d, b, e]. 
To move more than one element forward, increase the absolute value
of the rotation distance.  To move elements backward, use a positive
shift distance.

If the specified list is small or implements the RandomAccess interface, this implementation exchanges the first element into the location it should go, and then repeatedly exchanges the displaced element into the location it should go until a displaced element is swapped into the first element. If necessary, the process is repeated on the second and successive elements, until the rotation is complete. If the specified list is large and doesn't implement the RandomAccess interface, this implementation breaks the list into two sublist views around index -distance mod size. Then the reverse(List<?>) method is invoked on each sublist view, and finally it is invoked on the entire list. For a more complete description of both algorithms, see Section 2.3 of Jon Bentley's Programming Pearls (Addison-Wesley, 1986).

Params:
  • list – the list to be rotated.
  • distance – the distance to rotate the list. There are no constraints on this value; it may be zero, negative, or greater than list.size().
Throws:
Since:1.4
/**
     * Rotates the elements in the specified list by the specified distance.
     * After calling this method, the element at index {@code i} will be
     * the element previously at index {@code (i - distance)} mod
     * {@code list.size()}, for all values of {@code i} between {@code 0}
     * and {@code list.size()-1}, inclusive.  (This method has no effect on
     * the size of the list.)
     *
     * <p>For example, suppose {@code list} comprises{@code  [t, a, n, k, s]}.
     * After invoking {@code Collections.rotate(list, 1)} (or
     * {@code Collections.rotate(list, -4)}), {@code list} will comprise
     * {@code [s, t, a, n, k]}.
     *
     * <p>Note that this method can usefully be applied to sublists to
     * move one or more elements within a list while preserving the
     * order of the remaining elements.  For example, the following idiom
     * moves the element at index {@code j} forward to position
     * {@code k} (which must be greater than or equal to {@code j}):
     * <pre>
     *     Collections.rotate(list.subList(j, k+1), -1);
     * </pre>
     * To make this concrete, suppose {@code list} comprises
     * {@code [a, b, c, d, e]}.  To move the element at index {@code 1}
     * ({@code b}) forward two positions, perform the following invocation:
     * <pre>
     *     Collections.rotate(l.subList(1, 4), -1);
     * </pre>
     * The resulting list is {@code [a, c, d, b, e]}.
     *
     * <p>To move more than one element forward, increase the absolute value
     * of the rotation distance.  To move elements backward, use a positive
     * shift distance.
     *
     * <p>If the specified list is small or implements the {@link
     * RandomAccess} interface, this implementation exchanges the first
     * element into the location it should go, and then repeatedly exchanges
     * the displaced element into the location it should go until a displaced
     * element is swapped into the first element.  If necessary, the process
     * is repeated on the second and successive elements, until the rotation
     * is complete.  If the specified list is large and doesn't implement the
     * {@code RandomAccess} interface, this implementation breaks the
     * list into two sublist views around index {@code -distance mod size}.
     * Then the {@link #reverse(List)} method is invoked on each sublist view,
     * and finally it is invoked on the entire list.  For a more complete
     * description of both algorithms, see Section 2.3 of Jon Bentley's
     * <i>Programming Pearls</i> (Addison-Wesley, 1986).
     *
     * @param list the list to be rotated.
     * @param distance the distance to rotate the list.  There are no
     *        constraints on this value; it may be zero, negative, or
     *        greater than {@code list.size()}.
     * @throws UnsupportedOperationException if the specified list or
     *         its list-iterator does not support the {@code set} operation.
     * @since 1.4
     */
    public static void rotate(List<?> list, int distance) {
        if (list instanceof RandomAccess || list.size() < ROTATE_THRESHOLD)
            rotate1(list, distance);
        else
            rotate2(list, distance);
    }

    private static <T> void rotate1(List<T> list, int distance) {
        int size = list.size();
        if (size == 0)
            return;
        distance = distance % size;
        if (distance < 0)
            distance += size;
        if (distance == 0)
            return;

        for (int cycleStart = 0, nMoved = 0; nMoved != size; cycleStart++) {
            T displaced = list.get(cycleStart);
            int i = cycleStart;
            do {
                i += distance;
                if (i >= size)
                    i -= size;
                displaced = list.set(i, displaced);
                nMoved ++;
            } while (i != cycleStart);
        }
    }

    private static void rotate2(List<?> list, int distance) {
        int size = list.size();
        if (size == 0)
            return;
        int mid =  -distance % size;
        if (mid < 0)
            mid += size;
        if (mid == 0)
            return;

        reverse(list.subList(0, mid));
        reverse(list.subList(mid, size));
        reverse(list);
    }

    
Replaces all occurrences of one specified value in a list with another. More formally, replaces with newVal each element e in list such that (oldVal==null ? e==null : oldVal.equals(e)). (This method has no effect on the size of the list.) 
Params:
  • list – the list in which replacement is to occur.
  • oldVal – the old value to be replaced.
  • newVal – the new value with which oldVal is to be replaced.
Type parameters:
  • <T> – the class of the objects in the list
Throws:
Returns:true if list contained one or more elements e such that (oldVal==null ?  e==null : oldVal.equals(e)).
Since: 1.4
/**
     * Replaces all occurrences of one specified value in a list with another.
     * More formally, replaces with {@code newVal} each element {@code e}
     * in {@code list} such that
     * {@code (oldVal==null ? e==null : oldVal.equals(e))}.
     * (This method has no effect on the size of the list.)
     *
     * @param  <T> the class of the objects in the list
     * @param list the list in which replacement is to occur.
     * @param oldVal the old value to be replaced.
     * @param newVal the new value with which {@code oldVal} is to be
     *        replaced.
     * @return {@code true} if {@code list} contained one or more elements
     *         {@code e} such that
     *         {@code (oldVal==null ?  e==null : oldVal.equals(e))}.
     * @throws UnsupportedOperationException if the specified list or
     *         its list-iterator does not support the {@code set} operation.
     * @since  1.4
     */
    public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) {
        boolean result = false;
        int size = list.size();
        if (size < REPLACEALL_THRESHOLD || list instanceof RandomAccess) {
            if (oldVal==null) {
                for (int i=0; i<size; i++) {
                    if (list.get(i)==null) {
                        list.set(i, newVal);
                        result = true;
                    }
                }
            } else {
                for (int i=0; i<size; i++) {
                    if (oldVal.equals(list.get(i))) {
                        list.set(i, newVal);
                        result = true;
                    }
                }
            }
        } else {
            ListIterator<T> itr=list.listIterator();
            if (oldVal==null) {
                for (int i=0; i<size; i++) {
                    if (itr.next()==null) {
                        itr.set(newVal);
                        result = true;
                    }
                }
            } else {
                for (int i=0; i<size; i++) {
                    if (oldVal.equals(itr.next())) {
                        itr.set(newVal);
                        result = true;
                    }
                }
            }
        }
        return result;
    }

    
Returns the starting position of the first occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the lowest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size()) 
This implementation uses the "brute force" technique of scanning
over the source list, looking for a match with the target at each
location in turn.

Params:
  • source – the list in which to search for the first occurrence of target.
  • target – the list to search for as a subList of source.
Returns:the starting position of the first occurrence of the specified
        target list within the specified source list, or -1 if there
        is no such occurrence.
Since: 1.4
/**
     * Returns the starting position of the first occurrence of the specified
     * target list within the specified source list, or -1 if there is no
     * such occurrence.  More formally, returns the lowest index {@code i}
     * such that {@code source.subList(i, i+target.size()).equals(target)},
     * or -1 if there is no such index.  (Returns -1 if
     * {@code target.size() > source.size()})
     *
     * <p>This implementation uses the "brute force" technique of scanning
     * over the source list, looking for a match with the target at each
     * location in turn.
     *
     * @param source the list in which to search for the first occurrence
     *        of {@code target}.
     * @param target the list to search for as a subList of {@code source}.
     * @return the starting position of the first occurrence of the specified
     *         target list within the specified source list, or -1 if there
     *         is no such occurrence.
     * @since  1.4
     */
    public static int indexOfSubList(List<?> source, List<?> target) {
        int sourceSize = source.size();
        int targetSize = target.size();
        int maxCandidate = sourceSize - targetSize;

        if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
            (source instanceof RandomAccess&&target instanceof RandomAccess)) {
        nextCand:
            for (int candidate = 0; candidate <= maxCandidate; candidate++) {
                for (int i=0, j=candidate; i<targetSize; i++, j++)
                    if (!eq(target.get(i), source.get(j)))
                        continue nextCand;  // Element mismatch, try next cand
                return candidate;  // All elements of candidate matched target
            }
        } else {  // Iterator version of above algorithm
            ListIterator<?> si = source.listIterator();
        nextCand:
            for (int candidate = 0; candidate <= maxCandidate; candidate++) {
                ListIterator<?> ti = target.listIterator();
                for (int i=0; i<targetSize; i++) {
                    if (!eq(ti.next(), si.next())) {
                        // Back up source iterator to next candidate
                        for (int j=0; j<i; j++)
                            si.previous();
                        continue nextCand;
                    }
                }
                return candidate;
            }
        }
        return -1;  // No candidate matched the target
    }

    
Returns the starting position of the last occurrence of the specified target list within the specified source list, or -1 if there is no such occurrence. More formally, returns the highest index i such that source.subList(i, i+target.size()).equals(target), or -1 if there is no such index. (Returns -1 if target.size() > source.size()) 
This implementation uses the "brute force" technique of iterating
over the source list, looking for a match with the target at each
location in turn.

Params:
  • source – the list in which to search for the last occurrence of target.
  • target – the list to search for as a subList of source.
Returns:the starting position of the last occurrence of the specified
        target list within the specified source list, or -1 if there
        is no such occurrence.
Since: 1.4
/**
     * Returns the starting position of the last occurrence of the specified
     * target list within the specified source list, or -1 if there is no such
     * occurrence.  More formally, returns the highest index {@code i}
     * such that {@code source.subList(i, i+target.size()).equals(target)},
     * or -1 if there is no such index.  (Returns -1 if
     * {@code target.size() > source.size()})
     *
     * <p>This implementation uses the "brute force" technique of iterating
     * over the source list, looking for a match with the target at each
     * location in turn.
     *
     * @param source the list in which to search for the last occurrence
     *        of {@code target}.
     * @param target the list to search for as a subList of {@code source}.
     * @return the starting position of the last occurrence of the specified
     *         target list within the specified source list, or -1 if there
     *         is no such occurrence.
     * @since  1.4
     */
    public static int lastIndexOfSubList(List<?> source, List<?> target) {
        int sourceSize = source.size();
        int targetSize = target.size();
        int maxCandidate = sourceSize - targetSize;

        if (sourceSize < INDEXOFSUBLIST_THRESHOLD ||
            source instanceof RandomAccess) {   // Index access version
        nextCand:
            for (int candidate = maxCandidate; candidate >= 0; candidate--) {
                for (int i=0, j=candidate; i<targetSize; i++, j++)
                    if (!eq(target.get(i), source.get(j)))
                        continue nextCand;  // Element mismatch, try next cand
                return candidate;  // All elements of candidate matched target
            }
        } else {  // Iterator version of above algorithm
            if (maxCandidate < 0)
                return -1;
            ListIterator<?> si = source.listIterator(maxCandidate);
        nextCand:
            for (int candidate = maxCandidate; candidate >= 0; candidate--) {
                ListIterator<?> ti = target.listIterator();
                for (int i=0; i<targetSize; i++) {
                    if (!eq(ti.next(), si.next())) {
                        if (candidate != 0) {
                            // Back up source iterator to next candidate
                            for (int j=0; j<=i+1; j++)
                                si.previous();
                        }
                        continue nextCand;
                    }
                }
                return candidate;
            }
        }
        return -1;  // No candidate matched the target
    }


    // Unmodifiable Wrappers

    
Returns an unmodifiable view of the specified collection. Query operations on the returned collection "read through" to the specified collection, and attempts to modify the returned collection, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.

The returned collection will be serializable if the specified collection
is serializable.

Params:
  • c – the collection for which an unmodifiable view is to be
        returned.
Type parameters:
  • <T> – the class of the objects in the collection
Returns:an unmodifiable view of the specified collection.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified collection. Query operations on the returned collection "read through"
     * to the specified collection, and attempts to modify the returned
     * collection, whether direct or via its iterator, result in an
     * {@code UnsupportedOperationException}.<p>
     *
     * The returned collection does <i>not</i> pass the hashCode and equals
     * operations through to the backing collection, but relies on
     * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
     * is necessary to preserve the contracts of these operations in the case
     * that the backing collection is a set or a list.<p>
     *
     * The returned collection will be serializable if the specified collection
     * is serializable.
     *
     * @param  <T> the class of the objects in the collection
     * @param  c the collection for which an unmodifiable view is to be
     *         returned.
     * @return an unmodifiable view of the specified collection.
     */
    public static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c) {
        return new UnmodifiableCollection<>(c);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableCollection<E> implements Collection<E>, Serializable {
        private static final long serialVersionUID = 1820017752578914078L;

        final Collection<? extends E> c;

        UnmodifiableCollection(Collection<? extends E> c) {
            if (c==null)
                throw new NullPointerException();
            this.c = c;
        }

        public int size()                          {return c.size();}
        public boolean isEmpty()                   {return c.isEmpty();}
        public boolean contains(Object o)          {return c.contains(o);}
        public Object[] toArray()                  {return c.toArray();}
        public <T> T[] toArray(T[] a)              {return c.toArray(a);}
        public <T> T[] toArray(IntFunction<T[]> f) {return c.toArray(f);}
        public String toString()                   {return c.toString();}

        public Iterator<E> iterator() {
            return new Iterator<E>() {
                private final Iterator<? extends E> i = c.iterator();

                public boolean hasNext() {return i.hasNext();}
                public E next()          {return i.next();}
                public void remove() {
                    throw new UnsupportedOperationException();
                }
                @Override
                public void forEachRemaining(Consumer<? super E> action) {
                    // Use backing collection version
                    i.forEachRemaining(action);
                }
            };
        }

        public boolean add(E e) {
            throw new UnsupportedOperationException();
        }
        public boolean remove(Object o) {
            throw new UnsupportedOperationException();
        }

        public boolean containsAll(Collection<?> coll) {
            return c.containsAll(coll);
        }
        public boolean addAll(Collection<? extends E> coll) {
            throw new UnsupportedOperationException();
        }
        public boolean removeAll(Collection<?> coll) {
            throw new UnsupportedOperationException();
        }
        public boolean retainAll(Collection<?> coll) {
            throw new UnsupportedOperationException();
        }
        public void clear() {
            throw new UnsupportedOperationException();
        }

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            c.forEach(action);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            throw new UnsupportedOperationException();
        }
        @SuppressWarnings("unchecked")
        @Override
        public Spliterator<E> spliterator() {
            return (Spliterator<E>)c.spliterator();
        }
        @SuppressWarnings("unchecked")
        @Override
        public Stream<E> stream() {
            return (Stream<E>)c.stream();
        }
        @SuppressWarnings("unchecked")
        @Override
        public Stream<E> parallelStream() {
            return (Stream<E>)c.parallelStream();
        }
    }

    
Returns an unmodifiable view of the specified set. Query operations on the returned set "read through" to the specified set, and attempts to modify the returned set, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned set will be serializable if the specified set
is serializable.

Params:
  • s – the set for which an unmodifiable view is to be returned.
Type parameters:
  • <T> – the class of the objects in the set
Returns:an unmodifiable view of the specified set.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified set. Query operations on the returned set "read through" to the specified
     * set, and attempts to modify the returned set, whether direct or via its
     * iterator, result in an {@code UnsupportedOperationException}.<p>
     *
     * The returned set will be serializable if the specified set
     * is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param  s the set for which an unmodifiable view is to be returned.
     * @return an unmodifiable view of the specified set.
     */
    public static <T> Set<T> unmodifiableSet(Set<? extends T> s) {
        return new UnmodifiableSet<>(s);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableSet<E> extends UnmodifiableCollection<E>
                                 implements Set<E>, Serializable {
        private static final long serialVersionUID = -9215047833775013803L;

        UnmodifiableSet(Set<? extends E> s)     {super(s);}
        public boolean equals(Object o) {return o == this || c.equals(o);}
        public int hashCode()           {return c.hashCode();}
    }

    
Returns an unmodifiable view of the specified sorted set. Query operations on the returned sorted set "read through" to the specified sorted set. Attempts to modify the returned sorted set, whether direct, via its iterator, or via its subSet, headSet, or tailSet views, result in an UnsupportedOperationException.

The returned sorted set will be serializable if the specified sorted set
is serializable.

Params:
  • s – the sorted set for which an unmodifiable view is to be
       returned.
Type parameters:
  • <T> – the class of the objects in the set
Returns:an unmodifiable view of the specified sorted set.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified sorted set. Query operations on the returned sorted set "read
     * through" to the specified sorted set.  Attempts to modify the returned
     * sorted set, whether direct, via its iterator, or via its
     * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
     * an {@code UnsupportedOperationException}.<p>
     *
     * The returned sorted set will be serializable if the specified sorted set
     * is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param s the sorted set for which an unmodifiable view is to be
     *        returned.
     * @return an unmodifiable view of the specified sorted set.
     */
    public static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s) {
        return new UnmodifiableSortedSet<>(s);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableSortedSet<E>
                             extends UnmodifiableSet<E>
                             implements SortedSet<E>, Serializable {
        private static final long serialVersionUID = -4929149591599911165L;
        private final SortedSet<E> ss;

        UnmodifiableSortedSet(SortedSet<E> s) {super(s); ss = s;}

        public Comparator<? super E> comparator() {return ss.comparator();}

        public SortedSet<E> subSet(E fromElement, E toElement) {
            return new UnmodifiableSortedSet<>(ss.subSet(fromElement,toElement));
        }
        public SortedSet<E> headSet(E toElement) {
            return new UnmodifiableSortedSet<>(ss.headSet(toElement));
        }
        public SortedSet<E> tailSet(E fromElement) {
            return new UnmodifiableSortedSet<>(ss.tailSet(fromElement));
        }

        public E first()                   {return ss.first();}
        public E last()                    {return ss.last();}
    }

    
Returns an unmodifiable view of the specified navigable set. Query operations on the returned navigable set "read through" to the specified navigable set. Attempts to modify the returned navigable set, whether direct, via its iterator, or via its subSet, headSet, or tailSet views, result in an UnsupportedOperationException.

The returned navigable set will be serializable if the specified
navigable set is serializable.

Params:
  • s – the navigable set for which an unmodifiable view is to be
       returned
Type parameters:
  • <T> – the class of the objects in the set
Returns:an unmodifiable view of the specified navigable set
Since:1.8
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified navigable set. Query operations on the returned navigable set "read
     * through" to the specified navigable set.  Attempts to modify the returned
     * navigable set, whether direct, via its iterator, or via its
     * {@code subSet}, {@code headSet}, or {@code tailSet} views, result in
     * an {@code UnsupportedOperationException}.<p>
     *
     * The returned navigable set will be serializable if the specified
     * navigable set is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param s the navigable set for which an unmodifiable view is to be
     *        returned
     * @return an unmodifiable view of the specified navigable set
     * @since 1.8
     */
    public static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s) {
        return new UnmodifiableNavigableSet<>(s);
    }

    
Wraps a navigable set and disables all of the mutative operations.

Type parameters:
  • <E> – type of elements
@serialinclude
/**
     * Wraps a navigable set and disables all of the mutative operations.
     *
     * @param <E> type of elements
     * @serial include
     */
    static class UnmodifiableNavigableSet<E>
                             extends UnmodifiableSortedSet<E>
                             implements NavigableSet<E>, Serializable {

        private static final long serialVersionUID = -6027448201786391929L;

        
A singleton empty unmodifiable navigable set used for Collections.emptyNavigableSet(). 
Type parameters:
  • <E> – type of elements, if there were any, and bounds
/**
         * A singleton empty unmodifiable navigable set used for
         * {@link #emptyNavigableSet()}.
         *
         * @param <E> type of elements, if there were any, and bounds
         */
        private static class EmptyNavigableSet<E> extends UnmodifiableNavigableSet<E>
            implements Serializable {
            private static final long serialVersionUID = -6291252904449939134L;

            public EmptyNavigableSet() {
                super(new TreeSet<>());
            }

            private Object readResolve()        { return EMPTY_NAVIGABLE_SET; }
        }

        @SuppressWarnings("rawtypes")
        private static final NavigableSet<?> EMPTY_NAVIGABLE_SET =
                new EmptyNavigableSet<>();

        
The instance we are protecting.

