Collection
Collection
/* * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util; 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 <tt>Set</tt> and <tt>List</tt>. 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 <tt>Collection</tt> implementation classes (which * typically implement <tt>Collection</tt> 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 <tt>Collection</tt>, 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 <tt>Collection</tt> * implementations in the Java platform libraries comply. * * <p>The "destructive" methods contained in this interface, that is, the * methods that modify the collection on which they operate, are specified to * throw <tt>UnsupportedOperationException</tt> if this collection does not * support the operation. If this is the case, these methods may, but are not * required to, throw an <tt>UnsupportedOperationException</tt> if the * invocation would have no effect on the collection. For example, invoking * the {@link #addAll(Collection)} method on an unmodifiable collection may, * but is not required to, throw the exception if the collection to be added * is empty. * * <p><a name="optional-restrictions"> * Some collection implementations have restrictions on the elements that * they may contain.</a> 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 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>. 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 <tt>true</tt> if and only if this collection * contains at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>." This specification should * <i>not</i> be construed to imply that invoking <tt>Collection.contains</tt> * with a non-null argument <tt>o</tt> will cause <tt>o.equals(e)</tt> to be * invoked for any element <tt>e</tt>. Implementations are free to implement * optimizations whereby the <tt>equals</tt> 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. * * <p>This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * 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 <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of elements in this collection */ int size(); /** * Returns <tt>true</tt> if this collection contains no elements. * * @return <tt>true</tt> if this collection contains no elements */ boolean isEmpty(); /** * Returns <tt>true</tt> if this collection contains the specified element. * More formally, returns <tt>true</tt> if and only if this collection * contains at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this collection is to be tested * @return <tt>true</tt> if this collection contains the specified * element * @throws ClassCastException if the type of the specified element * is incompatible with this collection * (<a href="#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * collection does not permit null elements * (<a href="#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 <tt>Iterator</tt> 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. * * <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. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array 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 * <tt>null</tt>. (This is useful in determining the length of this * collection <i>only</i> if the caller knows that this collection does * not contain any <tt>null</tt> 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. * * <p>Like the {@link #toArray()} method, this method acts as bridge between * array-based and collection-based APIs. Further, this method allows * precise control over the runtime type of the output array, and may, * under certain circumstances, be used to save allocation costs. * * <p>Suppose <tt>x</tt> is a collection known to contain only strings. * The following code can be used to dump the collection into a newly * allocated array of <tt>String</tt>: * * <pre> * String[] y = x.toArray(new String[0]);</pre> * * Note that <tt>toArray(new Object[0])</tt> is identical in function to * <tt>toArray()</tt>. * * @param <T> the runtime 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 the specified array * is not a supertype of the runtime type of every element in * this collection * @throws NullPointerException if the specified array is null */ <T> T[] toArray(T[] a); // Modification Operations /** * Ensures that this collection contains the specified element (optional * operation). Returns <tt>true</tt> if this collection changed as a * result of the call. (Returns <tt>false</tt> 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 <tt>null</tt> 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 <tt>false</tt>). 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 <tt>true</tt> if this collection changed as a result of the * call * @throws UnsupportedOperationException if the <tt>add</tt> 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 <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>, if * this collection contains one or more such elements. Returns * <tt>true</tt> 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 <tt>true</tt> 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="#optional-restrictions">optional</a>) * @throws NullPointerException if the specified element is null and this * collection does not permit null elements * (<a href="#optional-restrictions">optional</a>) * @throws UnsupportedOperationException if the <tt>remove</tt> operation * is not supported by this collection */ boolean remove(Object o); // Bulk Operations /** * Returns <tt>true</tt> if this collection contains all of the elements * in the specified collection. * * @param c collection to be checked for containment in this collection * @return <tt>true</tt> 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="#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="#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 <tt>true</tt> if this collection changed as a result of the call * @throws UnsupportedOperationException if the <tt>addAll</tt> 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 <tt>true</tt> if this collection changed as a result of the * call * @throws UnsupportedOperationException if the <tt>removeAll</tt> 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="#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="#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 <tt>true</tt> if this collection changed as a result of the call * @throws UnsupportedOperationException if the <tt>retainAll</tt> 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="#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="#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 <tt>clear</tt> operation * is not supported by this collection */ void clear(); // Comparison and hashing /** * Compares the specified object with this collection for equality. <p> * * While the <tt>Collection</tt> interface adds no stipulations to the * general contract for the <tt>Object.equals</tt>, programmers who * implement the <tt>Collection</tt> interface "directly" (in other words, * create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt> * or a <tt>List</tt>) must exercise care if they choose to override the * <tt>Object.equals</tt>. It is not necessary to do so, and the simplest * course of action is to rely on <tt>Object</tt>'s implementation, but * the implementor may wish to implement a "value comparison" in place of * the default "reference comparison." (The <tt>List</tt> and * <tt>Set</tt> interfaces mandate such value comparisons.)<p> * * The general contract for the <tt>Object.equals</tt> method states that * equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and * only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt> * and <tt>Set.equals</tt> state that lists are only equal to other lists, * and sets to other sets. Thus, a custom <tt>equals</tt> method for a * collection class that implements neither the <tt>List</tt> nor * <tt>Set</tt> interface must return <tt>false</tt> 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 <tt>Set</tt> and * <tt>List</tt> interfaces.) * * @param o object to be compared for equality with this collection * @return <tt>true</tt> 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 * <tt>Collection</tt> interface adds no stipulations to the general * contract for the <tt>Object.hashCode</tt> method, programmers should * take note that any class that overrides the <tt>Object.equals</tt> * method must also override the <tt>Object.hashCode</tt> method in order * to satisfy the general contract for the <tt>Object.hashCode</tt> method. * In particular, <tt>c1.equals(c2)</tt> implies that * <tt>c1.hashCode()==c2.hashCode()</tt>. * * @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 collections'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); } }
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