java/11 : java.base/java/util/LinkedList.java

LinkedList
https://openjdk.java.net/
GPLv2 + Classpath Exception
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package java.util;

import java.util.function.Consumer;

Doubly-linked list implementation of the List and Deque interfaces. Implements all optional list operations, and permits all elements (including null). All of the operations perform as could be expected for a doubly-linked
list.  Operations that index into the list will traverse the list from
the beginning or the end, whichever is closer to the specified index.
Note that this implementation is not synchronized.
If multiple threads access a linked list concurrently, and at least
one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:
  List list = Collections.synchronizedList(new LinkedList(...));
The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. 
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
This class is a member of the

Java Collections Framework.
Author:  Josh Bloch
Type parameters: <E> – the type of elements held in this collection
See Also: List
ArrayList
Since: 1.2/**
 * Doubly-linked list implementation of the {@code List} and {@code Deque}
 * interfaces.  Implements all optional list operations, and permits all
 * elements (including {@code null}).
 *
 * <p>All of the operations perform as could be expected for a doubly-linked
 * list.  Operations that index into the list will traverse the list from
 * the beginning or the end, whichever is closer to the specified index.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked list concurrently, and at least
 * one of the threads modifies the list structurally, it <i>must</i> be
 * synchronized externally.  (A structural modification is any operation
 * that adds or deletes one or more elements; merely setting the value of
 * an element is not a structural modification.)  This is typically
 * accomplished by synchronizing on some object that naturally
 * encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new LinkedList(...));</pre>
 *
 * <p>The iterators returned by this class's {@code iterator} and
 * {@code listIterator} methods are <i>fail-fast</i>: if the list is
 * structurally modified at any time after the iterator is created, in
 * any way except through the Iterator's own {@code remove} or
 * {@code add} methods, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than
 * risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <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
 * @see     List
 * @see     ArrayList
 * @since 1.2
 * @param <E> the type of elements held in this collection
 */

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;

    Pointer to first node.
/**
     * Pointer to first node.
     */
    transient Node<E> first;

    Pointer to last node.
/**
     * Pointer to last node.
     */
    transient Node<E> last;

    /*
    void dataStructureInvariants() {
        assert (size == 0)
            ? (first == null && last == null)
            : (first.prev == null && last.next == null);
    }
    */

    Constructs an empty list.
/**
     * Constructs an empty list.
     */
    public LinkedList() {
    }

    Constructs a list containing the elements of the specified
collection, in the order they are returned by the collection's
iterator.
Params: c – the collection whose elements are to be placed into this list
Throws: NullPointerException – if the specified collection is null/**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param  c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    Links e as first element.
/**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    Links e as last element.
/**
     * Links e as last element.
     */
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
    }

    Inserts element e before non-null Node succ.
/**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    Unlinks non-null first node f.
/**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    Unlinks non-null last node l.
/**
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) {
        // assert l == last && l != null;
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null; // help GC
        last = prev;
        if (prev == null)
            first = null;
        else
            prev.next = null;
        size--;
        modCount++;
        return element;
    }

    Unlinks non-null node x.
/**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }

    Returns the first element in this list.
Throws: NoSuchElementException – if this list is empty
Returns: the first element in this list/**
     * Returns the first element in this list.
     *
     * @return the first element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    Returns the last element in this list.
Throws: NoSuchElementException – if this list is empty
Returns: the last element in this list/**
     * Returns the last element in this list.
     *
     * @return the last element in this list
     * @throws NoSuchElementException if this list is empty
     */
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    Removes and returns the first element from this list.
Throws: NoSuchElementException – if this list is empty
Returns: the first element from this list/**
     * Removes and returns the first element from this list.
     *
     * @return the first element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unlinkFirst(f);
    }

    Removes and returns the last element from this list.
Throws: NoSuchElementException – if this list is empty
Returns: the last element from this list/**
     * Removes and returns the last element from this list.
     *
     * @return the last element from this list
     * @throws NoSuchElementException if this list is empty
     */
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unlinkLast(l);
    }

    Inserts the specified element at the beginning of this list.
Params: e – the element to add/**
     * Inserts the specified element at the beginning of this list.
     *
     * @param e the element to add
     */
    public void addFirst(E e) {
        linkFirst(e);
    }

