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package java.util;

import java.util.function.IntFunction;
import java.util.function.Predicate;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;


The root interface in the collection hierarchy.  A collection
represents a group of objects, known as its elements.  Some
collections allow duplicate elements and others do not.  Some are ordered
and others unordered.  The JDK does not provide any direct implementations of this interface: it provides implementations of more specific subinterfaces like Set and List. This interface is typically used to pass collections around and manipulate them where maximum generality is desired. 
Bags or multisets (unordered collections that may contain
duplicate elements) should implement this interface directly.

All general-purpose Collection implementation classes (which typically implement Collection indirectly through one of its subinterfaces) should provide two "standard" constructors: a void (no arguments) constructor, which creates an empty collection, and a constructor with a single argument of type Collection, which creates a new collection with the same elements as its argument. In effect, the latter constructor allows the user to copy any collection, producing an equivalent collection of the desired implementation type. There is no way to enforce this convention (as interfaces cannot contain constructors) but all of the general-purpose Collection implementations in the Java platform libraries comply. 
Certain methods are specified to be
optional. If a collection implementation doesn't implement a particular operation, it should define the corresponding method to throw UnsupportedOperationException. Such methods are marked "optional operation" in method specifications of the collections interfaces. 
Some collection implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the collection may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface. 
It is up to each collection to determine its own synchronization
policy.  In the absence of a stronger guarantee by the
implementation, undefined behavior may result from the invocation
of any method on a collection that is being mutated by another
thread; this includes direct invocations, passing the collection to
a method that might perform invocations, and using an existing
iterator to examine the collection.

Many methods in Collections Framework interfaces are defined in terms of the equals method. For example, the specification for the contains(Object o) method says: "returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e))." This specification should not be construed to imply that invoking Collection.contains with a non-null argument o will cause o.equals(e) to be invoked for any element e. Implementations are free to implement optimizations whereby the equals invocation is avoided, for example, by first comparing the hash codes of the two elements. (The Object.hashCode() specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying Object methods wherever the implementor deems it appropriate. 
Some collection operations which perform recursive traversal of the collection may fail with an exception for self-referential instances where the collection directly or indirectly contains itself. This includes the clone(), equals(), hashCode() and toString() methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so. 
View Collections


Most collections manage storage for elements they contain. By contrast, view
collections themselves do not store elements, but instead they rely on a backing collection to store the actual elements. Operations that are not handled by the view collection itself are delegated to the backing collection. Examples of view collections include the wrapper collections returned by methods such as Collections.checkedCollection, Collections.synchronizedCollection, and Collections.unmodifiableCollection. Other examples of view collections include collections that provide a different representation of the same elements, for example, as provided by List.subList, NavigableSet.subSet, or Map.entrySet. Any changes made to the backing collection are visible in the view collection. Correspondingly, any changes made to the view collection — if changes are permitted — are written through to the backing collection. Although they technically aren't collections, instances of Iterator and ListIterator can also allow modifications to be written through to the backing collection, and in some cases, modifications to the backing collection will be visible to the Iterator during iteration. 
Unmodifiable Collections


Certain methods of this interface are considered "destructive" and are called "mutator" methods in that they modify the group of objects contained within the collection on which they operate. They can be specified to throw UnsupportedOperationException if this collection implementation does not support the operation. Such methods should (but are not required to) throw an UnsupportedOperationException if the invocation would have no effect on the collection. For example, consider a collection that does not support the add operation. What will happen if the addAll method is invoked on this collection, with an empty collection as the argument? The addition of zero elements has no effect, so it is permissible for this collection simply to do nothing and not to throw an exception. However, it is recommended that such cases throw an exception unconditionally, as throwing only in certain cases can lead to programming errors. 
An unmodifiable collection is a collection, all of whose mutator methods (as defined above) are specified to throw UnsupportedOperationException. Such a collection thus cannot be modified by calling any methods on it. For a collection to be properly unmodifiable, any view collections derived from it must also be unmodifiable. For example, if a List is unmodifiable, the List returned by List.subList is also unmodifiable. 
An unmodifiable collection is not necessarily immutable. If the contained elements are mutable, the entire collection is clearly mutable, even though it might be unmodifiable. For example, consider two unmodifiable lists containing mutable elements. The result of calling list1.equals(list2) might differ from one call to the next if the elements had been mutated, even though both lists are unmodifiable. However, if an unmodifiable collection contains all immutable elements, it can be considered effectively immutable. 
Unmodifiable View Collections


An unmodifiable view collection is a collection that is unmodifiable and that is also a view onto a backing collection. Its mutator methods throw UnsupportedOperationException, as described above, while reading and querying methods are delegated to the backing collection. The effect is to provide read-only access to the backing collection. This is useful for a component to provide users with read access to an internal collection, while preventing them from modifying such collections unexpectedly. Examples of unmodifiable view collections are those returned by the Collections.unmodifiableCollection, Collections.unmodifiableList, and related methods. 
Note that changes to the backing collection might still be possible,
and if they occur, they are visible through the unmodifiable view. Thus,
an unmodifiable view collection is not necessarily immutable. However,
if the backing collection of an unmodifiable view is effectively immutable,
or if the only reference to the backing collection is through an
unmodifiable view, the view can be considered effectively immutable.