/**
         * The instance we are protecting.
         */
        private final NavigableSet<E> ns;

        UnmodifiableNavigableSet(NavigableSet<E> s)         {super(s); ns = s;}

        public E lower(E e)                             { return ns.lower(e); }
        public E floor(E e)                             { return ns.floor(e); }
        public E ceiling(E e)                         { return ns.ceiling(e); }
        public E higher(E e)                           { return ns.higher(e); }
        public E pollFirst()     { throw new UnsupportedOperationException(); }
        public E pollLast()      { throw new UnsupportedOperationException(); }
        public NavigableSet<E> descendingSet()
                 { return new UnmodifiableNavigableSet<>(ns.descendingSet()); }
        public Iterator<E> descendingIterator()
                                         { return descendingSet().iterator(); }

        public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
            return new UnmodifiableNavigableSet<>(
                ns.subSet(fromElement, fromInclusive, toElement, toInclusive));
        }

        public NavigableSet<E> headSet(E toElement, boolean inclusive) {
            return new UnmodifiableNavigableSet<>(
                ns.headSet(toElement, inclusive));
        }

        public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
            return new UnmodifiableNavigableSet<>(
                ns.tailSet(fromElement, inclusive));
        }
    }

    
Returns an unmodifiable view of the specified list. Query operations on the returned list "read through" to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException.
 The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess if the specified list does. 
Params:
  • list – the list for which an unmodifiable view is to be returned.
Type parameters:
  • <T> – the class of the objects in the list
Returns:an unmodifiable view of the specified list.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified list. Query operations on the returned list "read through" to the
     * specified list, and attempts to modify the returned list, whether
     * direct or via its iterator, result in an
     * {@code UnsupportedOperationException}.<p>
     *
     * The returned list will be serializable if the specified list
     * is serializable. Similarly, the returned list will implement
     * {@link RandomAccess} if the specified list does.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list for which an unmodifiable view is to be returned.
     * @return an unmodifiable view of the specified list.
     */
    public static <T> List<T> unmodifiableList(List<? extends T> list) {
        return (list instanceof RandomAccess ?
                new UnmodifiableRandomAccessList<>(list) :
                new UnmodifiableList<>(list));
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableList<E> extends UnmodifiableCollection<E>
                                  implements List<E> {
        private static final long serialVersionUID = -283967356065247728L;

        final List<? extends E> list;

        UnmodifiableList(List<? extends E> list) {
            super(list);
            this.list = list;
        }

        public boolean equals(Object o) {return o == this || list.equals(o);}
        public int hashCode()           {return list.hashCode();}

        public E get(int index) {return list.get(index);}
        public E set(int index, E element) {
            throw new UnsupportedOperationException();
        }
        public void add(int index, E element) {
            throw new UnsupportedOperationException();
        }
        public E remove(int index) {
            throw new UnsupportedOperationException();
        }
        public int indexOf(Object o)            {return list.indexOf(o);}
        public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
        public boolean addAll(int index, Collection<? extends E> c) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {
            throw new UnsupportedOperationException();
        }

        public ListIterator<E> listIterator()   {return listIterator(0);}

        public ListIterator<E> listIterator(final int index) {
            return new ListIterator<E>() {
                private final ListIterator<? extends E> i
                    = list.listIterator(index);

                public boolean hasNext()     {return i.hasNext();}
                public E next()              {return i.next();}
                public boolean hasPrevious() {return i.hasPrevious();}
                public E previous()          {return i.previous();}
                public int nextIndex()       {return i.nextIndex();}
                public int previousIndex()   {return i.previousIndex();}

                public void remove() {
                    throw new UnsupportedOperationException();
                }
                public void set(E e) {
                    throw new UnsupportedOperationException();
                }
                public void add(E e) {
                    throw new UnsupportedOperationException();
                }

                @Override
                public void forEachRemaining(Consumer<? super E> action) {
                    i.forEachRemaining(action);
                }
            };
        }

        public List<E> subList(int fromIndex, int toIndex) {
            return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
        }

        
UnmodifiableRandomAccessList instances are serialized as
UnmodifiableList instances to allow them to be deserialized
in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
This method inverts the transformation.  As a beneficial
side-effect, it also grafts the RandomAccess marker onto
UnmodifiableList instances that were serialized in pre-1.4 JREs.
Note: Unfortunately, UnmodifiableRandomAccessList instances
serialized in 1.4.1 and deserialized in 1.4 will become
UnmodifiableList instances, as this method was missing in 1.4.

/**
         * UnmodifiableRandomAccessList instances are serialized as
         * UnmodifiableList instances to allow them to be deserialized
         * in pre-1.4 JREs (which do not have UnmodifiableRandomAccessList).
         * This method inverts the transformation.  As a beneficial
         * side-effect, it also grafts the RandomAccess marker onto
         * UnmodifiableList instances that were serialized in pre-1.4 JREs.
         *
         * Note: Unfortunately, UnmodifiableRandomAccessList instances
         * serialized in 1.4.1 and deserialized in 1.4 will become
         * UnmodifiableList instances, as this method was missing in 1.4.
         */
        private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new UnmodifiableRandomAccessList<>(list)
                    : this);
        }
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableRandomAccessList<E> extends UnmodifiableList<E>
                                              implements RandomAccess
    {
        UnmodifiableRandomAccessList(List<? extends E> list) {
            super(list);
        }

        public List<E> subList(int fromIndex, int toIndex) {
            return new UnmodifiableRandomAccessList<>(
                list.subList(fromIndex, toIndex));
        }

        private static final long serialVersionUID = -2542308836966382001L;

        
Allows instances to be deserialized in pre-1.4 JREs (which do
not have UnmodifiableRandomAccessList).  UnmodifiableList has
a readResolve method that inverts this transformation upon
deserialization.

/**
         * Allows instances to be deserialized in pre-1.4 JREs (which do
         * not have UnmodifiableRandomAccessList).  UnmodifiableList has
         * a readResolve method that inverts this transformation upon
         * deserialization.
         */
        private Object writeReplace() {
            return new UnmodifiableList<>(list);
        }
    }

    
Returns an unmodifiable view of the specified map. Query operations on the returned map "read through" to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException.

The returned map will be serializable if the specified map
is serializable.

Params:
  • m – the map for which an unmodifiable view is to be returned.
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:an unmodifiable view of the specified map.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified map. Query operations on the returned map "read through"
     * to the specified map, and attempts to modify the returned
     * map, whether direct or via its collection views, result in an
     * {@code UnsupportedOperationException}.<p>
     *
     * The returned map will be serializable if the specified map
     * is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param  m the map for which an unmodifiable view is to be returned.
     * @return an unmodifiable view of the specified map.
     */
    public static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m) {
        return new UnmodifiableMap<>(m);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class UnmodifiableMap<K,V> implements Map<K,V>, Serializable {
        private static final long serialVersionUID = -1034234728574286014L;

        private final Map<? extends K, ? extends V> m;

        UnmodifiableMap(Map<? extends K, ? extends V> m) {
            if (m==null)
                throw new NullPointerException();
            this.m = m;
        }

        public int size()                        {return m.size();}
        public boolean isEmpty()                 {return m.isEmpty();}
        public boolean containsKey(Object key)   {return m.containsKey(key);}
        public boolean containsValue(Object val) {return m.containsValue(val);}
        public V get(Object key)                 {return m.get(key);}

        public V put(K key, V value) {
            throw new UnsupportedOperationException();
        }
        public V remove(Object key) {
            throw new UnsupportedOperationException();
        }
        public void putAll(Map<? extends K, ? extends V> m) {
            throw new UnsupportedOperationException();
        }
        public void clear() {
            throw new UnsupportedOperationException();
        }

        private transient Set<K> keySet;
        private transient Set<Map.Entry<K,V>> entrySet;
        private transient Collection<V> values;

        public Set<K> keySet() {
            if (keySet==null)
                keySet = unmodifiableSet(m.keySet());
            return keySet;
        }

        public Set<Map.Entry<K,V>> entrySet() {
            if (entrySet==null)
                entrySet = new UnmodifiableEntrySet<>(m.entrySet());
            return entrySet;
        }

        public Collection<V> values() {
            if (values==null)
                values = unmodifiableCollection(m.values());
            return values;
        }

        public boolean equals(Object o) {return o == this || m.equals(o);}
        public int hashCode()           {return m.hashCode();}
        public String toString()        {return m.toString();}

        // Override default methods in Map
        @Override
        @SuppressWarnings("unchecked")
        public V getOrDefault(Object k, V defaultValue) {
            // Safe cast as we don't change the value
            return ((Map<K, V>)m).getOrDefault(k, defaultValue);
        }

        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            m.forEach(action);
        }

        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V putIfAbsent(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean remove(Object key, Object value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V replace(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        
We need this class in addition to UnmodifiableSet as
Map.Entries themselves permit modification of the backing Map
via their setValue operation.  This class is subtle: there are
many possible attacks that must be thwarted.

@serialinclude
/**
         * We need this class in addition to UnmodifiableSet as
         * Map.Entries themselves permit modification of the backing Map
         * via their setValue operation.  This class is subtle: there are
         * many possible attacks that must be thwarted.
         *
         * @serial include
         */
        static class UnmodifiableEntrySet<K,V>
            extends UnmodifiableSet<Map.Entry<K,V>> {
            private static final long serialVersionUID = 7854390611657943733L;

            @SuppressWarnings({"unchecked", "rawtypes"})
            UnmodifiableEntrySet(Set<? extends Map.Entry<? extends K, ? extends V>> s) {
                // Need to cast to raw in order to work around a limitation in the type system
                super((Set)s);
            }

            static <K, V> Consumer<Map.Entry<? extends K, ? extends V>> entryConsumer(
                    Consumer<? super Entry<K, V>> action) {
                return e -> action.accept(new UnmodifiableEntry<>(e));
            }

            public void forEach(Consumer<? super Entry<K, V>> action) {
                Objects.requireNonNull(action);
                c.forEach(entryConsumer(action));
            }

            static final class UnmodifiableEntrySetSpliterator<K, V>
                    implements Spliterator<Entry<K,V>> {
                final Spliterator<Map.Entry<K, V>> s;

                UnmodifiableEntrySetSpliterator(Spliterator<Entry<K, V>> s) {
                    this.s = s;
                }

                @Override
                public boolean tryAdvance(Consumer<? super Entry<K, V>> action) {
                    Objects.requireNonNull(action);
                    return s.tryAdvance(entryConsumer(action));
                }

                @Override
                public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
                    Objects.requireNonNull(action);
                    s.forEachRemaining(entryConsumer(action));
                }

                @Override
                public Spliterator<Entry<K, V>> trySplit() {
                    Spliterator<Entry<K, V>> split = s.trySplit();
                    return split == null
                           ? null
                           : new UnmodifiableEntrySetSpliterator<>(split);
                }

                @Override
                public long estimateSize() {
                    return s.estimateSize();
                }

                @Override
                public long getExactSizeIfKnown() {
                    return s.getExactSizeIfKnown();
                }

                @Override
                public int characteristics() {
                    return s.characteristics();
                }

                @Override
                public boolean hasCharacteristics(int characteristics) {
                    return s.hasCharacteristics(characteristics);
                }

                @Override
                public Comparator<? super Entry<K, V>> getComparator() {
                    return s.getComparator();
                }
            }

            @SuppressWarnings("unchecked")
            public Spliterator<Entry<K,V>> spliterator() {
                return new UnmodifiableEntrySetSpliterator<>(
                        (Spliterator<Map.Entry<K, V>>) c.spliterator());
            }

            @Override
            public Stream<Entry<K,V>> stream() {
                return StreamSupport.stream(spliterator(), false);
            }

            @Override
            public Stream<Entry<K,V>> parallelStream() {
                return StreamSupport.stream(spliterator(), true);
            }

            public Iterator<Map.Entry<K,V>> iterator() {
                return new Iterator<Map.Entry<K,V>>() {
                    private final Iterator<? extends Map.Entry<? extends K, ? extends V>> i = c.iterator();

                    public boolean hasNext() {
                        return i.hasNext();
                    }
                    public Map.Entry<K,V> next() {
                        return new UnmodifiableEntry<>(i.next());
                    }
                    public void remove() {
                        throw new UnsupportedOperationException();
                    }
                    public void forEachRemaining(Consumer<? super Map.Entry<K, V>> action) {
                        i.forEachRemaining(entryConsumer(action));
                    }
                };
            }

            @SuppressWarnings("unchecked")
            public Object[] toArray() {
                Object[] a = c.toArray();
                for (int i=0; i<a.length; i++)
                    a[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)a[i]);
                return a;
            }

            @SuppressWarnings("unchecked")
            public <T> T[] toArray(T[] a) {
                // We don't pass a to c.toArray, to avoid window of
                // vulnerability wherein an unscrupulous multithreaded client
                // could get his hands on raw (unwrapped) Entries from c.
                Object[] arr = c.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));

                for (int i=0; i<arr.length; i++)
                    arr[i] = new UnmodifiableEntry<>((Map.Entry<? extends K, ? extends V>)arr[i]);

                if (arr.length > a.length)
                    return (T[])arr;

                System.arraycopy(arr, 0, a, 0, arr.length);
                if (a.length > arr.length)
                    a[arr.length] = null;
                return a;
            }

            
This method is overridden to protect the backing set against
an object with a nefarious equals function that senses
that the equality-candidate is Map.Entry and calls its
setValue method.

/**
             * This method is overridden to protect the backing set against
             * an object with a nefarious equals function that senses
             * that the equality-candidate is Map.Entry and calls its
             * setValue method.
             */
            public boolean contains(Object o) {
                if (!(o instanceof Map.Entry))
                    return false;
                return c.contains(
                    new UnmodifiableEntry<>((Map.Entry<?,?>) o));
            }

            
The next two methods are overridden to protect against
an unscrupulous List whose contains(Object o) method senses
when o is a Map.Entry, and calls o.setValue.

/**
             * The next two methods are overridden to protect against
             * an unscrupulous List whose contains(Object o) method senses
             * when o is a Map.Entry, and calls o.setValue.
             */
            public boolean containsAll(Collection<?> coll) {
                for (Object e : coll) {
                    if (!contains(e)) // Invokes safe contains() above
                        return false;
                }
                return true;
            }
            public boolean equals(Object o) {
                if (o == this)
                    return true;

                if (!(o instanceof Set))
                    return false;
                Set<?> s = (Set<?>) o;
                if (s.size() != c.size())
                    return false;
                return containsAll(s); // Invokes safe containsAll() above
            }

            
This "wrapper class" serves two purposes: it prevents
the client from modifying the backing Map, by short-circuiting
the setValue method, and it protects the backing Map against
an ill-behaved Map.Entry that attempts to modify another
Map Entry when asked to perform an equality check.

/**
             * This "wrapper class" serves two purposes: it prevents
             * the client from modifying the backing Map, by short-circuiting
             * the setValue method, and it protects the backing Map against
             * an ill-behaved Map.Entry that attempts to modify another
             * Map Entry when asked to perform an equality check.
             */
            private static class UnmodifiableEntry<K,V> implements Map.Entry<K,V> {
                private Map.Entry<? extends K, ? extends V> e;

                UnmodifiableEntry(Map.Entry<? extends K, ? extends V> e)
                        {this.e = Objects.requireNonNull(e);}

                public K getKey()        {return e.getKey();}
                public V getValue()      {return e.getValue();}
                public V setValue(V value) {
                    throw new UnsupportedOperationException();
                }
                public int hashCode()    {return e.hashCode();}
                public boolean equals(Object o) {
                    if (this == o)
                        return true;
                    if (!(o instanceof Map.Entry))
                        return false;
                    Map.Entry<?,?> t = (Map.Entry<?,?>)o;
                    return eq(e.getKey(),   t.getKey()) &&
                           eq(e.getValue(), t.getValue());
                }
                public String toString() {return e.toString();}
            }
        }
    }

    
Returns an unmodifiable view of the specified sorted map. Query operations on the returned sorted map "read through" to the specified sorted map. Attempts to modify the returned sorted map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException.

The returned sorted map will be serializable if the specified sorted map
is serializable.

Params:
  • m – the sorted map for which an unmodifiable view is to be
       returned.
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:an unmodifiable view of the specified sorted map.
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified sorted map. Query operations on the returned sorted map "read through"
     * to the specified sorted map.  Attempts to modify the returned
     * sorted map, whether direct, via its collection views, or via its
     * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
     * an {@code UnsupportedOperationException}.<p>
     *
     * The returned sorted map will be serializable if the specified sorted map
     * is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param m the sorted map for which an unmodifiable view is to be
     *        returned.
     * @return an unmodifiable view of the specified sorted map.
     */
    public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
        return new UnmodifiableSortedMap<>(m);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableSortedMap<K,V>
          extends UnmodifiableMap<K,V>
          implements SortedMap<K,V>, Serializable {
        private static final long serialVersionUID = -8806743815996713206L;

        private final SortedMap<K, ? extends V> sm;

        UnmodifiableSortedMap(SortedMap<K, ? extends V> m) {super(m); sm = m; }
        public Comparator<? super K> comparator()   { return sm.comparator(); }
        public SortedMap<K,V> subMap(K fromKey, K toKey)
             { return new UnmodifiableSortedMap<>(sm.subMap(fromKey, toKey)); }
        public SortedMap<K,V> headMap(K toKey)
                     { return new UnmodifiableSortedMap<>(sm.headMap(toKey)); }
        public SortedMap<K,V> tailMap(K fromKey)
                   { return new UnmodifiableSortedMap<>(sm.tailMap(fromKey)); }
        public K firstKey()                           { return sm.firstKey(); }
        public K lastKey()                             { return sm.lastKey(); }
    }

    
Returns an unmodifiable view of the specified navigable map. Query operations on the returned navigable map "read through" to the specified navigable map. Attempts to modify the returned navigable map, whether direct, via its collection views, or via its subMap, headMap, or tailMap views, result in an UnsupportedOperationException.

The returned navigable map will be serializable if the specified
navigable map is serializable.

Params:
  • m – the navigable map for which an unmodifiable view is to be
       returned
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:an unmodifiable view of the specified navigable map
Since:1.8
/**
     * Returns an <a href="Collection.html#unmodview">unmodifiable view</a> of the
     * specified navigable map. Query operations on the returned navigable map "read
     * through" to the specified navigable map.  Attempts to modify the returned
     * navigable map, whether direct, via its collection views, or via its
     * {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
     * an {@code UnsupportedOperationException}.<p>
     *
     * The returned navigable map will be serializable if the specified
     * navigable map is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param m the navigable map for which an unmodifiable view is to be
     *        returned
     * @return an unmodifiable view of the specified navigable map
     * @since 1.8
     */
    public static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
        return new UnmodifiableNavigableMap<>(m);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class UnmodifiableNavigableMap<K,V>
          extends UnmodifiableSortedMap<K,V>
          implements NavigableMap<K,V>, Serializable {
        private static final long serialVersionUID = -4858195264774772197L;

        
A class for the EMPTY_NAVIGABLE_MAP which needs readResolve to preserve singleton property. 
Type parameters:
  • <K> – type of keys, if there were any, and of bounds
  • <V> – type of values, if there were any
/**
         * A class for the {@link EMPTY_NAVIGABLE_MAP} which needs readResolve
         * to preserve singleton property.
         *
         * @param <K> type of keys, if there were any, and of bounds
         * @param <V> type of values, if there were any
         */
        private static class EmptyNavigableMap<K,V> extends UnmodifiableNavigableMap<K,V>
            implements Serializable {

            private static final long serialVersionUID = -2239321462712562324L;

            EmptyNavigableMap()                       { super(new TreeMap<>()); }

            @Override
            public NavigableSet<K> navigableKeySet()
                                                { return emptyNavigableSet(); }

            private Object readResolve()        { return EMPTY_NAVIGABLE_MAP; }
        }

        
Singleton for emptyNavigableMap() which is also immutable. 
/**
         * Singleton for {@link emptyNavigableMap()} which is also immutable.
         */
        private static final EmptyNavigableMap<?,?> EMPTY_NAVIGABLE_MAP =
            new EmptyNavigableMap<>();

        
The instance we wrap and protect.

/**
         * The instance we wrap and protect.
         */
        private final NavigableMap<K, ? extends V> nm;

        UnmodifiableNavigableMap(NavigableMap<K, ? extends V> m)
                                                            {super(m); nm = m;}

        public K lowerKey(K key)                   { return nm.lowerKey(key); }
        public K floorKey(K key)                   { return nm.floorKey(key); }
        public K ceilingKey(K key)               { return nm.ceilingKey(key); }
        public K higherKey(K key)                 { return nm.higherKey(key); }

        @SuppressWarnings("unchecked")
        public Entry<K, V> lowerEntry(K key) {
            Entry<K,V> lower = (Entry<K, V>) nm.lowerEntry(key);
            return (null != lower)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(lower)
                : null;
        }

        @SuppressWarnings("unchecked")
        public Entry<K, V> floorEntry(K key) {
            Entry<K,V> floor = (Entry<K, V>) nm.floorEntry(key);
            return (null != floor)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(floor)
                : null;
        }

        @SuppressWarnings("unchecked")
        public Entry<K, V> ceilingEntry(K key) {
            Entry<K,V> ceiling = (Entry<K, V>) nm.ceilingEntry(key);
            return (null != ceiling)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(ceiling)
                : null;
        }


        @SuppressWarnings("unchecked")
        public Entry<K, V> higherEntry(K key) {
            Entry<K,V> higher = (Entry<K, V>) nm.higherEntry(key);
            return (null != higher)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(higher)
                : null;
        }

        @SuppressWarnings("unchecked")
        public Entry<K, V> firstEntry() {
            Entry<K,V> first = (Entry<K, V>) nm.firstEntry();
            return (null != first)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(first)
                : null;
        }

        @SuppressWarnings("unchecked")
        public Entry<K, V> lastEntry() {
            Entry<K,V> last = (Entry<K, V>) nm.lastEntry();
            return (null != last)
                ? new UnmodifiableEntrySet.UnmodifiableEntry<>(last)
                : null;
        }

        public Entry<K, V> pollFirstEntry()
                                 { throw new UnsupportedOperationException(); }
        public Entry<K, V> pollLastEntry()
                                 { throw new UnsupportedOperationException(); }
        public NavigableMap<K, V> descendingMap()
                       { return unmodifiableNavigableMap(nm.descendingMap()); }
        public NavigableSet<K> navigableKeySet()
                     { return unmodifiableNavigableSet(nm.navigableKeySet()); }
        public NavigableSet<K> descendingKeySet()
                    { return unmodifiableNavigableSet(nm.descendingKeySet()); }

        public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
            return unmodifiableNavigableMap(
                nm.subMap(fromKey, fromInclusive, toKey, toInclusive));
        }

        public NavigableMap<K, V> headMap(K toKey, boolean inclusive)
             { return unmodifiableNavigableMap(nm.headMap(toKey, inclusive)); }
        public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive)
           { return unmodifiableNavigableMap(nm.tailMap(fromKey, inclusive)); }
    }

    // Synch Wrappers

    
Returns a synchronized (thread-safe) collection backed by the specified
collection.  In order to guarantee serial access, it is critical that
all access to the backing collection is accomplished
through the returned collection.
 It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream: 
 Collection c = Collections.synchronizedCollection(myCollection);
    ...
 synchronized (c) {
     Iterator i = c.iterator(); // Must be in the synchronized block
     while (i.hasNext())
        foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list.