    Appends the specified element to the end of this list.
This method is equivalent to add. 
Params: e – the element to add/**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #add}.
     *
     * @param e the element to add
     */
    public void addLast(E e) {
        linkLast(e);
    }

    Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that Objects.equals(o, e). 
Params: o – element whose presence in this list is to be tested
Returns: true if this list contains the specified element/**
     * Returns {@code true} if this list contains the specified element.
     * More formally, returns {@code true} if and only if this list contains
     * at least one element {@code e} such that
     * {@code Objects.equals(o, e)}.
     *
     * @param o element whose presence in this list is to be tested
     * @return {@code true} if this list contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    Returns the number of elements in this list.
Returns: the number of elements in this list/**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
        return size;
    }

    Appends the specified element to the end of this list.
This method is equivalent to addLast. 
Params: e – element to be appended to this list
Returns: true (as specified by Collection.add)/**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    Removes the first occurrence of the specified element from this list, if it is present. If this list does not contain the element, it is unchanged. More formally, removes the element with the lowest index i such that Objects.equals(o, get(i)) (if such an element exists). Returns true if this list contained the specified element (or equivalently, if this list changed as a result of the call). 
Params: o – element to be removed from this list, if present
Returns: true if this list contained the specified element/**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If this list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * {@code i} such that
     * {@code Objects.equals(o, get(i))}
     * (if such an element exists).  Returns {@code true} if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    Appends all of the elements in the specified collection to the end of
this list, in the order that they are returned by the specified
collection's iterator.  The behavior of this operation is undefined if
the specified collection is modified while the operation is in
progress.  (Note that this will occur if the specified collection is
this list, and it's nonempty.)
Params: c – collection containing elements to be added to this list
Throws: NullPointerException – if the specified collection is null
Returns: true if this list changed as a result of the call/**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator.  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (Note that this will occur if the specified collection is
     * this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    Inserts all of the elements in the specified collection into this
list, starting at the specified position.  Shifts the element
currently at that position (if any) and any subsequent elements to
the right (increases their indices).  The new elements will appear
in the list in the order that they are returned by the
specified collection's iterator.
Params: index – index at which to insert the first element
             from the specified collection
c – collection containing elements to be added to this list
Throws: IndexOutOfBoundsException – {@inheritDoc}
NullPointerException – if the specified collection is null
Returns: true if this list changed as a result of the call/**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element
     *              from the specified collection
     * @param c collection containing elements to be added to this list
     * @return {@code true} if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        if (numNew == 0)
            return false;

        Node<E> pred, succ;
        if (index == size) {
            succ = null;
            pred = last;
        } else {
            succ = node(index);
            pred = succ.prev;
        }

        for (Object o : a) {
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(pred, e, null);
            if (pred == null)
                first = newNode;
            else
                pred.next = newNode;
            pred = newNode;
        }

        if (succ == null) {
            last = pred;
        } else {
            pred.next = succ;
            succ.prev = pred;
        }

        size += numNew;
        modCount++;
        return true;
    }

    Removes all of the elements from this list.
The list will be empty after this call returns.
/**
     * Removes all of the elements from this list.
     * The list will be empty after this call returns.
     */
    public void clear() {
        // Clearing all of the links between nodes is "unnecessary", but:
        // - helps a generational GC if the discarded nodes inhabit
        //   more than one generation
        // - is sure to free memory even if there is a reachable Iterator
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }


    // Positional Access Operations

    Returns the element at the specified position in this list.
Params: index – index of the element to return
Throws: IndexOutOfBoundsException – {@inheritDoc}
Returns: the element at the specified position in this list/**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