This interface is a member of the

Java Collections Framework.

Author: Josh Bloch,  Neal Gafter
Type parameters:
  • <E> – the type of elements in this collection
See Also:
Implementation Requirements: The default method implementations (inherited or otherwise) do not apply any synchronization protocol. If a Collection implementation has a specific synchronization protocol, then it must override default implementations to apply that protocol.
Since:1.2
/**
 * The root interface in the <i>collection hierarchy</i>.  A collection
 * represents a group of objects, known as its <i>elements</i>.  Some
 * collections allow duplicate elements and others do not.  Some are ordered
 * and others unordered.  The JDK does not provide any <i>direct</i>
 * implementations of this interface: it provides implementations of more
 * specific subinterfaces like {@code Set} and {@code List}.  This interface
 * is typically used to pass collections around and manipulate them where
 * maximum generality is desired.
 *
 * <p><i>Bags</i> or <i>multisets</i> (unordered collections that may contain
 * duplicate elements) should implement this interface directly.
 *
 * <p>All general-purpose {@code Collection} implementation classes (which
 * typically implement {@code Collection} indirectly through one of its
 * subinterfaces) should provide two "standard" constructors: a void (no
 * arguments) constructor, which creates an empty collection, and a
 * constructor with a single argument of type {@code Collection}, which
 * creates a new collection with the same elements as its argument.  In
 * effect, the latter constructor allows the user to copy any collection,
 * producing an equivalent collection of the desired implementation type.
 * There is no way to enforce this convention (as interfaces cannot contain
 * constructors) but all of the general-purpose {@code Collection}
 * implementations in the Java platform libraries comply.
 *
 * <p>Certain methods are specified to be
 * <i>optional</i>. If a collection implementation doesn't implement a
 * particular operation, it should define the corresponding method to throw
 * {@code UnsupportedOperationException}. Such methods are marked "optional
 * operation" in method specifications of the collections interfaces.
 *
 * <p><a id="optional-restrictions"></a>Some collection implementations
 * have restrictions on the elements that they may contain.
 * For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * {@code NullPointerException} or {@code ClassCastException}.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the collection may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>It is up to each collection to determine its own synchronization
 * policy.  In the absence of a stronger guarantee by the
 * implementation, undefined behavior may result from the invocation
 * of any method on a collection that is being mutated by another
 * thread; this includes direct invocations, passing the collection to
 * a method that might perform invocations, and using an existing
 * iterator to examine the collection.
 *
 * <p>Many methods in Collections Framework interfaces are defined in
 * terms of the {@link Object#equals(Object) equals} method.  For example,
 * the specification for the {@link #contains(Object) contains(Object o)}
 * method says: "returns {@code true} if and only if this collection
 * contains at least one element {@code e} such that
 * {@code (o==null ? e==null : o.equals(e))}."  This specification should
 * <i>not</i> be construed to imply that invoking {@code Collection.contains}
 * with a non-null argument {@code o} will cause {@code o.equals(e)} to be
 * invoked for any element {@code e}.  Implementations are free to implement
 * optimizations whereby the {@code equals} invocation is avoided, for
 * example, by first comparing the hash codes of the two elements.  (The
 * {@link Object#hashCode()} specification guarantees that two objects with
 * unequal hash codes cannot be equal.)  More generally, implementations of
 * the various Collections Framework interfaces are free to take advantage of
 * the specified behavior of underlying {@link Object} methods wherever the
 * implementor deems it appropriate.
 *
 * <p>Some collection operations which perform recursive traversal of the
 * collection may fail with an exception for self-referential instances where
 * the collection directly or indirectly contains itself. This includes the
 * {@code clone()}, {@code equals()}, {@code hashCode()} and {@code toString()}
 * methods. Implementations may optionally handle the self-referential scenario,
 * however most current implementations do not do so.
 *
 * <h2><a id="view">View Collections</a></h2>
 *
 * <p>Most collections manage storage for elements they contain. By contrast, <i>view
 * collections</i> themselves do not store elements, but instead they rely on a
 * backing collection to store the actual elements. Operations that are not handled
 * by the view collection itself are delegated to the backing collection. Examples of
 * view collections include the wrapper collections returned by methods such as