The returned collection will be serializable if the specified collection
is serializable.

Params:
  • c – the collection to be "wrapped" in a synchronized collection.
Type parameters:
  • <T> – the class of the objects in the collection
Returns:a synchronized view of the specified collection.
/**
     * Returns a synchronized (thread-safe) collection backed by the specified
     * collection.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing collection is accomplished
     * through the returned collection.<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * collection when traversing it via {@link Iterator}, {@link Spliterator}
     * or {@link Stream}:
     * <pre>
     *  Collection c = Collections.synchronizedCollection(myCollection);
     *     ...
     *  synchronized (c) {
     *      Iterator i = c.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *         foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned collection does <i>not</i> pass the {@code hashCode}
     * and {@code equals} operations through to the backing collection, but
     * relies on {@code Object}'s equals and hashCode methods.  This is
     * necessary to preserve the contracts of these operations in the case
     * that the backing collection is a set or a list.<p>
     *
     * The returned collection will be serializable if the specified collection
     * is serializable.
     *
     * @param  <T> the class of the objects in the collection
     * @param  c the collection to be "wrapped" in a synchronized collection.
     * @return a synchronized view of the specified collection.
     */
    public static <T> Collection<T> synchronizedCollection(Collection<T> c) {
        return new SynchronizedCollection<>(c);
    }

    static <T> Collection<T> synchronizedCollection(Collection<T> c, Object mutex) {
        return new SynchronizedCollection<>(c, mutex);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedCollection<E> implements Collection<E>, Serializable {
        private static final long serialVersionUID = 3053995032091335093L;

        final Collection<E> c;  // Backing Collection
        final Object mutex;     // Object on which to synchronize

        SynchronizedCollection(Collection<E> c) {
            this.c = Objects.requireNonNull(c);
            mutex = this;
        }

        SynchronizedCollection(Collection<E> c, Object mutex) {
            this.c = Objects.requireNonNull(c);
            this.mutex = Objects.requireNonNull(mutex);
        }

        public int size() {
            synchronized (mutex) {return c.size();}
        }
        public boolean isEmpty() {
            synchronized (mutex) {return c.isEmpty();}
        }
        public boolean contains(Object o) {
            synchronized (mutex) {return c.contains(o);}
        }
        public Object[] toArray() {
            synchronized (mutex) {return c.toArray();}
        }
        public <T> T[] toArray(T[] a) {
            synchronized (mutex) {return c.toArray(a);}
        }
        public <T> T[] toArray(IntFunction<T[]> f) {
            synchronized (mutex) {return c.toArray(f);}
        }

        public Iterator<E> iterator() {
            return c.iterator(); // Must be manually synched by user!
        }

        public boolean add(E e) {
            synchronized (mutex) {return c.add(e);}
        }
        public boolean remove(Object o) {
            synchronized (mutex) {return c.remove(o);}
        }

        public boolean containsAll(Collection<?> coll) {
            synchronized (mutex) {return c.containsAll(coll);}
        }
        public boolean addAll(Collection<? extends E> coll) {
            synchronized (mutex) {return c.addAll(coll);}
        }
        public boolean removeAll(Collection<?> coll) {
            synchronized (mutex) {return c.removeAll(coll);}
        }
        public boolean retainAll(Collection<?> coll) {
            synchronized (mutex) {return c.retainAll(coll);}
        }
        public void clear() {
            synchronized (mutex) {c.clear();}
        }
        public String toString() {
            synchronized (mutex) {return c.toString();}
        }
        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> consumer) {
            synchronized (mutex) {c.forEach(consumer);}
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            synchronized (mutex) {return c.removeIf(filter);}
        }
        @Override
        public Spliterator<E> spliterator() {
            return c.spliterator(); // Must be manually synched by user!
        }
        @Override
        public Stream<E> stream() {
            return c.stream(); // Must be manually synched by user!
        }
        @Override
        public Stream<E> parallelStream() {
            return c.parallelStream(); // Must be manually synched by user!
        }
        private void writeObject(ObjectOutputStream s) throws IOException {
            synchronized (mutex) {s.defaultWriteObject();}
        }
    }

    
Returns a synchronized (thread-safe) set backed by the specified
set.  In order to guarantee serial access, it is critical that
all access to the backing set is accomplished
through the returned set.
 It is imperative that the user manually synchronize on the returned collection when traversing it via Iterator, Spliterator or Stream: 
 Set s = Collections.synchronizedSet(new HashSet());
     ...
 synchronized (s) {
     Iterator i = s.iterator(); // Must be in the synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned set will be serializable if the specified set is
serializable.

Params:
  • s – the set to be "wrapped" in a synchronized set.
Type parameters:
  • <T> – the class of the objects in the set
Returns:a synchronized view of the specified set.
/**
     * Returns a synchronized (thread-safe) set backed by the specified
     * set.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing set is accomplished
     * through the returned set.<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * collection when traversing it via {@link Iterator}, {@link Spliterator}
     * or {@link Stream}:
     * <pre>
     *  Set s = Collections.synchronizedSet(new HashSet());
     *      ...
     *  synchronized (s) {
     *      Iterator i = s.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned set will be serializable if the specified set is
     * serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param  s the set to be "wrapped" in a synchronized set.
     * @return a synchronized view of the specified set.
     */
    public static <T> Set<T> synchronizedSet(Set<T> s) {
        return new SynchronizedSet<>(s);
    }

    static <T> Set<T> synchronizedSet(Set<T> s, Object mutex) {
        return new SynchronizedSet<>(s, mutex);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedSet<E>
          extends SynchronizedCollection<E>
          implements Set<E> {
        private static final long serialVersionUID = 487447009682186044L;

        SynchronizedSet(Set<E> s) {
            super(s);
        }
        SynchronizedSet(Set<E> s, Object mutex) {
            super(s, mutex);
        }

        public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized (mutex) {return c.equals(o);}
        }
        public int hashCode() {
            synchronized (mutex) {return c.hashCode();}
        }
    }

    
Returns a synchronized (thread-safe) sorted set backed by the specified
sorted set.  In order to guarantee serial access, it is critical that
all access to the backing sorted set is accomplished
through the returned sorted set (or its views).
 It is imperative that the user manually synchronize on the returned sorted set when traversing it or any of its subSet, headSet, or tailSet views via Iterator, Spliterator or Stream: 
 SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
     ...
 synchronized (s) {
     Iterator i = s.iterator(); // Must be in the synchronized block
     while (i.hasNext())
         foo(i.next());
 }


or:

 SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
 SortedSet s2 = s.headSet(foo);
     ...
 synchronized (s) {  // Note: s, not s2!!!
     Iterator i = s2.iterator(); // Must be in the synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned sorted set will be serializable if the specified
sorted set is serializable.

Params:
  • s – the sorted set to be "wrapped" in a synchronized sorted set.
Type parameters:
  • <T> – the class of the objects in the set
Returns:a synchronized view of the specified sorted set.
/**
     * Returns a synchronized (thread-safe) sorted set backed by the specified
     * sorted set.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing sorted set is accomplished
     * through the returned sorted set (or its views).<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * sorted set when traversing it or any of its {@code subSet},
     * {@code headSet}, or {@code tailSet} views via {@link Iterator},
     * {@link Spliterator} or {@link Stream}:
     * <pre>
     *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
     *      ...
     *  synchronized (s) {
     *      Iterator i = s.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * or:
     * <pre>
     *  SortedSet s = Collections.synchronizedSortedSet(new TreeSet());
     *  SortedSet s2 = s.headSet(foo);
     *      ...
     *  synchronized (s) {  // Note: s, not s2!!!
     *      Iterator i = s2.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned sorted set will be serializable if the specified
     * sorted set is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param  s the sorted set to be "wrapped" in a synchronized sorted set.
     * @return a synchronized view of the specified sorted set.
     */
    public static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s) {
        return new SynchronizedSortedSet<>(s);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedSortedSet<E>
        extends SynchronizedSet<E>
        implements SortedSet<E>
    {
        private static final long serialVersionUID = 8695801310862127406L;

        private final SortedSet<E> ss;

        SynchronizedSortedSet(SortedSet<E> s) {
            super(s);
            ss = s;
        }
        SynchronizedSortedSet(SortedSet<E> s, Object mutex) {
            super(s, mutex);
            ss = s;
        }

        public Comparator<? super E> comparator() {
            synchronized (mutex) {return ss.comparator();}
        }

        public SortedSet<E> subSet(E fromElement, E toElement) {
            synchronized (mutex) {
                return new SynchronizedSortedSet<>(
                    ss.subSet(fromElement, toElement), mutex);
            }
        }
        public SortedSet<E> headSet(E toElement) {
            synchronized (mutex) {
                return new SynchronizedSortedSet<>(ss.headSet(toElement), mutex);
            }
        }
        public SortedSet<E> tailSet(E fromElement) {
            synchronized (mutex) {
               return new SynchronizedSortedSet<>(ss.tailSet(fromElement),mutex);
            }
        }

        public E first() {
            synchronized (mutex) {return ss.first();}
        }
        public E last() {
            synchronized (mutex) {return ss.last();}
        }
    }

    
Returns a synchronized (thread-safe) navigable set backed by the
specified navigable set.  In order to guarantee serial access, it is
critical that all access to the backing navigable set is
accomplished through the returned navigable set (or its views).
 It is imperative that the user manually synchronize on the returned navigable set when traversing it, or any of its subSet, headSet, or tailSet views, via Iterator, Spliterator or Stream: 
 NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
     ...
 synchronized (s) {
     Iterator i = s.iterator(); // Must be in the synchronized block
     while (i.hasNext())
         foo(i.next());
 }


or:

 NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
 NavigableSet s2 = s.headSet(foo, true);
     ...
 synchronized (s) {  // Note: s, not s2!!!
     Iterator i = s2.iterator(); // Must be in the synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned navigable set will be serializable if the specified
navigable set is serializable.

Params:
  • s – the navigable set to be "wrapped" in a synchronized navigable
set
Type parameters:
  • <T> – the class of the objects in the set
Returns:a synchronized view of the specified navigable set
Since:1.8
/**
     * Returns a synchronized (thread-safe) navigable set backed by the
     * specified navigable set.  In order to guarantee serial access, it is
     * critical that <strong>all</strong> access to the backing navigable set is
     * accomplished through the returned navigable set (or its views).<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * navigable set when traversing it, or any of its {@code subSet},
     * {@code headSet}, or {@code tailSet} views, via {@link Iterator},
     * {@link Spliterator} or {@link Stream}:
     * <pre>
     *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
     *      ...
     *  synchronized (s) {
     *      Iterator i = s.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * or:
     * <pre>
     *  NavigableSet s = Collections.synchronizedNavigableSet(new TreeSet());
     *  NavigableSet s2 = s.headSet(foo, true);
     *      ...
     *  synchronized (s) {  // Note: s, not s2!!!
     *      Iterator i = s2.iterator(); // Must be in the synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned navigable set will be serializable if the specified
     * navigable set is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param  s the navigable set to be "wrapped" in a synchronized navigable
     * set
     * @return a synchronized view of the specified navigable set
     * @since 1.8
     */
    public static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s) {
        return new SynchronizedNavigableSet<>(s);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedNavigableSet<E>
        extends SynchronizedSortedSet<E>
        implements NavigableSet<E>
    {
        private static final long serialVersionUID = -5505529816273629798L;

        private final NavigableSet<E> ns;

        SynchronizedNavigableSet(NavigableSet<E> s) {
            super(s);
            ns = s;
        }

        SynchronizedNavigableSet(NavigableSet<E> s, Object mutex) {
            super(s, mutex);
            ns = s;
        }
        public E lower(E e)      { synchronized (mutex) {return ns.lower(e);} }
        public E floor(E e)      { synchronized (mutex) {return ns.floor(e);} }
        public E ceiling(E e)  { synchronized (mutex) {return ns.ceiling(e);} }
        public E higher(E e)    { synchronized (mutex) {return ns.higher(e);} }
        public E pollFirst()  { synchronized (mutex) {return ns.pollFirst();} }
        public E pollLast()    { synchronized (mutex) {return ns.pollLast();} }

        public NavigableSet<E> descendingSet() {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.descendingSet(), mutex);
            }
        }

        public Iterator<E> descendingIterator()
                 { synchronized (mutex) { return descendingSet().iterator(); } }

        public NavigableSet<E> subSet(E fromElement, E toElement) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.subSet(fromElement, true, toElement, false), mutex);
            }
        }
        public NavigableSet<E> headSet(E toElement) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.headSet(toElement, false), mutex);
            }
        }
        public NavigableSet<E> tailSet(E fromElement) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, true), mutex);
            }
        }

        public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), mutex);
            }
        }

        public NavigableSet<E> headSet(E toElement, boolean inclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.headSet(toElement, inclusive), mutex);
            }
        }

        public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(ns.tailSet(fromElement, inclusive), mutex);
            }
        }
    }

    
Returns a synchronized (thread-safe) list backed by the specified
list.  In order to guarantee serial access, it is critical that
all access to the backing list is accomplished
through the returned list.
 It is imperative that the user manually synchronize on the returned list when traversing it via Iterator, Spliterator or Stream: 
 List list = Collections.synchronizedList(new ArrayList());
     ...
 synchronized (list) {
     Iterator i = list.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned list will be serializable if the specified list is
serializable.

Params:
  • list – the list to be "wrapped" in a synchronized list.
Type parameters:
  • <T> – the class of the objects in the list
Returns:a synchronized view of the specified list.
/**
     * Returns a synchronized (thread-safe) list backed by the specified
     * list.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing list is accomplished
     * through the returned list.<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * list when traversing it via {@link Iterator}, {@link Spliterator}
     * or {@link Stream}:
     * <pre>
     *  List list = Collections.synchronizedList(new ArrayList());
     *      ...
     *  synchronized (list) {
     *      Iterator i = list.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned list will be serializable if the specified list is
     * serializable.
     *
     * @param  <T> the class of the objects in the list
     * @param  list the list to be "wrapped" in a synchronized list.
     * @return a synchronized view of the specified list.
     */
    public static <T> List<T> synchronizedList(List<T> list) {
        return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<>(list) :
                new SynchronizedList<>(list));
    }

    static <T> List<T> synchronizedList(List<T> list, Object mutex) {
        return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<>(list, mutex) :
                new SynchronizedList<>(list, mutex));
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedList<E>
        extends SynchronizedCollection<E>
        implements List<E> {
        private static final long serialVersionUID = -7754090372962971524L;

        final List<E> list;

        SynchronizedList(List<E> list) {
            super(list);
            this.list = list;
        }
        SynchronizedList(List<E> list, Object mutex) {
            super(list, mutex);
            this.list = list;
        }

        public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized (mutex) {return list.equals(o);}
        }
        public int hashCode() {
            synchronized (mutex) {return list.hashCode();}
        }

        public E get(int index) {
            synchronized (mutex) {return list.get(index);}
        }
        public E set(int index, E element) {
            synchronized (mutex) {return list.set(index, element);}
        }
        public void add(int index, E element) {
            synchronized (mutex) {list.add(index, element);}
        }
        public E remove(int index) {
            synchronized (mutex) {return list.remove(index);}
        }

        public int indexOf(Object o) {
            synchronized (mutex) {return list.indexOf(o);}
        }
        public int lastIndexOf(Object o) {
            synchronized (mutex) {return list.lastIndexOf(o);}
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            synchronized (mutex) {return list.addAll(index, c);}
        }

        public ListIterator<E> listIterator() {
            return list.listIterator(); // Must be manually synched by user
        }

        public ListIterator<E> listIterator(int index) {
            return list.listIterator(index); // Must be manually synched by user
        }

        public List<E> subList(int fromIndex, int toIndex) {
            synchronized (mutex) {
                return new SynchronizedList<>(list.subList(fromIndex, toIndex),
                                            mutex);
            }
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            synchronized (mutex) {list.replaceAll(operator);}
        }
        @Override
        public void sort(Comparator<? super E> c) {
            synchronized (mutex) {list.sort(c);}
        }

        
SynchronizedRandomAccessList instances are serialized as
SynchronizedList instances to allow them to be deserialized
in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
This method inverts the transformation.  As a beneficial
side-effect, it also grafts the RandomAccess marker onto
SynchronizedList instances that were serialized in pre-1.4 JREs.
Note: Unfortunately, SynchronizedRandomAccessList instances
serialized in 1.4.1 and deserialized in 1.4 will become
SynchronizedList instances, as this method was missing in 1.4.

/**
         * SynchronizedRandomAccessList instances are serialized as
         * SynchronizedList instances to allow them to be deserialized
         * in pre-1.4 JREs (which do not have SynchronizedRandomAccessList).
         * This method inverts the transformation.  As a beneficial
         * side-effect, it also grafts the RandomAccess marker onto
         * SynchronizedList instances that were serialized in pre-1.4 JREs.
         *
         * Note: Unfortunately, SynchronizedRandomAccessList instances
         * serialized in 1.4.1 and deserialized in 1.4 will become
         * SynchronizedList instances, as this method was missing in 1.4.
         */
        private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new SynchronizedRandomAccessList<>(list)
                    : this);
        }
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedRandomAccessList<E>
        extends SynchronizedList<E>
        implements RandomAccess {

        SynchronizedRandomAccessList(List<E> list) {
            super(list);
        }

        SynchronizedRandomAccessList(List<E> list, Object mutex) {
            super(list, mutex);
        }

        public List<E> subList(int fromIndex, int toIndex) {
            synchronized (mutex) {
                return new SynchronizedRandomAccessList<>(
                    list.subList(fromIndex, toIndex), mutex);
            }
        }

        private static final long serialVersionUID = 1530674583602358482L;

        
Allows instances to be deserialized in pre-1.4 JREs (which do
not have SynchronizedRandomAccessList).  SynchronizedList has
a readResolve method that inverts this transformation upon
deserialization.

/**
         * Allows instances to be deserialized in pre-1.4 JREs (which do
         * not have SynchronizedRandomAccessList).  SynchronizedList has
         * a readResolve method that inverts this transformation upon
         * deserialization.
         */
        private Object writeReplace() {
            return new SynchronizedList<>(list);
        }
    }

    
Returns a synchronized (thread-safe) map backed by the specified
map.  In order to guarantee serial access, it is critical that
all access to the backing map is accomplished
through the returned map.
 It is imperative that the user manually synchronize on the returned map when traversing any of its collection views via Iterator, Spliterator or Stream: 
 Map m = Collections.synchronizedMap(new HashMap());
     ...
 Set s = m.keySet();  // Needn't be in synchronized block
     ...
 synchronized (m) {  // Synchronizing on m, not s!
     Iterator i = s.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned map will be serializable if the specified map is
serializable.