    Replaces the element at the specified position in this list with the
specified element.
Params: index – index of the element to replace
element – element to be stored at the specified position
Throws: IndexOutOfBoundsException – {@inheritDoc}
Returns: the element previously at the specified position/**
     * Replaces the element at the specified position in this list with the
     * specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        checkElementIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    Inserts the specified element at the specified position in this list.
Shifts the element currently at that position (if any) and any
subsequent elements to the right (adds one to their indices).
Params: index – index at which the specified element is to be inserted
element – element to be inserted
Throws: IndexOutOfBoundsException – {@inheritDoc}/**
     * Inserts the specified element at the specified position in this list.
     * Shifts the element currently at that position (if any) and any
     * subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    Removes the element at the specified position in this list.  Shifts any
subsequent elements to the left (subtracts one from their indices).
Returns the element that was removed from the list.
Params: index – the index of the element to be removed
Throws: IndexOutOfBoundsException – {@inheritDoc}
Returns: the element previously at the specified position/**
     * Removes the element at the specified position in this list.  Shifts any
     * subsequent elements to the left (subtracts one from their indices).
     * Returns the element that was removed from the list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    Tells if the argument is the index of an existing element.
/**
     * Tells if the argument is the index of an existing element.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    Tells if the argument is the index of a valid position for an
iterator or an add operation.
/**
     * Tells if the argument is the index of a valid position for an
     * iterator or an add operation.
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    Constructs an IndexOutOfBoundsException detail message.
Of the many possible refactorings of the error handling code,
this "outlining" performs best with both server and client VMs.
/**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    private void checkElementIndex(int index) {
        if (!isElementIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    Returns the (non-null) Node at the specified element index.
/**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

    // Search Operations

    Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that Objects.equals(o, get(i)), or -1 if there is no such index. 
Params: o – element to search for
Returns: the index of the first occurrence of the specified element in
        this list, or -1 if this list does not contain the element/**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null)
                    return index;
                index++;
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

    Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index i such that Objects.equals(o, get(i)), or -1 if there is no such index. 
Params: o – element to search for
Returns: the index of the last occurrence of the specified element in
        this list, or -1 if this list does not contain the element/**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index {@code i} such that
     * {@code Objects.equals(o, get(i))},
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     */
    public int lastIndexOf(Object o) {
        int index = size;
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (x.item == null)
                    return index;
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (o.equals(x.item))
                    return index;
            }
        }
        return -1;
    }

    // Queue operations.

    Retrieves, but does not remove, the head (first element) of this list.
Returns: the head of this list, or null if this list is empty
Since: 1.5/**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    Retrieves, but does not remove, the head (first element) of this list.
Throws: NoSuchElementException – if this list is empty
Returns: the head of this list
Since: 1.5/**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E element() {
        return getFirst();
    }

    Retrieves and removes the head (first element) of this list.
Returns: the head of this list, or null if this list is empty
Since: 1.5/**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    Retrieves and removes the head (first element) of this list.
Throws: NoSuchElementException – if this list is empty
Returns: the head of this list
Since: 1.5/**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

    Adds the specified element as the tail (last element) of this list.
Params: e – the element to add
Returns: true (as specified by Queue.offer)
Since: 1.5/**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Queue#offer})
     * @since 1.5
     */
    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations
    Inserts the specified element at the front of this list.
Params: e – the element to insert
Returns: true (as specified by Deque.offerFirst)
Since: 1.6/**
     * Inserts the specified element at the front of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerFirst})
     * @since 1.6
     */
    public boolean offerFirst(E e) {
        addFirst(e);
        return true;
    }

    Inserts the specified element at the end of this list.
Params: e – the element to insert
Returns: true (as specified by Deque.offerLast)
Since: 1.6/**
     * Inserts the specified element at the end of this list.
     *
     * @param e the element to insert
     * @return {@code true} (as specified by {@link Deque#offerLast})
     * @since 1.6
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

    Retrieves, but does not remove, the first element of this list, or returns null if this list is empty. 
Returns: the first element of this list, or null if this list is empty
Since: 1.6/**
     * Retrieves, but does not remove, the first element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the first element of this list, or {@code null}
     *         if this list is empty
     * @since 1.6
     */
    public E peekFirst() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
     }

    Retrieves, but does not remove, the last element of this list, or returns null if this list is empty. 
Returns: the last element of this list, or null if this list is empty
Since: 1.6/**
     * Retrieves, but does not remove, the last element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the last element of this list, or {@code null}
     *         if this list is empty
     * @since 1.6
     */
    public E peekLast() {
        final Node<E> l = last;
        return (l == null) ? null : l.item;
    }

    Retrieves and removes the first element of this list, or returns null if this list is empty. 
Returns: the first element of this list, or null if this list is empty
Since: 1.6/**
     * Retrieves and removes the first element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the first element of this list, or {@code null} if
     *     this list is empty
     * @since 1.6
     */
    public E pollFirst() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    Retrieves and removes the last element of this list, or returns null if this list is empty. 
Returns: the last element of this list, or null if this list is empty
Since: 1.6/**
     * Retrieves and removes the last element of this list,
     * or returns {@code null} if this list is empty.
     *
     * @return the last element of this list, or {@code null} if
     *     this list is empty
     * @since 1.6
     */
    public E pollLast() {
        final Node<E> l = last;
        return (l == null) ? null : unlinkLast(l);
    }