 * {@link Collections#checkedCollection Collections.checkedCollection},
 * {@link Collections#synchronizedCollection Collections.synchronizedCollection}, and
 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection}.
 * Other examples of view collections include collections that provide a
 * different representation of the same elements, for example, as
 * provided by {@link List#subList List.subList},
 * {@link NavigableSet#subSet NavigableSet.subSet}, or
 * {@link Map#entrySet Map.entrySet}.
 * Any changes made to the backing collection are visible in the view collection.
 * Correspondingly, any changes made to the view collection &mdash; if changes
 * are permitted &mdash; are written through to the backing collection.
 * Although they technically aren't collections, instances of
 * {@link Iterator} and {@link ListIterator} can also allow modifications
 * to be written through to the backing collection, and in some cases,
 * modifications to the backing collection will be visible to the Iterator
 * during iteration.
 *
 * <h2><a id="unmodifiable">Unmodifiable Collections</a></h2>
 *
 * <p>Certain methods of this interface are considered "destructive" and are called
 * "mutator" methods in that they modify the group of objects contained within
 * the collection on which they operate. They can be specified to throw
 * {@code UnsupportedOperationException} if this collection implementation
 * does not support the operation. Such methods should (but are not required
 * to) throw an {@code UnsupportedOperationException} if the invocation would
 * have no effect on the collection. For example, consider a collection that
 * does not support the {@link #add add} operation. What will happen if the
 * {@link #addAll addAll} method is invoked on this collection, with an empty
 * collection as the argument? The addition of zero elements has no effect,
 * so it is permissible for this collection simply to do nothing and not to throw
 * an exception. However, it is recommended that such cases throw an exception
 * unconditionally, as throwing only in certain cases can lead to
 * programming errors.
 *
 * <p>An <i>unmodifiable collection</i> is a collection, all of whose
 * mutator methods (as defined above) are specified to throw
 * {@code UnsupportedOperationException}. Such a collection thus cannot be
 * modified by calling any methods on it. For a collection to be properly
 * unmodifiable, any view collections derived from it must also be unmodifiable.
 * For example, if a List is unmodifiable, the List returned by
 * {@link List#subList List.subList} is also unmodifiable.
 *
 * <p>An unmodifiable collection is not necessarily immutable. If the
 * contained elements are mutable, the entire collection is clearly
 * mutable, even though it might be unmodifiable. For example, consider
 * two unmodifiable lists containing mutable elements. The result of calling
 * {@code list1.equals(list2)} might differ from one call to the next if
 * the elements had been mutated, even though both lists are unmodifiable.
 * However, if an unmodifiable collection contains all immutable elements,
 * it can be considered effectively immutable.
 *
 * <h2><a id="unmodview">Unmodifiable View Collections</a></h2>
 *
 * <p>An <i>unmodifiable view collection</i> is a collection that is unmodifiable
 * and that is also a view onto a backing collection. Its mutator methods throw
 * {@code UnsupportedOperationException}, as described above, while
 * reading and querying methods are delegated to the backing collection.
 * The effect is to provide read-only access to the backing collection.
 * This is useful for a component to provide users with read access to
 * an internal collection, while preventing them from modifying such
 * collections unexpectedly. Examples of unmodifiable view collections
 * are those returned by the
 * {@link Collections#unmodifiableCollection Collections.unmodifiableCollection},
 * {@link Collections#unmodifiableList Collections.unmodifiableList}, and
 * related methods.
 *
 * <p>Note that changes to the backing collection might still be possible,
 * and if they occur, they are visible through the unmodifiable view. Thus,
 * an unmodifiable view collection is not necessarily immutable. However,
 * if the backing collection of an unmodifiable view is effectively immutable,
 * or if the only reference to the backing collection is through an
 * unmodifiable view, the view can be considered effectively immutable.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @implSpec
 * The default method implementations (inherited or otherwise) do not apply any
 * synchronization protocol.  If a {@code Collection} implementation has a
 * specific synchronization protocol, then it must override default
 * implementations to apply that protocol.
 *
 * @param <E> the type of elements in this collection
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Set
 * @see     List
 * @see     Map
 * @see     SortedSet
 * @see     SortedMap
 * @see     HashSet
 * @see     TreeSet
 * @see     ArrayList
 * @see     LinkedList
 * @see     Vector
 * @see     Collections
 * @see     Arrays
 * @see     AbstractCollection
 * @since 1.2
 */