Params:
  • m – the map to be "wrapped" in a synchronized map.
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:a synchronized view of the specified map.
/**
     * Returns a synchronized (thread-safe) map backed by the specified
     * map.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing map is accomplished
     * through the returned map.<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * map when traversing any of its collection views via {@link Iterator},
     * {@link Spliterator} or {@link Stream}:
     * <pre>
     *  Map m = Collections.synchronizedMap(new HashMap());
     *      ...
     *  Set s = m.keySet();  // Needn't be in synchronized block
     *      ...
     *  synchronized (m) {  // Synchronizing on m, not s!
     *      Iterator i = s.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned map will be serializable if the specified map is
     * serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param  m the map to be "wrapped" in a synchronized map.
     * @return a synchronized view of the specified map.
     */
    public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
        return new SynchronizedMap<>(m);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class SynchronizedMap<K,V>
        implements Map<K,V>, Serializable {
        private static final long serialVersionUID = 1978198479659022715L;

        private final Map<K,V> m;     // Backing Map
        final Object      mutex;        // Object on which to synchronize

        SynchronizedMap(Map<K,V> m) {
            this.m = Objects.requireNonNull(m);
            mutex = this;
        }

        SynchronizedMap(Map<K,V> m, Object mutex) {
            this.m = m;
            this.mutex = mutex;
        }

        public int size() {
            synchronized (mutex) {return m.size();}
        }
        public boolean isEmpty() {
            synchronized (mutex) {return m.isEmpty();}
        }
        public boolean containsKey(Object key) {
            synchronized (mutex) {return m.containsKey(key);}
        }
        public boolean containsValue(Object value) {
            synchronized (mutex) {return m.containsValue(value);}
        }
        public V get(Object key) {
            synchronized (mutex) {return m.get(key);}
        }

        public V put(K key, V value) {
            synchronized (mutex) {return m.put(key, value);}
        }
        public V remove(Object key) {
            synchronized (mutex) {return m.remove(key);}
        }
        public void putAll(Map<? extends K, ? extends V> map) {
            synchronized (mutex) {m.putAll(map);}
        }
        public void clear() {
            synchronized (mutex) {m.clear();}
        }

        private transient Set<K> keySet;
        private transient Set<Map.Entry<K,V>> entrySet;
        private transient Collection<V> values;

        public Set<K> keySet() {
            synchronized (mutex) {
                if (keySet==null)
                    keySet = new SynchronizedSet<>(m.keySet(), mutex);
                return keySet;
            }
        }

        public Set<Map.Entry<K,V>> entrySet() {
            synchronized (mutex) {
                if (entrySet==null)
                    entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
                return entrySet;
            }
        }

        public Collection<V> values() {
            synchronized (mutex) {
                if (values==null)
                    values = new SynchronizedCollection<>(m.values(), mutex);
                return values;
            }
        }

        public boolean equals(Object o) {
            if (this == o)
                return true;
            synchronized (mutex) {return m.equals(o);}
        }
        public int hashCode() {
            synchronized (mutex) {return m.hashCode();}
        }
        public String toString() {
            synchronized (mutex) {return m.toString();}
        }

        // Override default methods in Map
        @Override
        public V getOrDefault(Object k, V defaultValue) {
            synchronized (mutex) {return m.getOrDefault(k, defaultValue);}
        }
        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            synchronized (mutex) {m.forEach(action);}
        }
        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            synchronized (mutex) {m.replaceAll(function);}
        }
        @Override
        public V putIfAbsent(K key, V value) {
            synchronized (mutex) {return m.putIfAbsent(key, value);}
        }
        @Override
        public boolean remove(Object key, Object value) {
            synchronized (mutex) {return m.remove(key, value);}
        }
        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            synchronized (mutex) {return m.replace(key, oldValue, newValue);}
        }
        @Override
        public V replace(K key, V value) {
            synchronized (mutex) {return m.replace(key, value);}
        }
        @Override
        public V computeIfAbsent(K key,
                Function<? super K, ? extends V> mappingFunction) {
            synchronized (mutex) {return m.computeIfAbsent(key, mappingFunction);}
        }
        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            synchronized (mutex) {return m.computeIfPresent(key, remappingFunction);}
        }
        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            synchronized (mutex) {return m.compute(key, remappingFunction);}
        }
        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            synchronized (mutex) {return m.merge(key, value, remappingFunction);}
        }

        private void writeObject(ObjectOutputStream s) throws IOException {
            synchronized (mutex) {s.defaultWriteObject();}
        }
    }

    
Returns a synchronized (thread-safe) sorted map backed by the specified
sorted map.  In order to guarantee serial access, it is critical that
all access to the backing sorted map is accomplished
through the returned sorted map (or its views).
 It is imperative that the user manually synchronize on the returned sorted map when traversing any of its collection views, or the collections views of any of its subMap, headMap or tailMap views, via Iterator, Spliterator or Stream: 
 SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
     ...
 Set s = m.keySet();  // Needn't be in synchronized block
     ...
 synchronized (m) {  // Synchronizing on m, not s!
     Iterator i = s.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


or:

 SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
 SortedMap m2 = m.subMap(foo, bar);
     ...
 Set s2 = m2.keySet();  // Needn't be in synchronized block
     ...
 synchronized (m) {  // Synchronizing on m, not m2 or s2!
     Iterator i = s2.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned sorted map will be serializable if the specified
sorted map is serializable.

Params:
  • m – the sorted map to be "wrapped" in a synchronized sorted map.
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:a synchronized view of the specified sorted map.
/**
     * Returns a synchronized (thread-safe) sorted map backed by the specified
     * sorted map.  In order to guarantee serial access, it is critical that
     * <strong>all</strong> access to the backing sorted map is accomplished
     * through the returned sorted map (or its views).<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * sorted map when traversing any of its collection views, or the
     * collections views of any of its {@code subMap}, {@code headMap} or
     * {@code tailMap} views, via {@link Iterator}, {@link Spliterator} or
     * {@link Stream}:
     * <pre>
     *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
     *      ...
     *  Set s = m.keySet();  // Needn't be in synchronized block
     *      ...
     *  synchronized (m) {  // Synchronizing on m, not s!
     *      Iterator i = s.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * or:
     * <pre>
     *  SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
     *  SortedMap m2 = m.subMap(foo, bar);
     *      ...
     *  Set s2 = m2.keySet();  // Needn't be in synchronized block
     *      ...
     *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
     *      Iterator i = s2.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned sorted map will be serializable if the specified
     * sorted map is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param  m the sorted map to be "wrapped" in a synchronized sorted map.
     * @return a synchronized view of the specified sorted map.
     */
    public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
        return new SynchronizedSortedMap<>(m);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class SynchronizedSortedMap<K,V>
        extends SynchronizedMap<K,V>
        implements SortedMap<K,V>
    {
        private static final long serialVersionUID = -8798146769416483793L;

        private final SortedMap<K,V> sm;

        SynchronizedSortedMap(SortedMap<K,V> m) {
            super(m);
            sm = m;
        }
        SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
            super(m, mutex);
            sm = m;
        }

        public Comparator<? super K> comparator() {
            synchronized (mutex) {return sm.comparator();}
        }

        public SortedMap<K,V> subMap(K fromKey, K toKey) {
            synchronized (mutex) {
                return new SynchronizedSortedMap<>(
                    sm.subMap(fromKey, toKey), mutex);
            }
        }
        public SortedMap<K,V> headMap(K toKey) {
            synchronized (mutex) {
                return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
            }
        }
        public SortedMap<K,V> tailMap(K fromKey) {
            synchronized (mutex) {
               return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
            }
        }

        public K firstKey() {
            synchronized (mutex) {return sm.firstKey();}
        }
        public K lastKey() {
            synchronized (mutex) {return sm.lastKey();}
        }
    }

    
Returns a synchronized (thread-safe) navigable map backed by the
specified navigable map.  In order to guarantee serial access, it is
critical that all access to the backing navigable map is
accomplished through the returned navigable map (or its views).
 It is imperative that the user manually synchronize on the returned navigable map when traversing any of its collection views, or the collections views of any of its subMap, headMap or tailMap views, via Iterator, Spliterator or Stream: 
 NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
     ...
 Set s = m.keySet();  // Needn't be in synchronized block
     ...
 synchronized (m) {  // Synchronizing on m, not s!
     Iterator i = s.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


or:

 NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
 NavigableMap m2 = m.subMap(foo, true, bar, false);
     ...
 Set s2 = m2.keySet();  // Needn't be in synchronized block
     ...
 synchronized (m) {  // Synchronizing on m, not m2 or s2!
     Iterator i = s.iterator(); // Must be in synchronized block
     while (i.hasNext())
         foo(i.next());
 }


Failure to follow this advice may result in non-deterministic behavior.

The returned navigable map will be serializable if the specified
navigable map is serializable.

Params:
  • m – the navigable map to be "wrapped" in a synchronized navigable
             map
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:a synchronized view of the specified navigable map.
Since:1.8
/**
     * Returns a synchronized (thread-safe) navigable map backed by the
     * specified navigable map.  In order to guarantee serial access, it is
     * critical that <strong>all</strong> access to the backing navigable map is
     * accomplished through the returned navigable map (or its views).<p>
     *
     * It is imperative that the user manually synchronize on the returned
     * navigable map when traversing any of its collection views, or the
     * collections views of any of its {@code subMap}, {@code headMap} or
     * {@code tailMap} views, via {@link Iterator}, {@link Spliterator} or
     * {@link Stream}:
     * <pre>
     *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
     *      ...
     *  Set s = m.keySet();  // Needn't be in synchronized block
     *      ...
     *  synchronized (m) {  // Synchronizing on m, not s!
     *      Iterator i = s.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * or:
     * <pre>
     *  NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
     *  NavigableMap m2 = m.subMap(foo, true, bar, false);
     *      ...
     *  Set s2 = m2.keySet();  // Needn't be in synchronized block
     *      ...
     *  synchronized (m) {  // Synchronizing on m, not m2 or s2!
     *      Iterator i = s.iterator(); // Must be in synchronized block
     *      while (i.hasNext())
     *          foo(i.next());
     *  }
     * </pre>
     * Failure to follow this advice may result in non-deterministic behavior.
     *
     * <p>The returned navigable map will be serializable if the specified
     * navigable map is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param  m the navigable map to be "wrapped" in a synchronized navigable
     *              map
     * @return a synchronized view of the specified navigable map.
     * @since 1.8
     */
    public static <K,V> NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m) {
        return new SynchronizedNavigableMap<>(m);
    }

    
A synchronized NavigableMap.

@serialinclude
/**
     * A synchronized NavigableMap.
     *
     * @serial include
     */
    static class SynchronizedNavigableMap<K,V>
        extends SynchronizedSortedMap<K,V>
        implements NavigableMap<K,V>
    {
        private static final long serialVersionUID = 699392247599746807L;

        private final NavigableMap<K,V> nm;

        SynchronizedNavigableMap(NavigableMap<K,V> m) {
            super(m);
            nm = m;
        }
        SynchronizedNavigableMap(NavigableMap<K,V> m, Object mutex) {
            super(m, mutex);
            nm = m;
        }

        public Entry<K, V> lowerEntry(K key)
                        { synchronized (mutex) { return nm.lowerEntry(key); } }
        public K lowerKey(K key)
                          { synchronized (mutex) { return nm.lowerKey(key); } }
        public Entry<K, V> floorEntry(K key)
                        { synchronized (mutex) { return nm.floorEntry(key); } }
        public K floorKey(K key)
                          { synchronized (mutex) { return nm.floorKey(key); } }
        public Entry<K, V> ceilingEntry(K key)
                      { synchronized (mutex) { return nm.ceilingEntry(key); } }
        public K ceilingKey(K key)
                        { synchronized (mutex) { return nm.ceilingKey(key); } }
        public Entry<K, V> higherEntry(K key)
                       { synchronized (mutex) { return nm.higherEntry(key); } }
        public K higherKey(K key)
                         { synchronized (mutex) { return nm.higherKey(key); } }
        public Entry<K, V> firstEntry()
                           { synchronized (mutex) { return nm.firstEntry(); } }
        public Entry<K, V> lastEntry()
                            { synchronized (mutex) { return nm.lastEntry(); } }
        public Entry<K, V> pollFirstEntry()
                       { synchronized (mutex) { return nm.pollFirstEntry(); } }
        public Entry<K, V> pollLastEntry()
                        { synchronized (mutex) { return nm.pollLastEntry(); } }

        public NavigableMap<K, V> descendingMap() {
            synchronized (mutex) {
                return
                    new SynchronizedNavigableMap<>(nm.descendingMap(), mutex);
            }
        }

        public NavigableSet<K> keySet() {
            return navigableKeySet();
        }

        public NavigableSet<K> navigableKeySet() {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(nm.navigableKeySet(), mutex);
            }
        }

        public NavigableSet<K> descendingKeySet() {
            synchronized (mutex) {
                return new SynchronizedNavigableSet<>(nm.descendingKeySet(), mutex);
            }
        }


        public SortedMap<K,V> subMap(K fromKey, K toKey) {
            synchronized (mutex) {
                return new SynchronizedNavigableMap<>(
                    nm.subMap(fromKey, true, toKey, false), mutex);
            }
        }
        public SortedMap<K,V> headMap(K toKey) {
            synchronized (mutex) {
                return new SynchronizedNavigableMap<>(nm.headMap(toKey, false), mutex);
            }
        }
        public SortedMap<K,V> tailMap(K fromKey) {
            synchronized (mutex) {
        return new SynchronizedNavigableMap<>(nm.tailMap(fromKey, true),mutex);
            }
        }

        public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableMap<>(
                    nm.subMap(fromKey, fromInclusive, toKey, toInclusive), mutex);
            }
        }

        public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableMap<>(
                        nm.headMap(toKey, inclusive), mutex);
            }
        }

        public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
            synchronized (mutex) {
                return new SynchronizedNavigableMap<>(
                    nm.tailMap(fromKey, inclusive), mutex);
            }
        }
    }

    // Dynamically typesafe collection wrappers

    
Returns a dynamically typesafe view of the specified collection. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a collection contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the collection takes place through the view, it is guaranteed that the collection cannot contain an incorrectly
typed element.

The generics mechanism in the language provides compile-time
(static) type checking, but it is possible to defeat this mechanism
with unchecked casts.  Usually this is not a problem, as the compiler
issues warnings on all such unchecked operations.  There are, however,
times when static type checking alone is not sufficient.  For example,
suppose a collection is passed to a third-party library and it is
imperative that the library code not corrupt the collection by
inserting an element of the wrong type.

Another use of dynamically typesafe views is debugging. Suppose a program fails with a ClassCastException, indicating that an incorrectly typed element was put into a parameterized collection. Unfortunately, the exception can occur at any time after the erroneous element is inserted, so it typically provides little or no information as to the real source of the problem. If the problem is reproducible, one can quickly determine its source by temporarily modifying the program to wrap the collection with a dynamically typesafe view. For example, this declaration: 
 
    Collection<String> c = new HashSet<>();


may be replaced temporarily by this one:
 
 
    Collection<String> c = Collections.checkedCollection(
        new HashSet<>(), String.class);


Running the program again will cause it to fail at the point where
an incorrectly typed element is inserted into the collection, clearly
identifying the source of the problem.  Once the problem is fixed, the
modified declaration may be reverted back to the original.

The returned collection does not pass the hashCode and equals operations through to the backing collection, but relies on Object's equals and hashCode methods. This is necessary to preserve the contracts of these operations in the case that the backing collection is a set or a list. 
The returned collection will be serializable if the specified
collection is serializable.

Since null is considered to be a value of any reference type, the returned collection permits insertion of null elements whenever the backing collection does. 
Params:
  • c – the collection for which a dynamically typesafe view is to be
         returned
  • type – the type of element that c is permitted to hold
Type parameters:
  • <E> – the class of the objects in the collection
Returns:a dynamically typesafe view of the specified collection
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified collection.
     * Any attempt to insert an element of the wrong type will result in an
     * immediate {@link ClassCastException}.  Assuming a collection
     * contains no incorrectly typed elements prior to the time a
     * dynamically typesafe view is generated, and that all subsequent
     * access to the collection takes place through the view, it is
     * <i>guaranteed</i> that the collection cannot contain an incorrectly
     * typed element.
     *
     * <p>The generics mechanism in the language provides compile-time
     * (static) type checking, but it is possible to defeat this mechanism
     * with unchecked casts.  Usually this is not a problem, as the compiler
     * issues warnings on all such unchecked operations.  There are, however,
     * times when static type checking alone is not sufficient.  For example,
     * suppose a collection is passed to a third-party library and it is
     * imperative that the library code not corrupt the collection by
     * inserting an element of the wrong type.
     *
     * <p>Another use of dynamically typesafe views is debugging.  Suppose a
     * program fails with a {@code ClassCastException}, indicating that an
     * incorrectly typed element was put into a parameterized collection.
     * Unfortunately, the exception can occur at any time after the erroneous
     * element is inserted, so it typically provides little or no information
     * as to the real source of the problem.  If the problem is reproducible,
     * one can quickly determine its source by temporarily modifying the
     * program to wrap the collection with a dynamically typesafe view.
     * For example, this declaration:
     *  <pre> {@code
     *     Collection<String> c = new HashSet<>();
     * }</pre>
     * may be replaced temporarily by this one:
     *  <pre> {@code
     *     Collection<String> c = Collections.checkedCollection(
     *         new HashSet<>(), String.class);
     * }</pre>
     * Running the program again will cause it to fail at the point where
     * an incorrectly typed element is inserted into the collection, clearly
     * identifying the source of the problem.  Once the problem is fixed, the
     * modified declaration may be reverted back to the original.
     *
     * <p>The returned collection does <i>not</i> pass the hashCode and equals
     * operations through to the backing collection, but relies on
     * {@code Object}'s {@code equals} and {@code hashCode} methods.  This
     * is necessary to preserve the contracts of these operations in the case
     * that the backing collection is a set or a list.
     *
     * <p>The returned collection will be serializable if the specified
     * collection is serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned collection permits insertion of null elements
     * whenever the backing collection does.
     *
     * @param <E> the class of the objects in the collection
     * @param c the collection for which a dynamically typesafe view is to be
     *          returned
     * @param type the type of element that {@code c} is permitted to hold
     * @return a dynamically typesafe view of the specified collection
     * @since 1.5
     */
    public static <E> Collection<E> checkedCollection(Collection<E> c,
                                                      Class<E> type) {
        return new CheckedCollection<>(c, type);
    }

    @SuppressWarnings("unchecked")
    static <T> T[] zeroLengthArray(Class<T> type) {
        return (T[]) Array.newInstance(type, 0);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedCollection<E> implements Collection<E>, Serializable {
        private static final long serialVersionUID = 1578914078182001775L;

        final Collection<E> c;
        final Class<E> type;

        @SuppressWarnings("unchecked")
        E typeCheck(Object o) {
            if (o != null && !type.isInstance(o))
                throw new ClassCastException(badElementMsg(o));
            return (E) o;
        }

        private String badElementMsg(Object o) {
            return "Attempt to insert " + o.getClass() +
                " element into collection with element type " + type;
        }

        CheckedCollection(Collection<E> c, Class<E> type) {
            this.c = Objects.requireNonNull(c, "c");
            this.type = Objects.requireNonNull(type, "type");
        }

        public int size()                          { return c.size(); }
        public boolean isEmpty()                   { return c.isEmpty(); }
        public boolean contains(Object o)          { return c.contains(o); }
        public Object[] toArray()                  { return c.toArray(); }
        public <T> T[] toArray(T[] a)              { return c.toArray(a); }
        public <T> T[] toArray(IntFunction<T[]> f) { return c.toArray(f); }
        public String toString()                   { return c.toString(); }
        public boolean remove(Object o)            { return c.remove(o); }
        public void clear()                        {        c.clear(); }

        public boolean containsAll(Collection<?> coll) {
            return c.containsAll(coll);
        }
        public boolean removeAll(Collection<?> coll) {
            return c.removeAll(coll);
        }
        public boolean retainAll(Collection<?> coll) {
            return c.retainAll(coll);
        }

        public Iterator<E> iterator() {
            // JDK-6363904 - unwrapped iterator could be typecast to
            // ListIterator with unsafe set()
            final Iterator<E> it = c.iterator();
            return new Iterator<E>() {
                public boolean hasNext() { return it.hasNext(); }
                public E next()          { return it.next(); }
                public void remove()     {        it.remove(); }
                public void forEachRemaining(Consumer<? super E> action) {
                    it.forEachRemaining(action);
                }
            };
        }

        public boolean add(E e)          { return c.add(typeCheck(e)); }

        private E[] zeroLengthElementArray; // Lazily initialized

        private E[] zeroLengthElementArray() {
            return zeroLengthElementArray != null ? zeroLengthElementArray :
                (zeroLengthElementArray = zeroLengthArray(type));
        }

        @SuppressWarnings("unchecked")
        Collection<E> checkedCopyOf(Collection<? extends E> coll) {
            Object[] a;
            try {
                E[] z = zeroLengthElementArray();
                a = coll.toArray(z);
                // Defend against coll violating the toArray contract
                if (a.getClass() != z.getClass())
                    a = Arrays.copyOf(a, a.length, z.getClass());
            } catch (ArrayStoreException ignore) {
                // To get better and consistent diagnostics,
                // we call typeCheck explicitly on each element.
                // We call clone() to defend against coll retaining a
                // reference to the returned array and storing a bad
                // element into it after it has been type checked.
                a = coll.toArray().clone();
                for (Object o : a)
                    typeCheck(o);
            }
            // A slight abuse of the type system, but safe here.
            return (Collection<E>) Arrays.asList(a);
        }

        public boolean addAll(Collection<? extends E> coll) {
            // Doing things this way insulates us from concurrent changes
            // in the contents of coll and provides all-or-nothing
            // semantics (which we wouldn't get if we type-checked each
            // element as we added it)
            return c.addAll(checkedCopyOf(coll));
        }

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {c.forEach(action);}
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            return c.removeIf(filter);
        }
        @Override
        public Spliterator<E> spliterator() {return c.spliterator();}
        @Override
        public Stream<E> stream()           {return c.stream();}
        @Override
        public Stream<E> parallelStream()   {return c.parallelStream();}
    }

    
Returns a dynamically typesafe view of the specified queue. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a queue contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the queue takes place through the view, it is guaranteed that the
queue cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned queue will be serializable if the specified queue
is serializable.

