Collection.java

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * particular file as subject to the "Classpath" exception as provided
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 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 *
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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 <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.
 *
 * <h2><a id="serializable">Serializability of Collections</a></h2>
 *
 * <p>Serializability of collections is optional. As such, none of the collections
 * interfaces are declared to implement the {@link java.io.Serializable} interface.
 * However, serializability is regarded as being generally useful, so most collection
 * implementations are serializable.
 *
 * <p>The collection implementations that are public classes (such as {@code ArrayList}
 * or {@code HashMap}) are declared to implement the {@code Serializable} interface if they
 * are in fact serializable. Some collections implementations are not public classes,
 * such as the <a href="#unmodifiable">unmodifiable collections.</a> In such cases, the
 * serializability of such collections is described in the specification of the method
 * that creates them, or in some other suitable place. In cases where the serializability
 * of a collection is not specified, there is no guarantee about the serializability of such
 * collections. In particular, many <a href="#view">view collections</a> are not serializable.
 *
 * <p>A collection implementation that implements the {@code Serializable} interface cannot
 * be guaranteed to be serializable. The reason is that in general, collections
 * contain elements of other types, and it is not possible to determine statically
 * whether instances of some element type are actually serializable. For example, consider
 * a serializable {@code Collection<E>}, where {@code E} does not implement the
 * {@code Serializable} interface. The collection may be serializable, if it contains only
 * elements of some serializable subtype of {@code E}, or if it is empty. Collections are
 * thus said to be <i>conditionally serializable,</i> as the serializability of the collection
 * as a whole depends on whether the collection itself is serializable and on whether all
 * contained elements are also serializable.
 *
 * <p>An additional case occurs with instances of {@link SortedSet} and {@link SortedMap}.
 * These collections can be created with a {@link Comparator} that imposes an ordering on
 * the set elements or map keys. Such a collection is serializable only if the provided
 * {@code Comparator} is also serializable.
 *
 * <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 {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of elements in this collection
     */
    int size();

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

    /**
     * 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).
     *
     * @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 {@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.
     *
     * <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 {@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 {@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 {@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 {@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.)
     *
     * @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.
     *
     * @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.
     *
     * @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.
     *
     * @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 {@code clear} operation
     *         is not supported by this collection
     */
    void clear();


    // Comparison and hashing

    /**
     * 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
     * {@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 {@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 {@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 {@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);
    }
}