public interface Collection<E> extends Iterable<E> {
    // Query Operations

    
Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE. 
Returns:the number of elements in this collection
/**
     * Returns the number of elements in this collection.  If this collection
     * contains more than {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of elements in this collection
     */
    int size();

    
Returns true if this collection contains no elements. 
Returns:true if this collection contains no elements
/**
     * Returns {@code true} if this collection contains no elements.
     *
     * @return {@code true} if this collection contains no elements
     */
    boolean isEmpty();

    
Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that Objects.equals(o, e). 
Params:
  • o – element whose presence in this collection is to be tested
Throws:
Returns:true if this collection contains the specified element
/**
     * Returns {@code true} if this collection contains the specified element.
     * More formally, returns {@code true} if and only if this collection
     * contains at least one element {@code e} such that
     * {@code Objects.equals(o, e)}.
     *
     * @param o element whose presence in this collection is to be tested
     * @return {@code true} if this collection contains the specified
     *         element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     */
    boolean contains(Object o);

    
Returns an iterator over the elements in this collection.  There are no
guarantees concerning the order in which the elements are returned
(unless this collection is an instance of some class that provides a
guarantee).

Returns:an Iterator over the elements in this collection
/**
     * Returns an iterator over the elements in this collection.  There are no
     * guarantees concerning the order in which the elements are returned
     * (unless this collection is an instance of some class that provides a
     * guarantee).
     *
     * @return an {@code Iterator} over the elements in this collection
     */
    Iterator<E> iterator();

    
Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's 
runtime component type is Object. 
The returned array will be "safe" in that no references to it are
maintained by this collection.  (In other words, this method must
allocate a new array even if this collection is backed by an array).
The caller is thus free to modify the returned array.

API Note: This method acts as a bridge between array-based and collection-based APIs. It returns an array whose runtime type is Object[]. Use toArray(T[]) to reuse an existing array, or use toArray(IntFunction) to control the runtime type of the array.
Returns:an array, whose runtime component
type is Object, containing all of the elements in this collection
/**
     * Returns an array containing all of the elements in this collection.
     * If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order. The returned array's {@linkplain Class#getComponentType
     * runtime component type} is {@code Object}.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this collection.  (In other words, this method must
     * allocate a new array even if this collection is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * @apiNote
     * This method acts as a bridge between array-based and collection-based APIs.
     * It returns an array whose runtime type is {@code Object[]}.
     * Use {@link #toArray(Object[]) toArray(T[])} to reuse an existing
     * array, or use {@link #toArray(IntFunction)} to control the runtime type
     * of the array.
     *
     * @return an array, whose {@linkplain Class#getComponentType runtime component
     * type} is {@code Object}, containing all of the elements in this collection
     */
    Object[] toArray();

    
Returns an array containing all of the elements in this collection;
the runtime type of the returned array is that of the specified array.
If the collection 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 collection.

If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.) 
If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.

Params:
  • a – the array into which the elements of this collection are to be
       stored, if it is big enough; otherwise, a new array of the same
       runtime type is allocated for this purpose.
Type parameters:
  • <T> – the component type of the array to contain the collection
Throws:
API Note: This method acts as a bridge between array-based and collection-based APIs. It allows an existing array to be reused under certain circumstances. Use toArray() to create an array whose runtime type is Object[], or use toArray(IntFunction) to control the runtime type of the array. 
Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a previously allocated String array: 
    String[] y = new String[SIZE];
    ...
    y = x.toArray(y);


The return value is reassigned to the variable y, because a new array will be allocated and returned if the collection x has too many elements to fit into the existing array y. 
Note that toArray(new Object[0]) is identical in function to toArray().
Returns:an array containing all of the elements in this collection
/**
     * Returns an array containing all of the elements in this collection;
     * the runtime type of the returned array is that of the specified array.
     * If the collection 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 collection.
     *
     * <p>If this collection fits in the specified array with room to spare
     * (i.e., the array has more elements than this collection), the element
     * in the array immediately following the end of the collection is set to
     * {@code null}.  (This is useful in determining the length of this
     * collection <i>only</i> if the caller knows that this collection does
     * not contain any {@code null} elements.)
     *
     * <p>If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * @apiNote
     * This method acts as a bridge between array-based and collection-based APIs.
     * It allows an existing array to be reused under certain circumstances.
     * Use {@link #toArray()} to create an array whose runtime type is {@code Object[]},
     * or use {@link #toArray(IntFunction)} to control the runtime type of
     * the array.
     *
     * <p>Suppose {@code x} is a collection known to contain only strings.
     * The following code can be used to dump the collection into a previously
     * allocated {@code String} array:
     *
     * <pre>
     *     String[] y = new String[SIZE];
     *     ...
     *     y = x.toArray(y);</pre>
     *
     * <p>The return value is reassigned to the variable {@code y}, because a
     * new array will be allocated and returned if the collection {@code x} has
     * too many elements to fit into the existing array {@code y}.
     *
     * <p>Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param <T> the component type of the array to contain the collection
     * @param a the array into which the elements of this collection 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 all of the elements in this collection
     * @throws ArrayStoreException if the runtime type of any element in this
     *         collection is not assignable to the {@linkplain Class#getComponentType
     *         runtime component type} of the specified array
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);

    
Returns an array containing all of the elements in this collection, using the provided generator function to allocate the returned array. 
If this collection makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements in
the same order.