Since null is considered to be a value of any reference type, the returned queue permits insertion of null elements whenever the backing queue does. 
Params:
  • queue – the queue for which a dynamically typesafe view is to be
            returned
  • type – the type of element that queue is permitted to hold
Type parameters:
  • <E> – the class of the objects in the queue
Returns:a dynamically typesafe view of the specified queue
Since:1.8
/**
     * Returns a dynamically typesafe view of the specified queue.
     * Any attempt to insert an element of the wrong type will result in
     * an immediate {@link ClassCastException}.  Assuming a queue contains
     * no incorrectly typed elements prior to the time a dynamically typesafe
     * view is generated, and that all subsequent access to the queue
     * takes place through the view, it is <i>guaranteed</i> that the
     * queue cannot contain an incorrectly typed element.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned queue will be serializable if the specified queue
     * is serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned queue permits insertion of {@code null} elements
     * whenever the backing queue does.
     *
     * @param <E> the class of the objects in the queue
     * @param queue the queue for which a dynamically typesafe view is to be
     *             returned
     * @param type the type of element that {@code queue} is permitted to hold
     * @return a dynamically typesafe view of the specified queue
     * @since 1.8
     */
    public static <E> Queue<E> checkedQueue(Queue<E> queue, Class<E> type) {
        return new CheckedQueue<>(queue, type);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedQueue<E>
        extends CheckedCollection<E>
        implements Queue<E>, Serializable
    {
        private static final long serialVersionUID = 1433151992604707767L;
        final Queue<E> queue;

        CheckedQueue(Queue<E> queue, Class<E> elementType) {
            super(queue, elementType);
            this.queue = queue;
        }

        public E element()              {return queue.element();}
        public boolean equals(Object o) {return o == this || c.equals(o);}
        public int hashCode()           {return c.hashCode();}
        public E peek()                 {return queue.peek();}
        public E poll()                 {return queue.poll();}
        public E remove()               {return queue.remove();}
        public boolean offer(E e)       {return queue.offer(typeCheck(e));}
    }

    
Returns a dynamically typesafe view of the specified set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the set takes place through the view, it is guaranteed that the
set cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned set will be serializable if the specified set is
serializable.

Since null is considered to be a value of any reference type, the returned set permits insertion of null elements whenever the backing set does. 
Params:
  • s – the set for which a dynamically typesafe view is to be
         returned
  • type – the type of element that s is permitted to hold
Type parameters:
  • <E> – the class of the objects in the set
Returns:a dynamically typesafe view of the specified set
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified set.
     * Any attempt to insert an element of the wrong type will result in
     * an immediate {@link ClassCastException}.  Assuming a set contains
     * no incorrectly typed elements prior to the time a dynamically typesafe
     * view is generated, and that all subsequent access to the set
     * takes place through the view, it is <i>guaranteed</i> that the
     * set cannot contain an incorrectly typed element.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned set will be serializable if the specified set is
     * serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned set permits insertion of null elements whenever
     * the backing set does.
     *
     * @param <E> the class of the objects in the set
     * @param s the set for which a dynamically typesafe view is to be
     *          returned
     * @param type the type of element that {@code s} is permitted to hold
     * @return a dynamically typesafe view of the specified set
     * @since 1.5
     */
    public static <E> Set<E> checkedSet(Set<E> s, Class<E> type) {
        return new CheckedSet<>(s, type);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedSet<E> extends CheckedCollection<E>
                                 implements Set<E>, Serializable
    {
        private static final long serialVersionUID = 4694047833775013803L;

        CheckedSet(Set<E> s, Class<E> elementType) { super(s, elementType); }

        public boolean equals(Object o) { return o == this || c.equals(o); }
        public int hashCode()           { return c.hashCode(); }
    }

    
Returns a dynamically typesafe view of the specified sorted set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a sorted set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the sorted set takes place through the view, it is guaranteed that the sorted set cannot contain an incorrectly
typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned sorted set will be serializable if the specified sorted
set is serializable.

Since null is considered to be a value of any reference type, the returned sorted set permits insertion of null elements whenever the backing sorted set does. 
Params:
  • s – the sorted set for which a dynamically typesafe view is to be
         returned
  • type – the type of element that s is permitted to hold
Type parameters:
  • <E> – the class of the objects in the set
Returns:a dynamically typesafe view of the specified sorted set
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified sorted set.
     * Any attempt to insert an element of the wrong type will result in an
     * immediate {@link ClassCastException}.  Assuming a sorted set
     * contains no incorrectly typed elements prior to the time a
     * dynamically typesafe view is generated, and that all subsequent
     * access to the sorted set takes place through the view, it is
     * <i>guaranteed</i> that the sorted set cannot contain an incorrectly
     * typed element.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned sorted set will be serializable if the specified sorted
     * set is serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned sorted set permits insertion of null elements
     * whenever the backing sorted set does.
     *
     * @param <E> the class of the objects in the set
     * @param s the sorted set for which a dynamically typesafe view is to be
     *          returned
     * @param type the type of element that {@code s} is permitted to hold
     * @return a dynamically typesafe view of the specified sorted set
     * @since 1.5
     */
    public static <E> SortedSet<E> checkedSortedSet(SortedSet<E> s,
                                                    Class<E> type) {
        return new CheckedSortedSet<>(s, type);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedSortedSet<E> extends CheckedSet<E>
        implements SortedSet<E>, Serializable
    {
        private static final long serialVersionUID = 1599911165492914959L;

        private final SortedSet<E> ss;

        CheckedSortedSet(SortedSet<E> s, Class<E> type) {
            super(s, type);
            ss = s;
        }

        public Comparator<? super E> comparator() { return ss.comparator(); }
        public E first()                   { return ss.first(); }
        public E last()                    { return ss.last(); }

        public SortedSet<E> subSet(E fromElement, E toElement) {
            return checkedSortedSet(ss.subSet(fromElement,toElement), type);
        }
        public SortedSet<E> headSet(E toElement) {
            return checkedSortedSet(ss.headSet(toElement), type);
        }
        public SortedSet<E> tailSet(E fromElement) {
            return checkedSortedSet(ss.tailSet(fromElement), type);
        }
    }


Returns a dynamically typesafe view of the specified navigable set. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a navigable set contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the navigable set takes place through the view, it is guaranteed that the navigable set cannot contain an incorrectly
typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned navigable set will be serializable if the specified
navigable set is serializable.

Since null is considered to be a value of any reference type, the returned navigable set permits insertion of null elements whenever the backing sorted set does. 
Params:
  • s – the navigable set for which a dynamically typesafe view is to be
         returned
  • type – the type of element that s is permitted to hold
Type parameters:
  • <E> – the class of the objects in the set
Returns:a dynamically typesafe view of the specified navigable set
Since:1.8
/**
     * Returns a dynamically typesafe view of the specified navigable set.
     * Any attempt to insert an element of the wrong type will result in an
     * immediate {@link ClassCastException}.  Assuming a navigable set
     * contains no incorrectly typed elements prior to the time a
     * dynamically typesafe view is generated, and that all subsequent
     * access to the navigable set takes place through the view, it is
     * <em>guaranteed</em> that the navigable set cannot contain an incorrectly
     * typed element.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned navigable set will be serializable if the specified
     * navigable set is serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned navigable set permits insertion of null elements
     * whenever the backing sorted set does.
     *
     * @param <E> the class of the objects in the set
     * @param s the navigable set for which a dynamically typesafe view is to be
     *          returned
     * @param type the type of element that {@code s} is permitted to hold
     * @return a dynamically typesafe view of the specified navigable set
     * @since 1.8
     */
    public static <E> NavigableSet<E> checkedNavigableSet(NavigableSet<E> s,
                                                    Class<E> type) {
        return new CheckedNavigableSet<>(s, type);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedNavigableSet<E> extends CheckedSortedSet<E>
        implements NavigableSet<E>, Serializable
    {
        private static final long serialVersionUID = -5429120189805438922L;

        private final NavigableSet<E> ns;

        CheckedNavigableSet(NavigableSet<E> s, Class<E> type) {
            super(s, type);
            ns = s;
        }

        public E lower(E e)                             { return ns.lower(e); }
        public E floor(E e)                             { return ns.floor(e); }
        public E ceiling(E e)                         { return ns.ceiling(e); }
        public E higher(E e)                           { return ns.higher(e); }
        public E pollFirst()                         { return ns.pollFirst(); }
        public E pollLast()                            {return ns.pollLast(); }
        public NavigableSet<E> descendingSet()
                      { return checkedNavigableSet(ns.descendingSet(), type); }
        public Iterator<E> descendingIterator()
            {return checkedNavigableSet(ns.descendingSet(), type).iterator(); }

        public NavigableSet<E> subSet(E fromElement, E toElement) {
            return checkedNavigableSet(ns.subSet(fromElement, true, toElement, false), type);
        }
        public NavigableSet<E> headSet(E toElement) {
            return checkedNavigableSet(ns.headSet(toElement, false), type);
        }
        public NavigableSet<E> tailSet(E fromElement) {
            return checkedNavigableSet(ns.tailSet(fromElement, true), type);
        }

        public NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive) {
            return checkedNavigableSet(ns.subSet(fromElement, fromInclusive, toElement, toInclusive), type);
        }

        public NavigableSet<E> headSet(E toElement, boolean inclusive) {
            return checkedNavigableSet(ns.headSet(toElement, inclusive), type);
        }

        public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
            return checkedNavigableSet(ns.tailSet(fromElement, inclusive), type);
        }
    }

    
Returns a dynamically typesafe view of the specified list. Any attempt to insert an element of the wrong type will result in an immediate ClassCastException. Assuming a list contains no incorrectly typed elements prior to the time a dynamically typesafe view is generated, and that all subsequent access to the list takes place through the view, it is guaranteed that the
list cannot contain an incorrectly typed element.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned list will be serializable if the specified list
is serializable.

Since null is considered to be a value of any reference type, the returned list permits insertion of null elements whenever the backing list does. 
Params:
  • list – the list for which a dynamically typesafe view is to be
            returned
  • type – the type of element that list is permitted to hold
Type parameters:
  • <E> – the class of the objects in the list
Returns:a dynamically typesafe view of the specified list
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified list.
     * Any attempt to insert an element of the wrong type will result in
     * an immediate {@link ClassCastException}.  Assuming a list contains
     * no incorrectly typed elements prior to the time a dynamically typesafe
     * view is generated, and that all subsequent access to the list
     * takes place through the view, it is <i>guaranteed</i> that the
     * list cannot contain an incorrectly typed element.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned list will be serializable if the specified list
     * is serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned list permits insertion of null elements whenever
     * the backing list does.
     *
     * @param <E> the class of the objects in the list
     * @param list the list for which a dynamically typesafe view is to be
     *             returned
     * @param type the type of element that {@code list} is permitted to hold
     * @return a dynamically typesafe view of the specified list
     * @since 1.5
     */
    public static <E> List<E> checkedList(List<E> list, Class<E> type) {
        return (list instanceof RandomAccess ?
                new CheckedRandomAccessList<>(list, type) :
                new CheckedList<>(list, type));
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedList<E>
        extends CheckedCollection<E>
        implements List<E>
    {
        private static final long serialVersionUID = 65247728283967356L;
        final List<E> list;

        CheckedList(List<E> list, Class<E> type) {
            super(list, type);
            this.list = list;
        }

        public boolean equals(Object o)  { return o == this || list.equals(o); }
        public int hashCode()            { return list.hashCode(); }
        public E get(int index)          { return list.get(index); }
        public E remove(int index)       { return list.remove(index); }
        public int indexOf(Object o)     { return list.indexOf(o); }
        public int lastIndexOf(Object o) { return list.lastIndexOf(o); }

        public E set(int index, E element) {
            return list.set(index, typeCheck(element));
        }

        public void add(int index, E element) {
            list.add(index, typeCheck(element));
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            return list.addAll(index, checkedCopyOf(c));
        }
        public ListIterator<E> listIterator()   { return listIterator(0); }

        public ListIterator<E> listIterator(final int index) {
            final ListIterator<E> i = list.listIterator(index);

            return new ListIterator<E>() {
                public boolean hasNext()     { return i.hasNext(); }
                public E next()              { return i.next(); }
                public boolean hasPrevious() { return i.hasPrevious(); }
                public E previous()          { return i.previous(); }
                public int nextIndex()       { return i.nextIndex(); }
                public int previousIndex()   { return i.previousIndex(); }
                public void remove()         {        i.remove(); }

                public void set(E e) {
                    i.set(typeCheck(e));
                }

                public void add(E e) {
                    i.add(typeCheck(e));
                }

                @Override
                public void forEachRemaining(Consumer<? super E> action) {
                    i.forEachRemaining(action);
                }
            };
        }

        public List<E> subList(int fromIndex, int toIndex) {
            return new CheckedList<>(list.subList(fromIndex, toIndex), type);
        }

        
{@inheritDoc}

Throws:
  • ClassCastException – if the class of an element returned by the
        operator prevents it from being added to this collection. The
        exception may be thrown after some elements of the list have
        already been replaced.
/**
         * {@inheritDoc}
         *
         * @throws ClassCastException if the class of an element returned by the
         *         operator prevents it from being added to this collection. The
         *         exception may be thrown after some elements of the list have
         *         already been replaced.
         */
        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            Objects.requireNonNull(operator);
            list.replaceAll(e -> typeCheck(operator.apply(e)));
        }

        @Override
        public void sort(Comparator<? super E> c) {
            list.sort(c);
        }
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedRandomAccessList<E> extends CheckedList<E>
                                            implements RandomAccess
    {
        private static final long serialVersionUID = 1638200125423088369L;

        CheckedRandomAccessList(List<E> list, Class<E> type) {
            super(list, type);
        }

        public List<E> subList(int fromIndex, int toIndex) {
            return new CheckedRandomAccessList<>(
                    list.subList(fromIndex, toIndex), type);
        }
    }

    
Returns a dynamically typesafe view of the specified map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Entry instance obtained from the map's entry set view. 
Assuming a map contains no incorrectly typed keys or values
prior to the time a dynamically typesafe view is generated, and
that all subsequent access to the map takes place through the view
(or one of its collection views), it is guaranteed that the
map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned map will be serializable if the specified map is
serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does. 
Params:
  • m – the map for which a dynamically typesafe view is to be
         returned
  • keyType – the type of key that m is permitted to hold
  • valueType – the type of value that m is permitted to hold
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:a dynamically typesafe view of the specified map
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified map.
     * Any attempt to insert a mapping whose key or value have the wrong
     * type will result in an immediate {@link ClassCastException}.
     * Similarly, any attempt to modify the value currently associated with
     * a key will result in an immediate {@link ClassCastException},
     * whether the modification is attempted directly through the map
     * itself, or through a {@link Map.Entry} instance obtained from the
     * map's {@link Map#entrySet() entry set} view.
     *
     * <p>Assuming a map contains no incorrectly typed keys or values
     * prior to the time a dynamically typesafe view is generated, and
     * that all subsequent access to the map takes place through the view
     * (or one of its collection views), it is <i>guaranteed</i> that the
     * map cannot contain an incorrectly typed key or value.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned map will be serializable if the specified map is
     * serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned map permits insertion of null keys or values
     * whenever the backing map does.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param m the map for which a dynamically typesafe view is to be
     *          returned
     * @param keyType the type of key that {@code m} is permitted to hold
     * @param valueType the type of value that {@code m} is permitted to hold
     * @return a dynamically typesafe view of the specified map
     * @since 1.5
     */
    public static <K, V> Map<K, V> checkedMap(Map<K, V> m,
                                              Class<K> keyType,
                                              Class<V> valueType) {
        return new CheckedMap<>(m, keyType, valueType);
    }


    
@serialinclude
/**
     * @serial include
     */
    private static class CheckedMap<K,V>
        implements Map<K,V>, Serializable
    {
        private static final long serialVersionUID = 5742860141034234728L;

        private final Map<K, V> m;
        final Class<K> keyType;
        final Class<V> valueType;

        private void typeCheck(Object key, Object value) {
            if (key != null && !keyType.isInstance(key))
                throw new ClassCastException(badKeyMsg(key));

            if (value != null && !valueType.isInstance(value))
                throw new ClassCastException(badValueMsg(value));
        }

        private BiFunction<? super K, ? super V, ? extends V> typeCheck(
                BiFunction<? super K, ? super V, ? extends V> func) {
            Objects.requireNonNull(func);
            return (k, v) -> {
                V newValue = func.apply(k, v);
                typeCheck(k, newValue);
                return newValue;
            };
        }

        private String badKeyMsg(Object key) {
            return "Attempt to insert " + key.getClass() +
                    " key into map with key type " + keyType;
        }

        private String badValueMsg(Object value) {
            return "Attempt to insert " + value.getClass() +
                    " value into map with value type " + valueType;
        }

        CheckedMap(Map<K, V> m, Class<K> keyType, Class<V> valueType) {
            this.m = Objects.requireNonNull(m);
            this.keyType = Objects.requireNonNull(keyType);
            this.valueType = Objects.requireNonNull(valueType);
        }

        public int size()                      { return m.size(); }
        public boolean isEmpty()               { return m.isEmpty(); }
        public boolean containsKey(Object key) { return m.containsKey(key); }
        public boolean containsValue(Object v) { return m.containsValue(v); }
        public V get(Object key)               { return m.get(key); }
        public V remove(Object key)            { return m.remove(key); }
        public void clear()                    { m.clear(); }
        public Set<K> keySet()                 { return m.keySet(); }
        public Collection<V> values()          { return m.values(); }
        public boolean equals(Object o)        { return o == this || m.equals(o); }
        public int hashCode()                  { return m.hashCode(); }
        public String toString()               { return m.toString(); }

        public V put(K key, V value) {
            typeCheck(key, value);
            return m.put(key, value);
        }

        @SuppressWarnings("unchecked")
        public void putAll(Map<? extends K, ? extends V> t) {
            // Satisfy the following goals:
            // - good diagnostics in case of type mismatch
            // - all-or-nothing semantics
            // - protection from malicious t
            // - correct behavior if t is a concurrent map
            Object[] entries = t.entrySet().toArray();
            List<Map.Entry<K,V>> checked = new ArrayList<>(entries.length);
            for (Object o : entries) {
                Map.Entry<?,?> e = (Map.Entry<?,?>) o;
                Object k = e.getKey();
                Object v = e.getValue();
                typeCheck(k, v);
                checked.add(
                        new AbstractMap.SimpleImmutableEntry<>((K)k, (V)v));
            }
            for (Map.Entry<K,V> e : checked)
                m.put(e.getKey(), e.getValue());
        }

        private transient Set<Map.Entry<K,V>> entrySet;

        public Set<Map.Entry<K,V>> entrySet() {
            if (entrySet==null)
                entrySet = new CheckedEntrySet<>(m.entrySet(), valueType);
            return entrySet;
        }

        // Override default methods in Map
        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            m.forEach(action);
        }

        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            m.replaceAll(typeCheck(function));
        }

        @Override
        public V putIfAbsent(K key, V value) {
            typeCheck(key, value);
            return m.putIfAbsent(key, value);
        }

        @Override
        public boolean remove(Object key, Object value) {
            return m.remove(key, value);
        }

        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            typeCheck(key, newValue);
            return m.replace(key, oldValue, newValue);
        }

        @Override
        public V replace(K key, V value) {
            typeCheck(key, value);
            return m.replace(key, value);
        }

        @Override
        public V computeIfAbsent(K key,
                Function<? super K, ? extends V> mappingFunction) {
            Objects.requireNonNull(mappingFunction);
            return m.computeIfAbsent(key, k -> {
                V value = mappingFunction.apply(k);
                typeCheck(k, value);
                return value;
            });
        }

        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            return m.computeIfPresent(key, typeCheck(remappingFunction));
        }

        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            return m.compute(key, typeCheck(remappingFunction));
        }

        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            Objects.requireNonNull(remappingFunction);
            return m.merge(key, value, (v1, v2) -> {
                V newValue = remappingFunction.apply(v1, v2);
                typeCheck(null, newValue);
                return newValue;
            });
        }

        
We need this class in addition to CheckedSet as Map.Entry permits
modification of the backing Map via the setValue operation.  This
class is subtle: there are many possible attacks that must be
thwarted.