    Pushes an element onto the stack represented by this list.  In other
words, inserts the element at the front of this list.
This method is equivalent to addFirst. 
Params: e – the element to push
Since: 1.6/**
     * Pushes an element onto the stack represented by this list.  In other
     * words, inserts the element at the front of this list.
     *
     * <p>This method is equivalent to {@link #addFirst}.
     *
     * @param e the element to push
     * @since 1.6
     */
    public void push(E e) {
        addFirst(e);
    }

    Pops an element from the stack represented by this list.  In other
words, removes and returns the first element of this list.
This method is equivalent to removeFirst(). 
Throws: NoSuchElementException – if this list is empty
Returns: the element at the front of this list (which is the top
        of the stack represented by this list)
Since: 1.6/**
     * Pops an element from the stack represented by this list.  In other
     * words, removes and returns the first element of this list.
     *
     * <p>This method is equivalent to {@link #removeFirst()}.
     *
     * @return the element at the front of this list (which is the top
     *         of the stack represented by this list)
     * @throws NoSuchElementException if this list is empty
     * @since 1.6
     */
    public E pop() {
        return removeFirst();
    }

    Removes the first occurrence of the specified element in this
list (when traversing the list from head to tail).  If the list
does not contain the element, it is unchanged.
Params: o – element to be removed from this list, if present
Returns: true if the list contained the specified element
Since: 1.6/**
     * Removes the first occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if the list contained the specified element
     * @since 1.6
     */
    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    Removes the last occurrence of the specified element in this
list (when traversing the list from head to tail).  If the list
does not contain the element, it is unchanged.
Params: o – element to be removed from this list, if present
Returns: true if the list contained the specified element
Since: 1.6/**
     * Removes the last occurrence of the specified element in this
     * list (when traversing the list from head to tail).  If the list
     * does not contain the element, it is unchanged.
     *
     * @param o element to be removed from this list, if present
     * @return {@code true} if the list contained the specified element
     * @since 1.6
     */
    public boolean removeLastOccurrence(Object o) {
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list. Obeys the general contract of List.listIterator(int).
The list-iterator is fail-fast: if the list is structurally modified at any time after the Iterator is created, in any way except through the list-iterator's own remove or add methods, the list-iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. 
Params: index – index of the first element to be returned from the list-iterator (by a call to next)
Throws: IndexOutOfBoundsException – {@inheritDoc}
See Also: List.listIterator(int)
Returns: a ListIterator of the elements in this list (in proper
        sequence), starting at the specified position in the list/**
     * Returns a list-iterator of the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * Obeys the general contract of {@code List.listIterator(int)}.<p>
     *
     * The list-iterator is <i>fail-fast</i>: if the list is structurally
     * modified at any time after the Iterator is created, in any way except
     * through the list-iterator's own {@code remove} or {@code add}
     * methods, the list-iterator will throw a
     * {@code ConcurrentModificationException}.  Thus, in the face of
     * concurrent modification, the iterator fails quickly and cleanly, rather
     * than risking arbitrary, non-deterministic behavior at an undetermined
     * time in the future.
     *
     * @param index index of the first element to be returned from the
     *              list-iterator (by a call to {@code next})
     * @return a ListIterator of the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see List#listIterator(int)
     */
    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

        public boolean hasNext() {
            return nextIndex < size;
        }

        public E next() {
            checkForComodification();
            if (!hasNext())
                throw new NoSuchElementException();

            lastReturned = next;
            next = next.next;
            nextIndex++;
            return lastReturned.item;
        }

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

        public E previous() {
            checkForComodification();
            if (!hasPrevious())
                throw new NoSuchElementException();

            lastReturned = next = (next == null) ? last : next.prev;
            nextIndex--;
            return lastReturned.item;
        }

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

    Since: 1.6/**
     * @since 1.6
     */
    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    Adapter to provide descending iterators via ListItr.previous
/**
     * Adapter to provide descending iterators via ListItr.previous
     */
    private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }

    @SuppressWarnings("unchecked")
    private LinkedList<E> superClone() {
        try {
            return (LinkedList<E>) super.clone();
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    Returns a shallow copy of this LinkedList. (The elements themselves are not cloned.) 
Returns: a shallow copy of this LinkedList instance/**
     * Returns a shallow copy of this {@code LinkedList}. (The elements
     * themselves are not cloned.)
     *
     * @return a shallow copy of this {@code LinkedList} instance
     */
    public Object clone() {
        LinkedList<E> clone = superClone();

        // Put clone into "virgin" state
        clone.first = clone.last = null;
        clone.size = 0;
        clone.modCount = 0;

        // Initialize clone with our elements
        for (Node<E> x = first; x != null; x = x.next)
            clone.add(x.item);

        return clone;
    }

    Returns an array containing all of the elements in this list
in proper sequence (from first to last element).
The returned array will be "safe" in that no references to it are
maintained by this list.  (In other words, this method must allocate
a new array).  The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
Returns: an array containing all of the elements in this list
        in proper sequence/**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list
     *         in proper sequence
     */
    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }

    Returns an array containing all of the elements in this list in
proper sequence (from first to last element); the runtime type of
the returned array is that of the specified array.  If the list fits
in the specified array, it is returned therein.  Otherwise, a new
array is allocated with the runtime type of the specified array and
the size of this list.
If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the list is set to null. (This is useful in determining the length of the list only if
the caller knows that the list does not contain any null elements.)
Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs. 
Suppose x is a list known to contain only strings. The following code can be used to dump the list into a newly allocated array of String: 
    String[] y = x.toArray(new String[0]);
 Note that toArray(new Object[0]) is identical in function to toArray(). 
Params: a – the array into which the elements of the list are to
         be stored, if it is big enough; otherwise, a new array of the
         same runtime type is allocated for this purpose.
Throws: ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in
        this list
NullPointerException – if the specified array is null
Returns: an array containing the elements of the list/**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to {@code null}.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose {@code x} is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of {@code String}:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            a = (T[])java.lang.reflect.Array.newInstance(
                                a.getClass().getComponentType(), size);
        int i = 0;
        Object[] result = a;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;

        if (a.length > size)
            a[size] = null;

        return a;
    }

    private static final long serialVersionUID = 876323262645176354L;

    Saves the state of this LinkedList instance to a stream (that is, serializes it). 
@serialData The size of the list (the number of elements it
            contains) is emitted (int), followed by all of its
            elements (each an Object) in the proper order./**
     * Saves the state of this {@code LinkedList} instance to a stream
     * (that is, serializes it).
     *
     * @serialData The size of the list (the number of elements it
     *             contains) is emitted (int), followed by all of its
     *             elements (each an Object) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

    Reconstitutes this LinkedList instance from a stream (that is, deserializes it). /**
     * Reconstitutes this {@code LinkedList} instance from a stream
     * (that is, deserializes it).
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in any hidden serialization magic
        s.defaultReadObject();

        // Read in size
        int size = s.readInt();

        // Read in all elements in the proper order.
        for (int i = 0; i < size; i++)
            linkLast((E)s.readObject());
    }

    Creates a late-binding
and fail-fast Spliterator over the elements in this list. The Spliterator reports Spliterator.SIZED and Spliterator.ORDERED. Overriding implementations should document the reporting of additional characteristic values. 
Implementation Note:  The Spliterator additionally reports Spliterator.SUBSIZED and implements trySplit to permit limited parallelism..
Returns: a Spliterator over the elements in this list
Since: 1.8/**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#ORDERED}.  Overriding implementations should document
     * the reporting of additional characteristic values.
     *
     * @implNote
     * The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
     * and implements {@code trySplit} to permit limited parallelism..
     *
     * @return a {@code Spliterator} over the elements in this list
     * @since 1.8
     */
    @Override
    public Spliterator<E> spliterator() {
        return new LLSpliterator<>(this, -1, 0);
    }

    A customized variant of Spliterators.IteratorSpliterator /** A customized variant of Spliterators.IteratorSpliterator */
    static final class LLSpliterator<E> implements Spliterator<E> {
        static final int BATCH_UNIT = 1 << 10;  // batch array size increment
        static final int MAX_BATCH = 1 << 25;  // max batch array size;
        final LinkedList<E> list; // null OK unless traversed
        Node<E> current;      // current node; null until initialized
        int est;              // size estimate; -1 until first needed
        int expectedModCount; // initialized when est set
        int batch;            // batch size for splits

        LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
            this.list = list;
            this.est = est;
            this.expectedModCount = expectedModCount;
        }

        final int getEst() {
            int s; // force initialization
            final LinkedList<E> lst;
            if ((s = est) < 0) {
                if ((lst = list) == null)
                    s = est = 0;
                else {
                    expectedModCount = lst.modCount;
                    current = lst.first;
                    s = est = lst.size;
                }
            }
            return s;
        }

        public long estimateSize() { return (long) getEst(); }

        public Spliterator<E> trySplit() {
            Node<E> p;
            int s = getEst();
            if (s > 1 && (p = current) != null) {
                int n = batch + BATCH_UNIT;
                if (n > s)
                    n = s;
                if (n > MAX_BATCH)
                    n = MAX_BATCH;
                Object[] a = new Object[n];
                int j = 0;
                do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
                current = p;
                batch = j;
                est = s - j;
                return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
            }
            return null;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Node<E> p; int n;
            if (action == null) throw new NullPointerException();
            if ((n = getEst()) > 0 && (p = current) != null) {
                current = null;
                est = 0;
                do {
                    E e = p.item;
                    p = p.next;
                    action.accept(e);
                } while (p != null && --n > 0);
            }
            if (list.modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            Node<E> p;
            if (action == null) throw new NullPointerException();
            if (getEst() > 0 && (p = current) != null) {
                --est;
                E e = p.item;
                current = p.next;
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

}
Author:	Josh Bloch
Type parameters:	<E> – the type of elements held in this collection
See Also:	List ArrayList
Since:	1.2
Params:	c – the collection whose elements are to be placed into this list
Throws:	NullPointerException – if the specified collection is null
Throws:	NoSuchElementException – if this list is empty
Returns:	the first element in this list
Params:	o – element whose presence in this list is to be tested
Returns:	`true` if this list contains the specified element
Params:	e – element to be appended to this list
Returns:	`true` (as specified by `Collection.add`)
Params:	o – element to be removed from this list, if present
Returns:	`true` if this list contained the specified element
Params:	c – collection containing elements to be added to this list
Throws:	NullPointerException – if the specified collection is null
Returns:	`true` if this list changed as a result of the call
Params:	index – index at which to insert the first element from the specified collection c – collection containing elements to be added to this list
Throws:	IndexOutOfBoundsException – {@inheritDoc} NullPointerException – if the specified collection is null
Returns:	`true` if this list changed as a result of the call
Params:	index – index of the element to return
Throws:	IndexOutOfBoundsException – {@inheritDoc}
Returns:	the element at the specified position in this list
Params:	index – index of the element to replace element – element to be stored at the specified position
Throws:	IndexOutOfBoundsException – {@inheritDoc}
Returns:	the element previously at the specified position
Params:	index – index at which the specified element is to be inserted element – element to be inserted
Throws:	IndexOutOfBoundsException – {@inheritDoc}
Params:	index – the index of the element to be removed
Throws:	IndexOutOfBoundsException – {@inheritDoc}
Returns:	the element previously at the specified position
Params:	o – element to search for
Returns:	the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element
Returns:	the head of this list, or `null` if this list is empty
Since:	1.5
Params:	e – the element to add
Returns:	`true` (as specified by `Queue.offer`)
Since:	1.5
Params:	e – the element to insert
Returns:	`true` (as specified by `Deque.offerFirst`)
Since:	1.6
Returns:	the first element of this list, or `null` if this list is empty
Since:	1.6
Returns:	the last element of this list, or `null` if this list is empty
Since:	1.6
Params:	index – index of the first element to be returned from the list-iterator (by a call to `next`)
Throws:	IndexOutOfBoundsException – {@inheritDoc}
See Also:	List.listIterator(int)
Returns:	a ListIterator of the elements in this list (in proper sequence), starting at the specified position in the list
Params:	a – the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
Throws:	ArrayStoreException – if the runtime type of the specified array is not a supertype of the runtime type of every element in this list NullPointerException – if the specified array is null
Returns:	an array containing the elements of the list
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java/ 11/ java.base/java/util/LinkedList.java