Params:
  • generator – a function which produces a new array of the desired
                 type and the provided length
Type parameters:
  • <T> – the component type of the array to contain the collection
Throws:
API Note: This method acts as a bridge between array-based and collection-based APIs. It allows creation of an array of a particular runtime type. Use toArray() to create an array whose runtime type is Object[], or use toArray(T[]) to reuse an existing array. 
Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String: 
    String[] y = x.toArray(String[]::new);
Implementation Requirements: The default implementation calls the generator function with zero and then passes the resulting array to toArray(T[]).
Returns:an array containing all of the elements in this collection
Since:11
/**
     * Returns an array containing all of the elements in this collection,
     * using the provided {@code generator} function to allocate the returned array.
     *
     * <p>If this collection makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements in
     * the same order.
     *
     * @apiNote
     * This method acts as a bridge between array-based and collection-based APIs.
     * It allows creation of an array of a particular runtime type. Use
     * {@link #toArray()} to create an array whose runtime type is {@code Object[]},
     * or use {@link #toArray(Object[]) toArray(T[])} to reuse an existing array.
     *
     * <p>Suppose {@code x} is a collection known to contain only strings.
     * The following code can be used to dump the collection into a newly
     * allocated array of {@code String}:
     *
     * <pre>
     *     String[] y = x.toArray(String[]::new);</pre>
     *
     * @implSpec
     * The default implementation calls the generator function with zero
     * and then passes the resulting array to {@link #toArray(Object[]) toArray(T[])}.
     *
     * @param <T> the component type of the array to contain the collection
     * @param generator a function which produces a new array of the desired
     *                  type and the provided length
     * @return an array containing all of the elements in this collection
     * @throws ArrayStoreException if the runtime type of any element in this
     *         collection is not assignable to the {@linkplain Class#getComponentType
     *         runtime component type} of the generated array
     * @throws NullPointerException if the generator function is null
     * @since 11
     */
    default <T> T[] toArray(IntFunction<T[]> generator) {
        return toArray(generator.apply(0));
    }

    // Modification Operations

    
Ensures that this collection contains the specified element (optional operation). Returns true if this collection changed as a result of the call. (Returns false if this collection does not permit duplicates and already contains the specified element.)
 Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added.

If a collection refuses to add a particular element for any reason
other than that it already contains the element, it must throw an exception (rather than returning false). This preserves the invariant that a collection always contains the specified element after this call returns. 
Params:
  • e – element whose presence in this collection is to be ensured
Throws:
Returns:true if this collection changed as a result of the call
/**
     * Ensures that this collection contains the specified element (optional
     * operation).  Returns {@code true} if this collection changed as a
     * result of the call.  (Returns {@code false} if this collection does
     * not permit duplicates and already contains the specified element.)<p>
     *
     * Collections that support this operation may place limitations on what
     * elements may be added to this collection.  In particular, some
     * collections will refuse to add {@code null} elements, and others will
     * impose restrictions on the type of elements that may be added.
     * Collection classes should clearly specify in their documentation any
     * restrictions on what elements may be added.<p>
     *
     * If a collection refuses to add a particular element for any reason
     * other than that it already contains the element, it <i>must</i> throw
     * an exception (rather than returning {@code false}).  This preserves
     * the invariant that a collection always contains the specified element
     * after this call returns.
     *
     * @param e element whose presence in this collection is to be ensured
     * @return {@code true} if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this collection
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     * @throws IllegalArgumentException if some property of the element
     *         prevents it from being added to this collection
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to insertion restrictions
     */
    boolean add(E e);

    
Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element e such that Objects.equals(o, e), if this collection contains one or more such elements. Returns true if this collection contained the specified element (or equivalently, if this collection changed as a result of the call). 
Params:
  • o – element to be removed from this collection, if present
Throws:
Returns:true if an element was removed as a result of this call
/**
     * Removes a single instance of the specified element from this
     * collection, if it is present (optional operation).  More formally,
     * removes an element {@code e} such that
     * {@code Objects.equals(o, e)}, if
     * this collection contains one or more such elements.  Returns
     * {@code true} if this collection contained the specified element (or
     * equivalently, if this collection changed as a result of the call).
     *
     * @param o element to be removed from this collection, if present
     * @return {@code true} if an element was removed as a result of this call
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this collection
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this collection
     */
    boolean remove(Object o);


    // Bulk Operations

    
Returns true if this collection contains all of the elements in the specified collection. 
Params:
  • c – collection to be checked for containment in this collection
Throws:
  • ClassCastException – if the types of one or more elements
        in the specified collection are incompatible with this
        collection
        (optional)
  • NullPointerException – if the specified collection contains one
        or more null elements and this collection does not permit null
        elements
        (optional),
        or if the specified collection is null.
See Also:
Returns:true if this collection contains all of the elements in the specified collection
/**
     * Returns {@code true} if this collection contains all of the elements
     * in the specified collection.
     *
     * @param  c collection to be checked for containment in this collection
     * @return {@code true} if this collection contains all of the elements
     *         in the specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         collection
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this collection does not permit null
     *         elements
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null.
     * @see    #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    
Adds all of the elements in the specified collection to this collection
(optional operation).  The behavior of this operation is undefined if
the specified collection is modified while the operation is in progress.
(This implies that the behavior of this call is undefined if the
specified collection is this collection, and this collection is
nonempty.)