@serialexclude
/**
         * We need this class in addition to CheckedSet as Map.Entry permits
         * modification of the backing Map via the setValue operation.  This
         * class is subtle: there are many possible attacks that must be
         * thwarted.
         *
         * @serial exclude
         */
        static class CheckedEntrySet<K,V> implements Set<Map.Entry<K,V>> {
            private final Set<Map.Entry<K,V>> s;
            private final Class<V> valueType;

            CheckedEntrySet(Set<Map.Entry<K, V>> s, Class<V> valueType) {
                this.s = s;
                this.valueType = valueType;
            }

            public int size()        { return s.size(); }
            public boolean isEmpty() { return s.isEmpty(); }
            public String toString() { return s.toString(); }
            public int hashCode()    { return s.hashCode(); }
            public void clear()      {        s.clear(); }

            public boolean add(Map.Entry<K, V> e) {
                throw new UnsupportedOperationException();
            }
            public boolean addAll(Collection<? extends Map.Entry<K, V>> coll) {
                throw new UnsupportedOperationException();
            }

            public Iterator<Map.Entry<K,V>> iterator() {
                final Iterator<Map.Entry<K, V>> i = s.iterator();

                return new Iterator<Map.Entry<K,V>>() {
                    public boolean hasNext() { return i.hasNext(); }
                    public void remove()     { i.remove(); }

                    public Map.Entry<K,V> next() {
                        return checkedEntry(i.next(), valueType);
                    }

                    public void forEachRemaining(Consumer<? super Entry<K, V>> action) {
                        i.forEachRemaining(
                            e -> action.accept(checkedEntry(e, valueType)));
                    }
                };
            }

            @SuppressWarnings("unchecked")
            public Object[] toArray() {
                Object[] source = s.toArray();

                /*
                 * Ensure that we don't get an ArrayStoreException even if
                 * s.toArray returns an array of something other than Object
                 */
                Object[] dest = (source.getClass() == Object[].class)
                    ? source
                    : new Object[source.length];

                for (int i = 0; i < source.length; i++)
                    dest[i] = checkedEntry((Map.Entry<K,V>)source[i],
                                           valueType);
                return dest;
            }

            @SuppressWarnings("unchecked")
            public <T> T[] toArray(T[] a) {
                // We don't pass a to s.toArray, to avoid window of
                // vulnerability wherein an unscrupulous multithreaded client
                // could get his hands on raw (unwrapped) Entries from s.
                T[] arr = s.toArray(a.length==0 ? a : Arrays.copyOf(a, 0));

                for (int i=0; i<arr.length; i++)
                    arr[i] = (T) checkedEntry((Map.Entry<K,V>)arr[i],
                                              valueType);
                if (arr.length > a.length)
                    return arr;

                System.arraycopy(arr, 0, a, 0, arr.length);
                if (a.length > arr.length)
                    a[arr.length] = null;
                return a;
            }

            
This method is overridden to protect the backing set against
an object with a nefarious equals function that senses
that the equality-candidate is Map.Entry and calls its
setValue method.

/**
             * This method is overridden to protect the backing set against
             * an object with a nefarious equals function that senses
             * that the equality-candidate is Map.Entry and calls its
             * setValue method.
             */
            public boolean contains(Object o) {
                if (!(o instanceof Map.Entry))
                    return false;
                Map.Entry<?,?> e = (Map.Entry<?,?>) o;
                return s.contains(
                    (e instanceof CheckedEntry) ? e : checkedEntry(e, valueType));
            }

            
The bulk collection methods are overridden to protect
against an unscrupulous collection whose contains(Object o)
method senses when o is a Map.Entry, and calls o.setValue.

/**
             * The bulk collection methods are overridden to protect
             * against an unscrupulous collection whose contains(Object o)
             * method senses when o is a Map.Entry, and calls o.setValue.
             */
            public boolean containsAll(Collection<?> c) {
                for (Object o : c)
                    if (!contains(o)) // Invokes safe contains() above
                        return false;
                return true;
            }

            public boolean remove(Object o) {
                if (!(o instanceof Map.Entry))
                    return false;
                return s.remove(new AbstractMap.SimpleImmutableEntry
                                <>((Map.Entry<?,?>)o));
            }

            public boolean removeAll(Collection<?> c) {
                return batchRemove(c, false);
            }
            public boolean retainAll(Collection<?> c) {
                return batchRemove(c, true);
            }
            private boolean batchRemove(Collection<?> c, boolean complement) {
                Objects.requireNonNull(c);
                boolean modified = false;
                Iterator<Map.Entry<K,V>> it = iterator();
                while (it.hasNext()) {
                    if (c.contains(it.next()) != complement) {
                        it.remove();
                        modified = true;
                    }
                }
                return modified;
            }

            public boolean equals(Object o) {
                if (o == this)
                    return true;
                if (!(o instanceof Set))
                    return false;
                Set<?> that = (Set<?>) o;
                return that.size() == s.size()
                    && containsAll(that); // Invokes safe containsAll() above
            }

            static <K,V,T> CheckedEntry<K,V,T> checkedEntry(Map.Entry<K,V> e,
                                                            Class<T> valueType) {
                return new CheckedEntry<>(e, valueType);
            }

            
This "wrapper class" serves two purposes: it prevents
the client from modifying the backing Map, by short-circuiting
the setValue method, and it protects the backing Map against
an ill-behaved Map.Entry that attempts to modify another
Map.Entry when asked to perform an equality check.

/**
             * This "wrapper class" serves two purposes: it prevents
             * the client from modifying the backing Map, by short-circuiting
             * the setValue method, and it protects the backing Map against
             * an ill-behaved Map.Entry that attempts to modify another
             * Map.Entry when asked to perform an equality check.
             */
            private static class CheckedEntry<K,V,T> implements Map.Entry<K,V> {
                private final Map.Entry<K, V> e;
                private final Class<T> valueType;

                CheckedEntry(Map.Entry<K, V> e, Class<T> valueType) {
                    this.e = Objects.requireNonNull(e);
                    this.valueType = Objects.requireNonNull(valueType);
                }

                public K getKey()        { return e.getKey(); }
                public V getValue()      { return e.getValue(); }
                public int hashCode()    { return e.hashCode(); }
                public String toString() { return e.toString(); }

                public V setValue(V value) {
                    if (value != null && !valueType.isInstance(value))
                        throw new ClassCastException(badValueMsg(value));
                    return e.setValue(value);
                }

                private String badValueMsg(Object value) {
                    return "Attempt to insert " + value.getClass() +
                        " value into map with value type " + valueType;
                }

                public boolean equals(Object o) {
                    if (o == this)
                        return true;
                    if (!(o instanceof Map.Entry))
                        return false;
                    return e.equals(new AbstractMap.SimpleImmutableEntry
                                    <>((Map.Entry<?,?>)o));
                }
            }
        }
    }

    
Returns a dynamically typesafe view of the specified sorted map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Entry instance obtained from the map's entry set view. 
Assuming a map contains no incorrectly typed keys or values
prior to the time a dynamically typesafe view is generated, and
that all subsequent access to the map takes place through the view
(or one of its collection views), it is guaranteed that the
map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned map will be serializable if the specified map is
serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does. 
Params:
  • m – the map for which a dynamically typesafe view is to be
         returned
  • keyType – the type of key that m is permitted to hold
  • valueType – the type of value that m is permitted to hold
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:a dynamically typesafe view of the specified map
Since:1.5
/**
     * Returns a dynamically typesafe view of the specified sorted map.
     * Any attempt to insert a mapping whose key or value have the wrong
     * type will result in an immediate {@link ClassCastException}.
     * Similarly, any attempt to modify the value currently associated with
     * a key will result in an immediate {@link ClassCastException},
     * whether the modification is attempted directly through the map
     * itself, or through a {@link Map.Entry} instance obtained from the
     * map's {@link Map#entrySet() entry set} view.
     *
     * <p>Assuming a map contains no incorrectly typed keys or values
     * prior to the time a dynamically typesafe view is generated, and
     * that all subsequent access to the map takes place through the view
     * (or one of its collection views), it is <i>guaranteed</i> that the
     * map cannot contain an incorrectly typed key or value.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned map will be serializable if the specified map is
     * serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned map permits insertion of null keys or values
     * whenever the backing map does.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param m the map for which a dynamically typesafe view is to be
     *          returned
     * @param keyType the type of key that {@code m} is permitted to hold
     * @param valueType the type of value that {@code m} is permitted to hold
     * @return a dynamically typesafe view of the specified map
     * @since 1.5
     */
    public static <K,V> SortedMap<K,V> checkedSortedMap(SortedMap<K, V> m,
                                                        Class<K> keyType,
                                                        Class<V> valueType) {
        return new CheckedSortedMap<>(m, keyType, valueType);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedSortedMap<K,V> extends CheckedMap<K,V>
        implements SortedMap<K,V>, Serializable
    {
        private static final long serialVersionUID = 1599671320688067438L;

        private final SortedMap<K, V> sm;

        CheckedSortedMap(SortedMap<K, V> m,
                         Class<K> keyType, Class<V> valueType) {
            super(m, keyType, valueType);
            sm = m;
        }

        public Comparator<? super K> comparator() { return sm.comparator(); }
        public K firstKey()                       { return sm.firstKey(); }
        public K lastKey()                        { return sm.lastKey(); }

        public SortedMap<K,V> subMap(K fromKey, K toKey) {
            return checkedSortedMap(sm.subMap(fromKey, toKey),
                                    keyType, valueType);
        }
        public SortedMap<K,V> headMap(K toKey) {
            return checkedSortedMap(sm.headMap(toKey), keyType, valueType);
        }
        public SortedMap<K,V> tailMap(K fromKey) {
            return checkedSortedMap(sm.tailMap(fromKey), keyType, valueType);
        }
    }

    
Returns a dynamically typesafe view of the specified navigable map. Any attempt to insert a mapping whose key or value have the wrong type will result in an immediate ClassCastException. Similarly, any attempt to modify the value currently associated with a key will result in an immediate ClassCastException, whether the modification is attempted directly through the map itself, or through a Entry instance obtained from the map's entry set view. 
Assuming a map contains no incorrectly typed keys or values
prior to the time a dynamically typesafe view is generated, and
that all subsequent access to the map takes place through the view
(or one of its collection views), it is guaranteed that the
map cannot contain an incorrectly typed key or value.

A discussion of the use of dynamically typesafe views may be found in the documentation for the 
checkedCollection method. 
The returned map will be serializable if the specified map is
serializable.

Since null is considered to be a value of any reference type, the returned map permits insertion of null keys or values whenever the backing map does. 
Params:
  • m – the map for which a dynamically typesafe view is to be
         returned
  • keyType – the type of key that m is permitted to hold
  • valueType – the type of value that m is permitted to hold
Type parameters:
  • <K> – type of map keys
  • <V> – type of map values
Returns:a dynamically typesafe view of the specified map
Since:1.8
/**
     * Returns a dynamically typesafe view of the specified navigable map.
     * Any attempt to insert a mapping whose key or value have the wrong
     * type will result in an immediate {@link ClassCastException}.
     * Similarly, any attempt to modify the value currently associated with
     * a key will result in an immediate {@link ClassCastException},
     * whether the modification is attempted directly through the map
     * itself, or through a {@link Map.Entry} instance obtained from the
     * map's {@link Map#entrySet() entry set} view.
     *
     * <p>Assuming a map contains no incorrectly typed keys or values
     * prior to the time a dynamically typesafe view is generated, and
     * that all subsequent access to the map takes place through the view
     * (or one of its collection views), it is <em>guaranteed</em> that the
     * map cannot contain an incorrectly typed key or value.
     *
     * <p>A discussion of the use of dynamically typesafe views may be
     * found in the documentation for the {@link #checkedCollection
     * checkedCollection} method.
     *
     * <p>The returned map will be serializable if the specified map is
     * serializable.
     *
     * <p>Since {@code null} is considered to be a value of any reference
     * type, the returned map permits insertion of null keys or values
     * whenever the backing map does.
     *
     * @param <K> type of map keys
     * @param <V> type of map values
     * @param m the map for which a dynamically typesafe view is to be
     *          returned
     * @param keyType the type of key that {@code m} is permitted to hold
     * @param valueType the type of value that {@code m} is permitted to hold
     * @return a dynamically typesafe view of the specified map
     * @since 1.8
     */
    public static <K,V> NavigableMap<K,V> checkedNavigableMap(NavigableMap<K, V> m,
                                                        Class<K> keyType,
                                                        Class<V> valueType) {
        return new CheckedNavigableMap<>(m, keyType, valueType);
    }

    
@serialinclude
/**
     * @serial include
     */
    static class CheckedNavigableMap<K,V> extends CheckedSortedMap<K,V>
        implements NavigableMap<K,V>, Serializable
    {
        private static final long serialVersionUID = -4852462692372534096L;

        private final NavigableMap<K, V> nm;

        CheckedNavigableMap(NavigableMap<K, V> m,
                         Class<K> keyType, Class<V> valueType) {
            super(m, keyType, valueType);
            nm = m;
        }

        public Comparator<? super K> comparator()   { return nm.comparator(); }
        public K firstKey()                           { return nm.firstKey(); }
        public K lastKey()                             { return nm.lastKey(); }

        public Entry<K, V> lowerEntry(K key) {
            Entry<K,V> lower = nm.lowerEntry(key);
            return (null != lower)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(lower, valueType)
                : null;
        }

        public K lowerKey(K key)                   { return nm.lowerKey(key); }

        public Entry<K, V> floorEntry(K key) {
            Entry<K,V> floor = nm.floorEntry(key);
            return (null != floor)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(floor, valueType)
                : null;
        }

        public K floorKey(K key)                   { return nm.floorKey(key); }

        public Entry<K, V> ceilingEntry(K key) {
            Entry<K,V> ceiling = nm.ceilingEntry(key);
            return (null != ceiling)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(ceiling, valueType)
                : null;
        }

        public K ceilingKey(K key)               { return nm.ceilingKey(key); }

        public Entry<K, V> higherEntry(K key) {
            Entry<K,V> higher = nm.higherEntry(key);
            return (null != higher)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(higher, valueType)
                : null;
        }

        public K higherKey(K key)                 { return nm.higherKey(key); }

        public Entry<K, V> firstEntry() {
            Entry<K,V> first = nm.firstEntry();
            return (null != first)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(first, valueType)
                : null;
        }

        public Entry<K, V> lastEntry() {
            Entry<K,V> last = nm.lastEntry();
            return (null != last)
                ? new CheckedMap.CheckedEntrySet.CheckedEntry<>(last, valueType)
                : null;
        }

        public Entry<K, V> pollFirstEntry() {
            Entry<K,V> entry = nm.pollFirstEntry();
            return (null == entry)
                ? null
                : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
        }

        public Entry<K, V> pollLastEntry() {
            Entry<K,V> entry = nm.pollLastEntry();
            return (null == entry)
                ? null
                : new CheckedMap.CheckedEntrySet.CheckedEntry<>(entry, valueType);
        }

        public NavigableMap<K, V> descendingMap() {
            return checkedNavigableMap(nm.descendingMap(), keyType, valueType);
        }

        public NavigableSet<K> keySet() {
            return navigableKeySet();
        }

        public NavigableSet<K> navigableKeySet() {
            return checkedNavigableSet(nm.navigableKeySet(), keyType);
        }

        public NavigableSet<K> descendingKeySet() {
            return checkedNavigableSet(nm.descendingKeySet(), keyType);
        }

        @Override
        public NavigableMap<K,V> subMap(K fromKey, K toKey) {
            return checkedNavigableMap(nm.subMap(fromKey, true, toKey, false),
                                    keyType, valueType);
        }

        @Override
        public NavigableMap<K,V> headMap(K toKey) {
            return checkedNavigableMap(nm.headMap(toKey, false), keyType, valueType);
        }

        @Override
        public NavigableMap<K,V> tailMap(K fromKey) {
            return checkedNavigableMap(nm.tailMap(fromKey, true), keyType, valueType);
        }

        public NavigableMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
            return checkedNavigableMap(nm.subMap(fromKey, fromInclusive, toKey, toInclusive), keyType, valueType);
        }

        public NavigableMap<K, V> headMap(K toKey, boolean inclusive) {
            return checkedNavigableMap(nm.headMap(toKey, inclusive), keyType, valueType);
        }

        public NavigableMap<K, V> tailMap(K fromKey, boolean inclusive) {
            return checkedNavigableMap(nm.tailMap(fromKey, inclusive), keyType, valueType);
        }
    }

    // Empty collections

    
Returns an iterator that has no elements.  More precisely,


Implementations of this method are permitted, but not
required, to return the same object from multiple invocations.

Type parameters:
  • <T> – type of elements, if there were any, in the iterator
Returns:an empty iterator
Since:1.7
/**
     * Returns an iterator that has no elements.  More precisely,
     *
     * <ul>
     * <li>{@link Iterator#hasNext hasNext} always returns {@code
     * false}.</li>
     * <li>{@link Iterator#next next} always throws {@link
     * NoSuchElementException}.</li>
     * <li>{@link Iterator#remove remove} always throws {@link
     * IllegalStateException}.</li>
     * </ul>
     *
     * <p>Implementations of this method are permitted, but not
     * required, to return the same object from multiple invocations.
     *
     * @param <T> type of elements, if there were any, in the iterator
     * @return an empty iterator
     * @since 1.7
     */
    @SuppressWarnings("unchecked")
    public static <T> Iterator<T> emptyIterator() {
        return (Iterator<T>) EmptyIterator.EMPTY_ITERATOR;
    }

    private static class EmptyIterator<E> implements Iterator<E> {
        static final EmptyIterator<Object> EMPTY_ITERATOR
            = new EmptyIterator<>();

        public boolean hasNext() { return false; }
        public E next() { throw new NoSuchElementException(); }
        public void remove() { throw new IllegalStateException(); }
        @Override
        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
        }
    }

    
Returns a list iterator that has no elements.  More precisely,


Implementations of this method are permitted, but not
required, to return the same object from multiple invocations.

Type parameters:
  • <T> – type of elements, if there were any, in the iterator
Returns:an empty list iterator
Since:1.7
/**
     * Returns a list iterator that has no elements.  More precisely,
     *
     * <ul>
     * <li>{@link Iterator#hasNext hasNext} and {@link
     * ListIterator#hasPrevious hasPrevious} always return {@code
     * false}.</li>
     * <li>{@link Iterator#next next} and {@link ListIterator#previous
     * previous} always throw {@link NoSuchElementException}.</li>
     * <li>{@link Iterator#remove remove} and {@link ListIterator#set
     * set} always throw {@link IllegalStateException}.</li>
     * <li>{@link ListIterator#add add} always throws {@link
     * UnsupportedOperationException}.</li>
     * <li>{@link ListIterator#nextIndex nextIndex} always returns
     * {@code 0}.</li>
     * <li>{@link ListIterator#previousIndex previousIndex} always
     * returns {@code -1}.</li>
     * </ul>
     *
     * <p>Implementations of this method are permitted, but not
     * required, to return the same object from multiple invocations.
     *
     * @param <T> type of elements, if there were any, in the iterator
     * @return an empty list iterator
     * @since 1.7
     */
    @SuppressWarnings("unchecked")
    public static <T> ListIterator<T> emptyListIterator() {
        return (ListIterator<T>) EmptyListIterator.EMPTY_ITERATOR;
    }

    private static class EmptyListIterator<E>
        extends EmptyIterator<E>
        implements ListIterator<E>
    {
        static final EmptyListIterator<Object> EMPTY_ITERATOR
            = new EmptyListIterator<>();

        public boolean hasPrevious() { return false; }
        public E previous() { throw new NoSuchElementException(); }
        public int nextIndex()     { return 0; }
        public int previousIndex() { return -1; }
        public void set(E e) { throw new IllegalStateException(); }
        public void add(E e) { throw new UnsupportedOperationException(); }
    }

    
Returns an enumeration that has no elements.  More precisely,


Implementations of this method are permitted, but not
required, to return the same object from multiple invocations.

Type parameters:
  • <T> – the class of the objects in the enumeration
Returns:an empty enumeration
Since:1.7
/**
     * Returns an enumeration that has no elements.  More precisely,
     *
     * <ul>
     * <li>{@link Enumeration#hasMoreElements hasMoreElements} always
     * returns {@code false}.</li>
     * <li> {@link Enumeration#nextElement nextElement} always throws
     * {@link NoSuchElementException}.</li>
     * </ul>
     *
     * <p>Implementations of this method are permitted, but not
     * required, to return the same object from multiple invocations.
     *
     * @param  <T> the class of the objects in the enumeration
     * @return an empty enumeration
     * @since 1.7
     */
    @SuppressWarnings("unchecked")
    public static <T> Enumeration<T> emptyEnumeration() {
        return (Enumeration<T>) EmptyEnumeration.EMPTY_ENUMERATION;
    }

    private static class EmptyEnumeration<E> implements Enumeration<E> {
        static final EmptyEnumeration<Object> EMPTY_ENUMERATION
            = new EmptyEnumeration<>();

        public boolean hasMoreElements() { return false; }
        public E nextElement() { throw new NoSuchElementException(); }
        public Iterator<E> asIterator() { return emptyIterator(); }
    }

    
The empty set (immutable).  This set is serializable.

See Also:
  • emptySet()
/**
     * The empty set (immutable).  This set is serializable.
     *
     * @see #emptySet()
     */
    @SuppressWarnings("rawtypes")
    public static final Set EMPTY_SET = new EmptySet<>();

    
Returns an empty set (immutable).  This set is serializable.
Unlike the like-named field, this method is parameterized.

This example illustrates the type-safe way to obtain an empty set:

    Set<String> s = Collections.emptySet();



Type parameters:
  • <T> – the class of the objects in the set
See Also:
Implementation Note:Implementations of this method need not create a separate Set object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns:the empty set
Since:1.5
/**
     * Returns an empty set (immutable).  This set is serializable.
     * Unlike the like-named field, this method is parameterized.
     *
     * <p>This example illustrates the type-safe way to obtain an empty set:
     * <pre>
     *     Set&lt;String&gt; s = Collections.emptySet();
     * </pre>
     * @implNote Implementations of this method need not create a separate
     * {@code Set} object for each call.  Using this method is likely to have
     * comparable cost to using the like-named field.  (Unlike this method, the
     * field does not provide type safety.)
     *
     * @param  <T> the class of the objects in the set
     * @return the empty set
     *
     * @see #EMPTY_SET
     * @since 1.5
     */
    @SuppressWarnings("unchecked")
    public static final <T> Set<T> emptySet() {
        return (Set<T>) EMPTY_SET;
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class EmptySet<E>
        extends AbstractSet<E>
        implements Serializable
    {
        private static final long serialVersionUID = 1582296315990362920L;

        public Iterator<E> iterator() { return emptyIterator(); }

        public int size() {return 0;}
        public boolean isEmpty() {return true;}
        public void clear() {}

        public boolean contains(Object obj) {return false;}
        public boolean containsAll(Collection<?> c) { return c.isEmpty(); }

        public Object[] toArray() { return new Object[0]; }

        public <T> T[] toArray(T[] a) {
            if (a.length > 0)
                a[0] = null;
            return a;
        }

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            Objects.requireNonNull(filter);
            return false;
        }
        @Override
        public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }

        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_SET;
        }

        @Override
        public int hashCode() {
            return 0;
        }
    }

    
Returns an empty sorted set (immutable).  This set is serializable.

