Java tutorial
/* * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.util; import java.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 — if changes * are permitted — 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 {@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); } }