Params:
  • c – collection containing elements to be added to this collection
Throws:
  • UnsupportedOperationException – if the addAll operation is not supported by this collection
  • ClassCastException – if the class of an element of the specified
        collection prevents it from being added to this collection
  • NullPointerException – if the specified collection contains a
        null element and this collection does not permit null elements,
        or if the specified collection is null
  • IllegalArgumentException – if some property of an element of the
        specified collection prevents it from being added to this
        collection
  • IllegalStateException – if not all the elements can be added at
        this time due to insertion restrictions
See Also:
Returns:true if this collection changed as a result of the call
/**
     * Adds all of the elements in the specified collection to this collection
     * (optional operation).  The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in progress.
     * (This implies that the behavior of this call is undefined if the
     * specified collection is this collection, and this collection is
     * nonempty.)
     *
     * @param c collection containing elements to be added to this collection
     * @return {@code true} if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the {@code addAll} operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this collection
     * @throws NullPointerException if the specified collection contains a
     *         null element and this collection does not permit null elements,
     *         or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this
     *         collection
     * @throws IllegalStateException if not all the elements can be added at
     *         this time due to insertion restrictions
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    
Removes all of this collection's elements that are also contained in the
specified collection (optional operation).  After this call returns,
this collection will contain no elements in common with the specified
collection.

Params:
  • c – collection containing elements to be removed from this collection
Throws:
  • UnsupportedOperationException – if the removeAll method is not supported by this collection
  • ClassCastException – if the types of one or more elements
        in this collection are incompatible with the specified
        collection
        (optional)
  • NullPointerException – if this collection contains one or more
        null elements and the specified collection does not support
        null elements
        (optional),
        or if the specified collection is null
See Also:
Returns:true if this collection changed as a result of the call
/**
     * Removes all of this collection's elements that are also contained in the
     * specified collection (optional operation).  After this call returns,
     * this collection will contain no elements in common with the specified
     * collection.
     *
     * @param c collection containing elements to be removed from this collection
     * @return {@code true} if this collection changed as a result of the
     *         call
     * @throws UnsupportedOperationException if the {@code removeAll} method
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not support
     *         null elements
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    
Removes all of the elements of this collection that satisfy the given
predicate.  Errors or runtime exceptions thrown during iteration or by
the predicate are relayed to the caller.

Params:
  • filter – a predicate which returns true for elements to be removed
Throws:
  • NullPointerException – if the specified filter is null
  • UnsupportedOperationException – if elements cannot be removed
        from this collection.  Implementations may throw this exception if a
        matching element cannot be removed or if, in general, removal is not
        supported.
Implementation Requirements: The default implementation traverses all elements of the collection using its iterator. Each matching element is removed using Iterator.remove(). If the collection's iterator does not support removal then an UnsupportedOperationException will be thrown on the first matching element.
Returns:true if any elements were removed
Since:1.8
/**
     * Removes all of the elements of this collection that satisfy the given
     * predicate.  Errors or runtime exceptions thrown during iteration or by
     * the predicate are relayed to the caller.
     *
     * @implSpec
     * The default implementation traverses all elements of the collection using
     * its {@link #iterator}.  Each matching element is removed using
     * {@link Iterator#remove()}.  If the collection's iterator does not
     * support removal then an {@code UnsupportedOperationException} will be
     * thrown on the first matching element.
     *
     * @param filter a predicate which returns {@code true} for elements to be
     *        removed
     * @return {@code true} if any elements were removed
     * @throws NullPointerException if the specified filter is null
     * @throws UnsupportedOperationException if elements cannot be removed
     *         from this collection.  Implementations may throw this exception if a
     *         matching element cannot be removed or if, in general, removal is not
     *         supported.
     * @since 1.8
     */
    default boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        boolean removed = false;
        final Iterator<E> each = iterator();
        while (each.hasNext()) {
            if (filter.test(each.next())) {
                each.remove();
                removed = true;
            }
        }
        return removed;
    }

    
Retains only the elements in this collection that are contained in the
specified collection (optional operation).  In other words, removes from
this collection all of its elements that are not contained in the
specified collection.