This example illustrates the type-safe way to obtain an empty
sorted set:

 
    SortedSet<String> s = Collections.emptySortedSet();



Type parameters:
  • <E> – type of elements, if there were any, in the set
Implementation Note:Implementations of this method need not create a separate SortedSet object for each call.
Returns:the empty sorted set
Since:1.8
/**
     * Returns an empty sorted set (immutable).  This set is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty
     * sorted set:
     * <pre> {@code
     *     SortedSet<String> s = Collections.emptySortedSet();
     * }</pre>
     *
     * @implNote Implementations of this method need not create a separate
     * {@code SortedSet} object for each call.
     *
     * @param <E> type of elements, if there were any, in the set
     * @return the empty sorted set
     * @since 1.8
     */
    @SuppressWarnings("unchecked")
    public static <E> SortedSet<E> emptySortedSet() {
        return (SortedSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
    }

    
Returns an empty navigable set (immutable).  This set is serializable.

This example illustrates the type-safe way to obtain an empty
navigable set:

 
    NavigableSet<String> s = Collections.emptyNavigableSet();



Type parameters:
  • <E> – type of elements, if there were any, in the set
Implementation Note:Implementations of this method need not create a separate NavigableSet object for each call.
Returns:the empty navigable set
Since:1.8
/**
     * Returns an empty navigable set (immutable).  This set is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty
     * navigable set:
     * <pre> {@code
     *     NavigableSet<String> s = Collections.emptyNavigableSet();
     * }</pre>
     *
     * @implNote Implementations of this method need not
     * create a separate {@code NavigableSet} object for each call.
     *
     * @param <E> type of elements, if there were any, in the set
     * @return the empty navigable set
     * @since 1.8
     */
    @SuppressWarnings("unchecked")
    public static <E> NavigableSet<E> emptyNavigableSet() {
        return (NavigableSet<E>) UnmodifiableNavigableSet.EMPTY_NAVIGABLE_SET;
    }

    
The empty list (immutable).  This list is serializable.

See Also:
  • emptyList()
/**
     * The empty list (immutable).  This list is serializable.
     *
     * @see #emptyList()
     */
    @SuppressWarnings("rawtypes")
    public static final List EMPTY_LIST = new EmptyList<>();

    
Returns an empty list (immutable).  This list is serializable.

This example illustrates the type-safe way to obtain an empty list:

    List<String> s = Collections.emptyList();



Type parameters:
  • <T> – type of elements, if there were any, in the list
See Also:
Implementation Note: Implementations of this method need not create a separate List object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns:an empty immutable list
Since:1.5
/**
     * Returns an empty list (immutable).  This list is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty list:
     * <pre>
     *     List&lt;String&gt; s = Collections.emptyList();
     * </pre>
     *
     * @implNote
     * Implementations of this method need not create a separate {@code List}
     * object for each call.   Using this method is likely to have comparable
     * cost to using the like-named field.  (Unlike this method, the field does
     * not provide type safety.)
     *
     * @param <T> type of elements, if there were any, in the list
     * @return an empty immutable list
     *
     * @see #EMPTY_LIST
     * @since 1.5
     */
    @SuppressWarnings("unchecked")
    public static final <T> List<T> emptyList() {
        return (List<T>) EMPTY_LIST;
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class EmptyList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {
        private static final long serialVersionUID = 8842843931221139166L;

        public Iterator<E> iterator() {
            return emptyIterator();
        }
        public ListIterator<E> listIterator() {
            return emptyListIterator();
        }

        public int size() {return 0;}
        public boolean isEmpty() {return true;}
        public void clear() {}

        public boolean contains(Object obj) {return false;}
        public boolean containsAll(Collection<?> c) { return c.isEmpty(); }

        public Object[] toArray() { return new Object[0]; }

        public <T> T[] toArray(T[] a) {
            if (a.length > 0)
                a[0] = null;
            return a;
        }

        public E get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }

        public boolean equals(Object o) {
            return (o instanceof List) && ((List<?>)o).isEmpty();
        }

        public int hashCode() { return 1; }

        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            Objects.requireNonNull(filter);
            return false;
        }
        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            Objects.requireNonNull(operator);
        }
        @Override
        public void sort(Comparator<? super E> c) {
        }

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            Objects.requireNonNull(action);
        }

        @Override
        public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }

        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_LIST;
        }
    }

    
The empty map (immutable).  This map is serializable.

See Also:
  • emptyMap()
Since:1.3
/**
     * The empty map (immutable).  This map is serializable.
     *
     * @see #emptyMap()
     * @since 1.3
     */
    @SuppressWarnings("rawtypes")
    public static final Map EMPTY_MAP = new EmptyMap<>();

    
Returns an empty map (immutable).  This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

    Map<String, Date> s = Collections.emptyMap();



Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
See Also:
Implementation Note:Implementations of this method need not create a separate Map object for each call. Using this method is likely to have comparable cost to using the like-named field. (Unlike this method, the field does not provide type safety.)
Returns:an empty map
Since:1.5
/**
     * Returns an empty map (immutable).  This map is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty map:
     * <pre>
     *     Map&lt;String, Date&gt; s = Collections.emptyMap();
     * </pre>
     * @implNote Implementations of this method need not create a separate
     * {@code Map} object for each call.  Using this method is likely to have
     * comparable cost to using the like-named field.  (Unlike this method, the
     * field does not provide type safety.)
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @return an empty map
     * @see #EMPTY_MAP
     * @since 1.5
     */
    @SuppressWarnings("unchecked")
    public static final <K,V> Map<K,V> emptyMap() {
        return (Map<K,V>) EMPTY_MAP;
    }

    
Returns an empty sorted map (immutable).  This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

 
    SortedMap<String, Date> s = Collections.emptySortedMap();



Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Implementation Note:Implementations of this method need not create a separate SortedMap object for each call.
Returns:an empty sorted map
Since:1.8
/**
     * Returns an empty sorted map (immutable).  This map is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty map:
     * <pre> {@code
     *     SortedMap<String, Date> s = Collections.emptySortedMap();
     * }</pre>
     *
     * @implNote Implementations of this method need not create a separate
     * {@code SortedMap} object for each call.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @return an empty sorted map
     * @since 1.8
     */
    @SuppressWarnings("unchecked")
    public static final <K,V> SortedMap<K,V> emptySortedMap() {
        return (SortedMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
    }

    
Returns an empty navigable map (immutable).  This map is serializable.

This example illustrates the type-safe way to obtain an empty map:

 
    NavigableMap<String, Date> s = Collections.emptyNavigableMap();



Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Implementation Note:Implementations of this method need not create a separate NavigableMap object for each call.
Returns:an empty navigable map
Since:1.8
/**
     * Returns an empty navigable map (immutable).  This map is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty map:
     * <pre> {@code
     *     NavigableMap<String, Date> s = Collections.emptyNavigableMap();
     * }</pre>
     *
     * @implNote Implementations of this method need not create a separate
     * {@code NavigableMap} object for each call.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @return an empty navigable map
     * @since 1.8
     */
    @SuppressWarnings("unchecked")
    public static final <K,V> NavigableMap<K,V> emptyNavigableMap() {
        return (NavigableMap<K,V>) UnmodifiableNavigableMap.EMPTY_NAVIGABLE_MAP;
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class EmptyMap<K,V>
        extends AbstractMap<K,V>
        implements Serializable
    {
        private static final long serialVersionUID = 6428348081105594320L;

        public int size()                          {return 0;}
        public boolean isEmpty()                   {return true;}
        public void clear()                        {}
        public boolean containsKey(Object key)     {return false;}
        public boolean containsValue(Object value) {return false;}
        public V get(Object key)                   {return null;}
        public Set<K> keySet()                     {return emptySet();}
        public Collection<V> values()              {return emptySet();}
        public Set<Map.Entry<K,V>> entrySet()      {return emptySet();}

        public boolean equals(Object o) {
            return (o instanceof Map) && ((Map<?,?>)o).isEmpty();
        }

        public int hashCode()                      {return 0;}

        // Override default methods in Map
        @Override
        @SuppressWarnings("unchecked")
        public V getOrDefault(Object k, V defaultValue) {
            return defaultValue;
        }

        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            Objects.requireNonNull(action);
        }

        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            Objects.requireNonNull(function);
        }

        @Override
        public V putIfAbsent(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean remove(Object key, Object value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V replace(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfAbsent(K key,
                Function<? super K, ? extends V> mappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_MAP;
        }
    }

    // Singleton collections

    
Returns an immutable set containing only the specified object.
The returned set is serializable.

Params:
  • o – the sole object to be stored in the returned set.
Type parameters:
  • <T> – the class of the objects in the set
Returns:an immutable set containing only the specified object.
/**
     * Returns an immutable set containing only the specified object.
     * The returned set is serializable.
     *
     * @param  <T> the class of the objects in the set
     * @param o the sole object to be stored in the returned set.
     * @return an immutable set containing only the specified object.
     */
    public static <T> Set<T> singleton(T o) {
        return new SingletonSet<>(o);
    }

    static <E> Iterator<E> singletonIterator(final E e) {
        return new Iterator<E>() {
            private boolean hasNext = true;
            public boolean hasNext() {
                return hasNext;
            }
            public E next() {
                if (hasNext) {
                    hasNext = false;
                    return e;
                }
                throw new NoSuchElementException();
            }
            public void remove() {
                throw new UnsupportedOperationException();
            }
            @Override
            public void forEachRemaining(Consumer<? super E> action) {
                Objects.requireNonNull(action);
                if (hasNext) {
                    hasNext = false;
                    action.accept(e);
                }
            }
        };
    }

    
Creates a Spliterator with only the specified element 
Type parameters:
  • <T> – Type of elements
Returns:A singleton Spliterator
/**
     * Creates a {@code Spliterator} with only the specified element
     *
     * @param <T> Type of elements
     * @return A singleton {@code Spliterator}
     */
    static <T> Spliterator<T> singletonSpliterator(final T element) {
        return new Spliterator<T>() {
            long est = 1;

            @Override
            public Spliterator<T> trySplit() {
                return null;
            }

            @Override
            public boolean tryAdvance(Consumer<? super T> consumer) {
                Objects.requireNonNull(consumer);
                if (est > 0) {
                    est--;
                    consumer.accept(element);
                    return true;
                }
                return false;
            }

            @Override
            public void forEachRemaining(Consumer<? super T> consumer) {
                tryAdvance(consumer);
            }

            @Override
            public long estimateSize() {
                return est;
            }

            @Override
            public int characteristics() {
                int value = (element != null) ? Spliterator.NONNULL : 0;

                return value | Spliterator.SIZED | Spliterator.SUBSIZED | Spliterator.IMMUTABLE |
                       Spliterator.DISTINCT | Spliterator.ORDERED;
            }
        };
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class SingletonSet<E>
        extends AbstractSet<E>
        implements Serializable
    {
        private static final long serialVersionUID = 3193687207550431679L;

        private final E element;

        SingletonSet(E e) {element = e;}

        public Iterator<E> iterator() {
            return singletonIterator(element);
        }

        public int size() {return 1;}

        public boolean contains(Object o) {return eq(o, element);}

        // Override default methods for Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            action.accept(element);
        }
        @Override
        public Spliterator<E> spliterator() {
            return singletonSpliterator(element);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            throw new UnsupportedOperationException();
        }
        @Override
        public int hashCode() {
            return Objects.hashCode(element);
        }
    }

    
Returns an immutable list containing only the specified object.
The returned list is serializable.

Params:
  • o – the sole object to be stored in the returned list.
Type parameters:
  • <T> – the class of the objects in the list
Returns:an immutable list containing only the specified object.
Since:1.3
/**
     * Returns an immutable list containing only the specified object.
     * The returned list is serializable.
     *
     * @param  <T> the class of the objects in the list
     * @param o the sole object to be stored in the returned list.
     * @return an immutable list containing only the specified object.
     * @since 1.3
     */
    public static <T> List<T> singletonList(T o) {
        return new SingletonList<>(o);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class SingletonList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {

        private static final long serialVersionUID = 3093736618740652951L;

        private final E element;

        SingletonList(E obj)                {element = obj;}

        public Iterator<E> iterator() {
            return singletonIterator(element);
        }

        public int size()                   {return 1;}

        public boolean contains(Object obj) {return eq(obj, element);}

        public E get(int index) {
            if (index != 0)
              throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
            return element;
        }

        // Override default methods for Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            action.accept(element);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {
        }
        @Override
        public Spliterator<E> spliterator() {
            return singletonSpliterator(element);
        }
        @Override
        public int hashCode() {
            return 31 + Objects.hashCode(element);
        }
    }

    
Returns an immutable map, mapping only the specified key to the
specified value.  The returned map is serializable.

Params:
  • key – the sole key to be stored in the returned map.
  • value – the value to which the returned map maps key.
Type parameters:
  • <K> – the class of the map keys
  • <V> – the class of the map values
Returns:an immutable map containing only the specified key-value
        mapping.
Since:1.3
/**
     * Returns an immutable map, mapping only the specified key to the
     * specified value.  The returned map is serializable.
     *
     * @param <K> the class of the map keys
     * @param <V> the class of the map values
     * @param key the sole key to be stored in the returned map.
     * @param value the value to which the returned map maps {@code key}.
     * @return an immutable map containing only the specified key-value
     *         mapping.
     * @since 1.3
     */
    public static <K,V> Map<K,V> singletonMap(K key, V value) {
        return new SingletonMap<>(key, value);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class SingletonMap<K,V>
          extends AbstractMap<K,V>
          implements Serializable {
        private static final long serialVersionUID = -6979724477215052911L;

        private final K k;
        private final V v;

        SingletonMap(K key, V value) {
            k = key;
            v = value;
        }

        public int size()                                           {return 1;}
        public boolean isEmpty()                                {return false;}
        public boolean containsKey(Object key)             {return eq(key, k);}
        public boolean containsValue(Object value)       {return eq(value, v);}
        public V get(Object key)              {return (eq(key, k) ? v : null);}

        private transient Set<K> keySet;
        private transient Set<Map.Entry<K,V>> entrySet;
        private transient Collection<V> values;

        public Set<K> keySet() {
            if (keySet==null)
                keySet = singleton(k);
            return keySet;
        }

        public Set<Map.Entry<K,V>> entrySet() {
            if (entrySet==null)
                entrySet = Collections.<Map.Entry<K,V>>singleton(
                    new SimpleImmutableEntry<>(k, v));
            return entrySet;
        }

        public Collection<V> values() {
            if (values==null)
                values = singleton(v);
            return values;
        }

        // Override default methods in Map
        @Override
        public V getOrDefault(Object key, V defaultValue) {
            return eq(key, k) ? v : defaultValue;
        }

        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            action.accept(k, v);
        }

        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V putIfAbsent(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean remove(Object key, Object value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V replace(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfAbsent(K key,
                Function<? super K, ? extends V> mappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public int hashCode() {
            return Objects.hashCode(k) ^ Objects.hashCode(v);
        }
    }

    // Miscellaneous

    
Returns an immutable list consisting of n copies of the specified object. The newly allocated data object is tiny (it contains a single reference to the data object). This method is useful in combination with the List.addAll method to grow lists. The returned list is serializable. 
Params:
  • n – the number of elements in the returned list.
  • o – the element to appear repeatedly in the returned list.
Type parameters:
  • <T> – the class of the object to copy and of the objects
        in the returned list.
Throws:
See Also:
Returns:an immutable list consisting of n copies of the specified object.
/**
     * Returns an immutable list consisting of {@code n} copies of the
     * specified object.  The newly allocated data object is tiny (it contains
     * a single reference to the data object).  This method is useful in
     * combination with the {@code List.addAll} method to grow lists.
     * The returned list is serializable.
     *
     * @param  <T> the class of the object to copy and of the objects
     *         in the returned list.
     * @param  n the number of elements in the returned list.
     * @param  o the element to appear repeatedly in the returned list.
     * @return an immutable list consisting of {@code n} copies of the
     *         specified object.
     * @throws IllegalArgumentException if {@code n < 0}
     * @see    List#addAll(Collection)
     * @see    List#addAll(int, Collection)
     */
    public static <T> List<T> nCopies(int n, T o) {
        if (n < 0)
            throw new IllegalArgumentException("List length = " + n);
        return new CopiesList<>(n, o);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class CopiesList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable
    {
        private static final long serialVersionUID = 2739099268398711800L;

        final int n;
        final E element;

        CopiesList(int n, E e) {
            assert n >= 0;
            this.n = n;
            element = e;
        }

        public int size() {
            return n;
        }

        public boolean contains(Object obj) {
            return n != 0 && eq(obj, element);
        }

        public int indexOf(Object o) {
            return contains(o) ? 0 : -1;
        }

        public int lastIndexOf(Object o) {
            return contains(o) ? n - 1 : -1;
        }

        public E get(int index) {
            if (index < 0 || index >= n)
                throw new IndexOutOfBoundsException("Index: "+index+
                                                    ", Size: "+n);
            return element;
        }

        public Object[] toArray() {
            final Object[] a = new Object[n];
            if (element != null)
                Arrays.fill(a, 0, n, element);
            return a;
        }

        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            final int n = this.n;
            if (a.length < n) {
                a = (T[])java.lang.reflect.Array
                    .newInstance(a.getClass().getComponentType(), n);
                if (element != null)
                    Arrays.fill(a, 0, n, element);
            } else {
                Arrays.fill(a, 0, n, element);
                if (a.length > n)
                    a[n] = null;
            }
            return a;
        }

        public List<E> subList(int fromIndex, int toIndex) {
            if (fromIndex < 0)
                throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
            if (toIndex > n)
                throw new IndexOutOfBoundsException("toIndex = " + toIndex);
            if (fromIndex > toIndex)
                throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                                   ") > toIndex(" + toIndex + ")");
            return new CopiesList<>(toIndex - fromIndex, element);
        }

        @Override
        public int hashCode() {
            if (n == 0) return 1;
            // hashCode of n repeating elements is 31^n + elementHash * Sum(31^k, k = 0..n-1)
            // this implementation completes in O(log(n)) steps taking advantage of
            // 31^(2*n) = (31^n)^2 and Sum(31^k, k = 0..(2*n-1)) = Sum(31^k, k = 0..n-1) * (31^n + 1)
            int pow = 31;
            int sum = 1;
            for (int i = Integer.numberOfLeadingZeros(n) + 1; i < Integer.SIZE; i++) {
                sum *= pow + 1;
                pow *= pow;
                if ((n << i) < 0) {
                    pow *= 31;
                    sum = sum * 31 + 1;
                }
            }
            return pow + sum * (element == null ? 0 : element.hashCode());
        }

        @Override
        public boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof CopiesList) {
                CopiesList<?> other = (CopiesList<?>) o;
                return n == other.n && (n == 0 || eq(element, other.element));
            }
            if (!(o instanceof List))
                return false;

            int remaining = n;
            E e = element;
            Iterator<?> itr = ((List<?>) o).iterator();
            if (e == null) {
                while (itr.hasNext() && remaining-- > 0) {
                    if (itr.next() != null)
                        return false;
                }
            } else {
                while (itr.hasNext() && remaining-- > 0) {
                    if (!e.equals(itr.next()))
                        return false;
                }
            }
            return remaining == 0 && !itr.hasNext();
        }

        // Override default methods in Collection
        @Override
        public Stream<E> stream() {
            return IntStream.range(0, n).mapToObj(i -> element);
        }

        @Override
        public Stream<E> parallelStream() {
            return IntStream.range(0, n).parallel().mapToObj(i -> element);
        }

        @Override
        public Spliterator<E> spliterator() {
            return stream().spliterator();
        }

        private void readObject(ObjectInputStream ois) throws IOException, ClassNotFoundException {
            ois.defaultReadObject();
            SharedSecrets.getJavaObjectInputStreamAccess().checkArray(ois, Object[].class, n);
        }
    }

    
Returns a comparator that imposes the reverse of the natural
ordering on a collection of objects that implement the Comparable interface. (The natural ordering is the ordering imposed by the objects' own compareTo method.) This enables a simple idiom for sorting (or maintaining) collections (or arrays) of objects that implement the Comparable interface in reverse-natural-order. For example, suppose a is an array of strings. Then: 
         Arrays.sort(a, Collections.reverseOrder());

 sorts the array in reverse-lexicographic (alphabetical) order.

The returned comparator is serializable.