Params:
  • c – collection containing elements to be retained in this collection
Throws:
  • UnsupportedOperationException – if the retainAll operation is not supported by this collection
  • ClassCastException – if the types of one or more elements
        in this collection are incompatible with the specified
        collection
        (optional)
  • NullPointerException – if this collection contains one or more
        null elements and the specified collection does not permit null
        elements
        (optional),
        or if the specified collection is null
See Also:
Returns:true if this collection changed as a result of the call
/**
     * Retains only the elements in this collection that are contained in the
     * specified collection (optional operation).  In other words, removes from
     * this collection all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this collection
     * @return {@code true} if this collection changed as a result of the call
     * @throws UnsupportedOperationException if the {@code retainAll} operation
     *         is not supported by this collection
     * @throws ClassCastException if the types of one or more elements
     *         in this collection are incompatible with the specified
     *         collection
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this collection contains one or more
     *         null elements and the specified collection does not permit null
     *         elements
     *         (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    
Removes all of the elements from this collection (optional operation).
The collection will be empty after this method returns.

Throws:
  • UnsupportedOperationException – if the clear operation is not supported by this collection
/**
     * Removes all of the elements from this collection (optional operation).
     * The collection will be empty after this method returns.
     *
     * @throws UnsupportedOperationException if the {@code clear} operation
     *         is not supported by this collection
     */
    void clear();


    // Comparison and hashing

    
Compares the specified object with this collection for equality. 
 While the Collection interface adds no stipulations to the general contract for the Object.equals, programmers who implement the Collection interface "directly" (in other words, create a class that is a Collection but is not a Set or a List) must exercise care if they choose to override the Object.equals. It is not necessary to do so, and the simplest course of action is to rely on Object's implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (The List and Set interfaces mandate such value comparisons.)
 The general contract for the Object.equals method states that equals must be symmetric (in other words, a.equals(b) if and only if b.equals(a)). The contracts for List.equals and Set.equals state that lists are only equal to other lists, and sets to other sets. Thus, a custom equals method for a collection class that implements neither the List nor Set interface must return false when this collection is compared to any list or set. (By the same logic, it is not possible to write a class that correctly implements both the Set and List interfaces.) 
Params:
  • o – object to be compared for equality with this collection
See Also:
Returns:true if the specified object is equal to this collection
/**
     * Compares the specified object with this collection for equality. <p>
     *
     * While the {@code Collection} interface adds no stipulations to the
     * general contract for the {@code Object.equals}, programmers who
     * implement the {@code Collection} interface "directly" (in other words,
     * create a class that is a {@code Collection} but is not a {@code Set}
     * or a {@code List}) must exercise care if they choose to override the
     * {@code Object.equals}.  It is not necessary to do so, and the simplest
     * course of action is to rely on {@code Object}'s implementation, but
     * the implementor may wish to implement a "value comparison" in place of
     * the default "reference comparison."  (The {@code List} and
     * {@code Set} interfaces mandate such value comparisons.)<p>
     *
     * The general contract for the {@code Object.equals} method states that
     * equals must be symmetric (in other words, {@code a.equals(b)} if and
     * only if {@code b.equals(a)}).  The contracts for {@code List.equals}
     * and {@code Set.equals} state that lists are only equal to other lists,
     * and sets to other sets.  Thus, a custom {@code equals} method for a
     * collection class that implements neither the {@code List} nor
     * {@code Set} interface must return {@code false} when this collection
     * is compared to any list or set.  (By the same logic, it is not possible
     * to write a class that correctly implements both the {@code Set} and
     * {@code List} interfaces.)
     *
     * @param o object to be compared for equality with this collection
     * @return {@code true} if the specified object is equal to this
     * collection
     *
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     * @see List#equals(Object)
     */
    boolean equals(Object o);

    
Returns the hash code value for this collection. While the Collection interface adds no stipulations to the general contract for the Object.hashCode method, programmers should take note that any class that overrides the Object.equals method must also override the Object.hashCode method in order to satisfy the general contract for the Object.hashCode method. In particular, c1.equals(c2) implies that c1.hashCode()==c2.hashCode(). 
See Also:
Returns:the hash code value for this collection
/**
     * Returns the hash code value for this collection.  While the
     * {@code Collection} interface adds no stipulations to the general
     * contract for the {@code Object.hashCode} method, programmers should
     * take note that any class that overrides the {@code Object.equals}
     * method must also override the {@code Object.hashCode} method in order
     * to satisfy the general contract for the {@code Object.hashCode} method.
     * In particular, {@code c1.equals(c2)} implies that
     * {@code c1.hashCode()==c2.hashCode()}.
     *
     * @return the hash code value for this collection
     *
     * @see Object#hashCode()
     * @see Object#equals(Object)
     */
    int hashCode();

    
Creates a Spliterator over the elements in this collection. Implementations should document characteristic values reported by the spliterator. Such characteristic values are not required to be reported if the spliterator reports Spliterator.SIZED and this collection contains no elements. 
The default implementation should be overridden by subclasses that can return a more efficient spliterator. In order to preserve expected laziness behavior for the stream() and parallelStream() methods, spliterators should either have the characteristic of IMMUTABLE or CONCURRENT, or be late-binding. If none of these is practical, the overriding class should describe the spliterator's documented policy of binding and structural interference, and should override the stream() and parallelStream() methods to create streams using a Supplier of the spliterator, as in: 

    Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)



These requirements ensure that streams produced by the stream() and parallelStream() methods will reflect the contents of the collection as of initiation of the terminal stream operation. 
Implementation Requirements:
The default implementation creates a
late-binding spliterator from the collection's Iterator. The spliterator inherits the fail-fast properties of the collection's iterator.

 The created Spliterator reports Spliterator.SIZED.
Implementation Note: The created Spliterator additionally reports Spliterator.SUBSIZED. 
If a spliterator covers no elements then the reporting of additional characteristic values, beyond that of SIZED and SUBSIZED, does not aid clients to control, specialize or simplify computation. However, this does enable shared use of an immutable and empty spliterator instance (see Spliterators.emptySpliterator()) for empty collections, and enables clients to determine if such a spliterator covers no elements.
Returns:a Spliterator over the elements in this collection
Since:1.8
/**
     * Creates a {@link Spliterator} over the elements in this collection.
     *
     * Implementations should document characteristic values reported by the
     * spliterator.  Such characteristic values are not required to be reported
     * if the spliterator reports {@link Spliterator#SIZED} and this collection
     * contains no elements.
     *
     * <p>The default implementation should be overridden by subclasses that
     * can return a more efficient spliterator.  In order to
     * preserve expected laziness behavior for the {@link #stream()} and
     * {@link #parallelStream()} methods, spliterators should either have the
     * characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
     * <em><a href="Spliterator.html#binding">late-binding</a></em>.
     * If none of these is practical, the overriding class should describe the
     * spliterator's documented policy of binding and structural interference,
     * and should override the {@link #stream()} and {@link #parallelStream()}
     * methods to create streams using a {@code Supplier} of the spliterator,
     * as in:
     * <pre>{@code
     *     Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
     * }</pre>
     * <p>These requirements ensure that streams produced by the
     * {@link #stream()} and {@link #parallelStream()} methods will reflect the
     * contents of the collection as of initiation of the terminal stream
     * operation.
     *
     * @implSpec
     * The default implementation creates a
     * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     * from the collection's {@code Iterator}.  The spliterator inherits the
     * <em>fail-fast</em> properties of the collection's iterator.
     * <p>
     * The created {@code Spliterator} reports {@link Spliterator#SIZED}.
     *
     * @implNote
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SUBSIZED}.
     *
     * <p>If a spliterator covers no elements then the reporting of additional
     * characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
     * does not aid clients to control, specialize or simplify computation.
     * However, this does enable shared use of an immutable and empty
     * spliterator instance (see {@link Spliterators#emptySpliterator()}) for
     * empty collections, and enables clients to determine if such a spliterator
     * covers no elements.
     *
     * @return a {@code Spliterator} over the elements in this collection
     * @since 1.8
     */
    @Override
    default Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, 0);
    }

    
Returns a sequential Stream with this collection as its source. 
This method should be overridden when the spliterator() method cannot return a spliterator that is IMMUTABLE, CONCURRENT, or late-binding. (See spliterator() for details.) 
Implementation Requirements: The default implementation creates a sequential Stream from the collection's Spliterator.
Returns:a sequential Stream over the elements in this collection
Since:1.8
/**
     * Returns a sequential {@code Stream} with this collection as its source.
     *
     * <p>This method should be overridden when the {@link #spliterator()}
     * method cannot return a spliterator that is {@code IMMUTABLE},
     * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     * for details.)
     *
     * @implSpec
     * The default implementation creates a sequential {@code Stream} from the
     * collection's {@code Spliterator}.
     *
     * @return a sequential {@code Stream} over the elements in this collection
     * @since 1.8
     */
    default Stream<E> stream() {
        return StreamSupport.stream(spliterator(), false);
    }

    
Returns a possibly parallel Stream with this collection as its source. It is allowable for this method to return a sequential stream. 
This method should be overridden when the spliterator() method cannot return a spliterator that is IMMUTABLE, CONCURRENT, or late-binding. (See spliterator() for details.) 
Implementation Requirements: The default implementation creates a parallel Stream from the collection's Spliterator.
Returns:a possibly parallel Stream over the elements in this collection
Since:1.8
/**
     * Returns a possibly parallel {@code Stream} with this collection as its
     * source.  It is allowable for this method to return a sequential stream.
     *
     * <p>This method should be overridden when the {@link #spliterator()}
     * method cannot return a spliterator that is {@code IMMUTABLE},
     * {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
     * for details.)
     *
     * @implSpec
     * The default implementation creates a parallel {@code Stream} from the
     * collection's {@code Spliterator}.
     *
     * @return a possibly parallel {@code Stream} over the elements in this
     * collection
     * @since 1.8
     */
    default Stream<E> parallelStream() {
        return StreamSupport.stream(spliterator(), true);
    }
}