Type parameters:
  • <T> – the class of the objects compared by the comparator
See Also:
Returns:A comparator that imposes the reverse of the natural
        ordering on a collection of objects that implement the Comparable interface.
/**
     * Returns a comparator that imposes the reverse of the <em>natural
     * ordering</em> on a collection of objects that implement the
     * {@code Comparable} interface.  (The natural ordering is the ordering
     * imposed by the objects' own {@code compareTo} method.)  This enables a
     * simple idiom for sorting (or maintaining) collections (or arrays) of
     * objects that implement the {@code Comparable} interface in
     * reverse-natural-order.  For example, suppose {@code a} is an array of
     * strings. Then: <pre>
     *          Arrays.sort(a, Collections.reverseOrder());
     * </pre> sorts the array in reverse-lexicographic (alphabetical) order.<p>
     *
     * The returned comparator is serializable.
     *
     * @param  <T> the class of the objects compared by the comparator
     * @return A comparator that imposes the reverse of the <i>natural
     *         ordering</i> on a collection of objects that implement
     *         the {@code Comparable} interface.
     * @see Comparable
     */
    @SuppressWarnings("unchecked")
    public static <T> Comparator<T> reverseOrder() {
        return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class ReverseComparator
        implements Comparator<Comparable<Object>>, Serializable {

        private static final long serialVersionUID = 7207038068494060240L;

        static final ReverseComparator REVERSE_ORDER
            = new ReverseComparator();

        public int compare(Comparable<Object> c1, Comparable<Object> c2) {
            return c2.compareTo(c1);
        }

        private Object readResolve() { return Collections.reverseOrder(); }

        @Override
        public Comparator<Comparable<Object>> reversed() {
            return Comparator.naturalOrder();
        }
    }

    
Returns a comparator that imposes the reverse ordering of the specified comparator. If the specified comparator is null, this method is equivalent to reverseOrder() (in other words, it returns a comparator that imposes the reverse of the natural ordering on
a collection of objects that implement the Comparable interface).

The returned comparator is serializable (assuming the specified comparator is also serializable or null). 
Params:
  • cmp – a comparator who's ordering is to be reversed by the returned comparator or null
Type parameters:
  • <T> – the class of the objects compared by the comparator
Returns:A comparator that imposes the reverse ordering of the
        specified comparator.
Since:1.5
/**
     * Returns a comparator that imposes the reverse ordering of the specified
     * comparator.  If the specified comparator is {@code null}, this method is
     * equivalent to {@link #reverseOrder()} (in other words, it returns a
     * comparator that imposes the reverse of the <em>natural ordering</em> on
     * a collection of objects that implement the Comparable interface).
     *
     * <p>The returned comparator is serializable (assuming the specified
     * comparator is also serializable or {@code null}).
     *
     * @param <T> the class of the objects compared by the comparator
     * @param cmp a comparator who's ordering is to be reversed by the returned
     * comparator or {@code null}
     * @return A comparator that imposes the reverse ordering of the
     *         specified comparator.
     * @since 1.5
     */
    @SuppressWarnings("unchecked")
    public static <T> Comparator<T> reverseOrder(Comparator<T> cmp) {
        if (cmp == null) {
            return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
        } else if (cmp == ReverseComparator.REVERSE_ORDER) {
            return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
        } else if (cmp == Comparators.NaturalOrderComparator.INSTANCE) {
            return (Comparator<T>) ReverseComparator.REVERSE_ORDER;
        } else if (cmp instanceof ReverseComparator2) {
            return ((ReverseComparator2<T>) cmp).cmp;
        } else {
            return new ReverseComparator2<>(cmp);
        }
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class ReverseComparator2<T> implements Comparator<T>,
        Serializable
    {
        private static final long serialVersionUID = 4374092139857L;

        
The comparator specified in the static factory.  This will never
be null, as the static factory returns a ReverseComparator
instance if its argument is null.

@serial
/**
         * The comparator specified in the static factory.  This will never
         * be null, as the static factory returns a ReverseComparator
         * instance if its argument is null.
         *
         * @serial
         */
        final Comparator<T> cmp;

        ReverseComparator2(Comparator<T> cmp) {
            assert cmp != null;
            this.cmp = cmp;
        }

        public int compare(T t1, T t2) {
            return cmp.compare(t2, t1);
        }

        public boolean equals(Object o) {
            return (o == this) ||
                (o instanceof ReverseComparator2 &&
                 cmp.equals(((ReverseComparator2)o).cmp));
        }

        public int hashCode() {
            return cmp.hashCode() ^ Integer.MIN_VALUE;
        }

        @Override
        public Comparator<T> reversed() {
            return cmp;
        }
    }

    
Returns an enumeration over the specified collection.  This provides
interoperability with legacy APIs that require an enumeration
as input.

The iterator returned from a call to Enumeration.asIterator() does not support removal of elements from the specified collection. This is necessary to avoid unintentionally increasing the capabilities of the returned enumeration. 
Params:
  • c – the collection for which an enumeration is to be returned.
Type parameters:
  • <T> – the class of the objects in the collection
See Also:
Returns:an enumeration over the specified collection.
/**
     * Returns an enumeration over the specified collection.  This provides
     * interoperability with legacy APIs that require an enumeration
     * as input.
     *
     * <p>The iterator returned from a call to {@link Enumeration#asIterator()}
     * does not support removal of elements from the specified collection.  This
     * is necessary to avoid unintentionally increasing the capabilities of the
     * returned enumeration.
     *
     * @param  <T> the class of the objects in the collection
     * @param c the collection for which an enumeration is to be returned.
     * @return an enumeration over the specified collection.
     * @see Enumeration
     */
    public static <T> Enumeration<T> enumeration(final Collection<T> c) {
        return new Enumeration<T>() {
            private final Iterator<T> i = c.iterator();

            public boolean hasMoreElements() {
                return i.hasNext();
            }

            public T nextElement() {
                return i.next();
            }
        };
    }

    
Returns an array list containing the elements returned by the
specified enumeration in the order they are returned by the
enumeration.  This method provides interoperability between
legacy APIs that return enumerations and new APIs that require
collections.

Params:
  • e – enumeration providing elements for the returned
         array list
Type parameters:
  • <T> – the class of the objects returned by the enumeration
See Also:
Returns:an array list containing the elements returned
        by the specified enumeration.
Since:1.4
/**
     * Returns an array list containing the elements returned by the
     * specified enumeration in the order they are returned by the
     * enumeration.  This method provides interoperability between
     * legacy APIs that return enumerations and new APIs that require
     * collections.
     *
     * @param <T> the class of the objects returned by the enumeration
     * @param e enumeration providing elements for the returned
     *          array list
     * @return an array list containing the elements returned
     *         by the specified enumeration.
     * @since 1.4
     * @see Enumeration
     * @see ArrayList
     */
    public static <T> ArrayList<T> list(Enumeration<T> e) {
        ArrayList<T> l = new ArrayList<>();
        while (e.hasMoreElements())
            l.add(e.nextElement());
        return l;
    }

    
Returns true if the specified arguments are equal, or both null.
NB: Do not replace with Object.equals until JDK-8015417 is resolved.

/**
     * Returns true if the specified arguments are equal, or both null.
     *
     * NB: Do not replace with Object.equals until JDK-8015417 is resolved.
     */
    static boolean eq(Object o1, Object o2) {
        return o1==null ? o2==null : o1.equals(o2);
    }

    
Returns the number of elements in the specified collection equal to the specified object. More formally, returns the number of elements e in the collection such that Objects.equals(o, e). 
Params:
  • c – the collection in which to determine the frequency of o
  • o – the object whose frequency is to be determined
Throws:
Returns:the number of elements in c equal to o
Since:1.5
/**
     * Returns the number of elements in the specified collection equal to the
     * specified object.  More formally, returns the number of elements
     * {@code e} in the collection such that
     * {@code Objects.equals(o, e)}.
     *
     * @param c the collection in which to determine the frequency
     *     of {@code o}
     * @param o the object whose frequency is to be determined
     * @return the number of elements in {@code c} equal to {@code o}
     * @throws NullPointerException if {@code c} is null
     * @since 1.5
     */
    public static int frequency(Collection<?> c, Object o) {
        int result = 0;
        if (o == null) {
            for (Object e : c)
                if (e == null)
                    result++;
        } else {
            for (Object e : c)
                if (o.equals(e))
                    result++;
        }
        return result;
    }

    
Returns true if the two specified collections have no elements in common. 
Care must be exercised if this method is used on collections that do not comply with the general contract for Collection. Implementations may elect to iterate over either collection and test for containment in the other collection (or to perform any equivalent computation). If either collection uses a nonstandard equality test (as does a SortedSet whose ordering is not compatible with
equals, or the key set of an IdentityHashMap), both collections must use the same nonstandard equality test, or the result of this method is undefined. 
Care must also be exercised when using collections that have
restrictions on the elements that they may contain. Collection
implementations are allowed to throw exceptions for any operation
involving elements they deem ineligible. For absolute safety the
specified collections should contain only elements which are
eligible elements for both collections.

Note that it is permissible to pass the same collection in both parameters, in which case the method will return true if and only if the collection is empty. 
Params:
  • c1 – a collection
  • c2 – a collection
Throws:
Returns:true if the two specified collections have no elements in common.
Since:1.5
/**
     * Returns {@code true} if the two specified collections have no
     * elements in common.
     *
     * <p>Care must be exercised if this method is used on collections that
     * do not comply with the general contract for {@code Collection}.
     * Implementations may elect to iterate over either collection and test
     * for containment in the other collection (or to perform any equivalent
     * computation).  If either collection uses a nonstandard equality test
     * (as does a {@link SortedSet} whose ordering is not <em>compatible with
     * equals</em>, or the key set of an {@link IdentityHashMap}), both
     * collections must use the same nonstandard equality test, or the
     * result of this method is undefined.
     *
     * <p>Care must also be exercised when using collections that have
     * restrictions on the elements that they may contain. Collection
     * implementations are allowed to throw exceptions for any operation
     * involving elements they deem ineligible. For absolute safety the
     * specified collections should contain only elements which are
     * eligible elements for both collections.
     *
     * <p>Note that it is permissible to pass the same collection in both
     * parameters, in which case the method will return {@code true} if and
     * only if the collection is empty.
     *
     * @param c1 a collection
     * @param c2 a collection
     * @return {@code true} if the two specified collections have no
     * elements in common.
     * @throws NullPointerException if either collection is {@code null}.
     * @throws NullPointerException if one collection contains a {@code null}
     * element and {@code null} is not an eligible element for the other collection.
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws ClassCastException if one collection contains an element that is
     * of a type which is ineligible for the other collection.
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @since 1.5
     */
    public static boolean disjoint(Collection<?> c1, Collection<?> c2) {
        // The collection to be used for contains(). Preference is given to
        // the collection who's contains() has lower O() complexity.
        Collection<?> contains = c2;
        // The collection to be iterated. If the collections' contains() impl
        // are of different O() complexity, the collection with slower
        // contains() will be used for iteration. For collections who's
        // contains() are of the same complexity then best performance is
        // achieved by iterating the smaller collection.
        Collection<?> iterate = c1;

        // Performance optimization cases. The heuristics:
        //   1. Generally iterate over c1.
        //   2. If c1 is a Set then iterate over c2.
        //   3. If either collection is empty then result is always true.
        //   4. Iterate over the smaller Collection.
        if (c1 instanceof Set) {
            // Use c1 for contains as a Set's contains() is expected to perform
            // better than O(N/2)
            iterate = c2;
            contains = c1;
        } else if (!(c2 instanceof Set)) {
            // Both are mere Collections. Iterate over smaller collection.
            // Example: If c1 contains 3 elements and c2 contains 50 elements and
            // assuming contains() requires ceiling(N/2) comparisons then
            // checking for all c1 elements in c2 would require 75 comparisons
            // (3 * ceiling(50/2)) vs. checking all c2 elements in c1 requiring
            // 100 comparisons (50 * ceiling(3/2)).
            int c1size = c1.size();
            int c2size = c2.size();
            if (c1size == 0 || c2size == 0) {
                // At least one collection is empty. Nothing will match.
                return true;
            }

            if (c1size > c2size) {
                iterate = c2;
                contains = c1;
            }
        }

        for (Object e : iterate) {
            if (contains.contains(e)) {
               // Found a common element. Collections are not disjoint.
                return false;
            }
        }

        // No common elements were found.
        return true;
    }

    
Adds all of the specified elements to the specified collection. Elements to be added may be specified individually or as an array. The behavior of this convenience method is identical to that of c.addAll(Arrays.asList(elements)), but this method is likely to run significantly faster under most implementations. 
When elements are specified individually, this method provides a
convenient way to add a few elements to an existing collection:

    Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");



Params:
  • c – the collection into which elements are to be inserted
  • elements – the elements to insert into c
Type parameters:
  • <T> – the class of the elements to add and of the collection
Throws:
See Also:
Returns:true if the collection changed as a result of the call
Since:1.5
/**
     * Adds all of the specified elements to the specified collection.
     * Elements to be added may be specified individually or as an array.
     * The behavior of this convenience method is identical to that of
     * {@code c.addAll(Arrays.asList(elements))}, but this method is likely
     * to run significantly faster under most implementations.
     *
     * <p>When elements are specified individually, this method provides a
     * convenient way to add a few elements to an existing collection:
     * <pre>
     *     Collections.addAll(flavors, "Peaches 'n Plutonium", "Rocky Racoon");
     * </pre>
     *
     * @param  <T> the class of the elements to add and of the collection
     * @param c the collection into which {@code elements} are to be inserted
     * @param elements the elements to insert into {@code c}
     * @return {@code true} if the collection changed as a result of the call
     * @throws UnsupportedOperationException if {@code c} does not support
     *         the {@code add} operation
     * @throws NullPointerException if {@code elements} contains one or more
     *         null values and {@code c} does not permit null elements, or
     *         if {@code c} or {@code elements} are {@code null}
     * @throws IllegalArgumentException if some property of a value in
     *         {@code elements} prevents it from being added to {@code c}
     * @see Collection#addAll(Collection)
     * @since 1.5
     */
    @SafeVarargs
    public static <T> boolean addAll(Collection<? super T> c, T... elements) {
        boolean result = false;
        for (T element : elements)
            result |= c.add(element);
        return result;
    }

    
Returns a set backed by the specified map. The resulting set displays the same ordering, concurrency, and performance characteristics as the backing map. In essence, this factory method provides a Set implementation corresponding to any Map implementation. There is no need to use this method on a Map implementation that already has a corresponding Set implementation (such as HashMap or TreeMap). 
Each method invocation on the set returned by this method results in exactly one method invocation on the backing map or its keySet view, with one exception. The addAll method is implemented as a sequence of put invocations on the backing map. 
The specified map must be empty at the time this method is invoked,
and should not be accessed directly after this method returns.  These
conditions are ensured if the map is created empty, passed directly
to this method, and no reference to the map is retained, as illustrated
in the following code fragment:

   Set<Object> weakHashSet = Collections.newSetFromMap(
       new WeakHashMap<Object, Boolean>());



Params:
  • map – the backing map
Type parameters:
  • <E> – the class of the map keys and of the objects in the
       returned set
Throws:
Returns:the set backed by the map
Since:1.6
/**
     * Returns a set backed by the specified map.  The resulting set displays
     * the same ordering, concurrency, and performance characteristics as the
     * backing map.  In essence, this factory method provides a {@link Set}
     * implementation corresponding to any {@link Map} implementation.  There
     * is no need to use this method on a {@link Map} implementation that
     * already has a corresponding {@link Set} implementation (such as {@link
     * HashMap} or {@link TreeMap}).
     *
     * <p>Each method invocation on the set returned by this method results in
     * exactly one method invocation on the backing map or its {@code keySet}
     * view, with one exception.  The {@code addAll} method is implemented
     * as a sequence of {@code put} invocations on the backing map.
     *
     * <p>The specified map must be empty at the time this method is invoked,
     * and should not be accessed directly after this method returns.  These
     * conditions are ensured if the map is created empty, passed directly
     * to this method, and no reference to the map is retained, as illustrated
     * in the following code fragment:
     * <pre>
     *    Set&lt;Object&gt; weakHashSet = Collections.newSetFromMap(
     *        new WeakHashMap&lt;Object, Boolean&gt;());
     * </pre>
     *
     * @param <E> the class of the map keys and of the objects in the
     *        returned set
     * @param map the backing map
     * @return the set backed by the map
     * @throws IllegalArgumentException if {@code map} is not empty
     * @since 1.6
     */
    public static <E> Set<E> newSetFromMap(Map<E, Boolean> map) {
        return new SetFromMap<>(map);
    }

    
@serialinclude
/**
     * @serial include
     */
    private static class SetFromMap<E> extends AbstractSet<E>
        implements Set<E>, Serializable
    {
        private final Map<E, Boolean> m;  // The backing map
        private transient Set<E> s;       // Its keySet

        SetFromMap(Map<E, Boolean> map) {
            if (!map.isEmpty())
                throw new IllegalArgumentException("Map is non-empty");
            m = map;
            s = map.keySet();
        }

        public void clear()               {        m.clear(); }
        public int size()                 { return m.size(); }
        public boolean isEmpty()          { return m.isEmpty(); }
        public boolean contains(Object o) { return m.containsKey(o); }
        public boolean remove(Object o)   { return m.remove(o) != null; }
        public boolean add(E e) { return m.put(e, Boolean.TRUE) == null; }
        public Iterator<E> iterator()     { return s.iterator(); }
        public Object[] toArray()         { return s.toArray(); }
        public <T> T[] toArray(T[] a)     { return s.toArray(a); }
        public String toString()          { return s.toString(); }
        public int hashCode()             { return s.hashCode(); }
        public boolean equals(Object o)   { return o == this || s.equals(o); }
        public boolean containsAll(Collection<?> c) {return s.containsAll(c);}
        public boolean removeAll(Collection<?> c)   {return s.removeAll(c);}
        public boolean retainAll(Collection<?> c)   {return s.retainAll(c);}
        // addAll is the only inherited implementation

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            s.forEach(action);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            return s.removeIf(filter);
        }

        @Override
        public Spliterator<E> spliterator() {return s.spliterator();}
        @Override
        public Stream<E> stream()           {return s.stream();}
        @Override
        public Stream<E> parallelStream()   {return s.parallelStream();}

        private static final long serialVersionUID = 2454657854757543876L;

        private void readObject(java.io.ObjectInputStream stream)
            throws IOException, ClassNotFoundException
        {
            stream.defaultReadObject();
            s = m.keySet();
        }
    }

    
Returns a view of a Deque as a Last-in-first-out (Lifo) Queue. Method add is mapped to push, remove is mapped to pop and so on. This view can be useful when you would like to use a method requiring a Queue but you need Lifo ordering. 
Each method invocation on the queue returned by this method results in exactly one method invocation on the backing deque, with one exception. The addAll method is implemented as a sequence of addFirst invocations on the backing deque. 
Params:
  • deque – the deque
Type parameters:
  • <T> – the class of the objects in the deque
Returns:the queue
Since: 1.6
/**
     * Returns a view of a {@link Deque} as a Last-in-first-out (Lifo)
     * {@link Queue}. Method {@code add} is mapped to {@code push},
     * {@code remove} is mapped to {@code pop} and so on. This
     * view can be useful when you would like to use a method
     * requiring a {@code Queue} but you need Lifo ordering.
     *
     * <p>Each method invocation on the queue returned by this method
     * results in exactly one method invocation on the backing deque, with
     * one exception.  The {@link Queue#addAll addAll} method is
     * implemented as a sequence of {@link Deque#addFirst addFirst}
     * invocations on the backing deque.
     *
     * @param  <T> the class of the objects in the deque
     * @param deque the deque
     * @return the queue
     * @since  1.6
     */
    public static <T> Queue<T> asLifoQueue(Deque<T> deque) {
        return new AsLIFOQueue<>(Objects.requireNonNull(deque));
    }

    
@serialinclude
/**
     * @serial include
     */
    static class AsLIFOQueue<E> extends AbstractQueue<E>
        implements Queue<E>, Serializable {
        private static final long serialVersionUID = 1802017725587941708L;
        private final Deque<E> q;
        AsLIFOQueue(Deque<E> q)                     { this.q = q; }
        public boolean add(E e)                     { q.addFirst(e); return true; }
        public boolean offer(E e)                   { return q.offerFirst(e); }
        public E poll()                             { return q.pollFirst(); }
        public E remove()                           { return q.removeFirst(); }
        public E peek()                             { return q.peekFirst(); }
        public E element()                          { return q.getFirst(); }
        public void clear()                         {        q.clear(); }
        public int size()                           { return q.size(); }
        public boolean isEmpty()                    { return q.isEmpty(); }
        public boolean contains(Object o)           { return q.contains(o); }
        public boolean remove(Object o)             { return q.remove(o); }
        public Iterator<E> iterator()               { return q.iterator(); }
        public Object[] toArray()                   { return q.toArray(); }
        public <T> T[] toArray(T[] a)               { return q.toArray(a); }
        public <T> T[] toArray(IntFunction<T[]> f)  { return q.toArray(f); }
        public String toString()                    { return q.toString(); }
        public boolean containsAll(Collection<?> c) { return q.containsAll(c); }
        public boolean removeAll(Collection<?> c)   { return q.removeAll(c); }
        public boolean retainAll(Collection<?> c)   { return q.retainAll(c); }
        // We use inherited addAll; forwarding addAll would be wrong

        // Override default methods in Collection
        @Override
        public void forEach(Consumer<? super E> action) {q.forEach(action);}
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            return q.removeIf(filter);
        }
        @Override
        public Spliterator<E> spliterator() {return q.spliterator();}
        @Override
        public Stream<E> stream()           {return q.stream();}
        @Override
        public Stream<E> parallelStream()   {return q.parallelStream();}
    }
}