/* * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; /** * The root interface in the collection hierarchy. A collection * represents a group of objects, known as its elements. Some * collections allow duplicate elements and others do not. Some are ordered * and others unordered. The JDK does not provide any direct * implementations of this interface: it provides implementations of more * specific subinterfaces like Set and List. This interface * is typically used to pass collections around and manipulate them where * maximum generality is desired. * *

Bags or multisets (unordered collections that may contain * duplicate elements) should implement this interface directly. * *

All general-purpose Collection implementation classes (which * typically implement Collection indirectly through one of its * subinterfaces) should provide two "standard" constructors: a void (no * arguments) constructor, which creates an empty collection, and a * constructor with a single argument of type Collection, which * creates a new collection with the same elements as its argument. In * effect, the latter constructor allows the user to copy any collection, * producing an equivalent collection of the desired implementation type. * There is no way to enforce this convention (as interfaces cannot contain * constructors) but all of the general-purpose Collection * implementations in the Java platform libraries comply. * *

The "destructive" methods contained in this interface, that is, the * methods that modify the collection on which they operate, are specified to * throw UnsupportedOperationException if this collection does not * support the operation. If this is the case, these methods may, but are not * required to, throw an UnsupportedOperationException 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. * *

* Some collection implementations have restrictions on the elements that * they may contain. For example, some implementations prohibit null elements, * and some have restrictions on the types of their elements. Attempting to * add an ineligible element throws an unchecked exception, typically * NullPointerException or ClassCastException. Attempting * to query the presence of an ineligible element may throw an exception, * or it may simply return false; some implementations will exhibit the former * behavior and some will exhibit the latter. More generally, attempting an * operation on an ineligible element whose completion would not result in * the insertion of an ineligible element into the collection may throw an * exception or it may succeed, at the option of the implementation. * Such exceptions are marked as "optional" in the specification for this * interface. * *

It is up to each collection to determine its own synchronization * policy. In the absence of a stronger guarantee by the * implementation, undefined behavior may result from the invocation * of any method on a collection that is being mutated by another * thread; this includes direct invocations, passing the collection to * a method that might perform invocations, and using an existing * iterator to examine the collection. * *

Many methods in Collections Framework interfaces are defined in * terms of the {@link Object#equals(Object) equals} method. For example, * the specification for the {@link #contains(Object) contains(Object o)} * method says: "returns true if and only if this collection * contains at least one element e such that * (o==null ? e==null : o.equals(e))." This specification should * not be construed to imply that invoking Collection.contains * with a non-null argument o will cause o.equals(e) to be * invoked for any element e. Implementations are free to implement * optimizations whereby the equals invocation is avoided, for * example, by first comparing the hash codes of the two elements. (The * {@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. * *

This interface is a member of the * * Java Collections Framework. * * @param 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 extends Iterable { // Query Operations /** * Returns the number of elements in this collection. If this collection * contains more than Integer.MAX_VALUE elements, returns * Integer.MAX_VALUE. * * @return the number of elements in this collection */ int size(); /** * Returns true if this collection contains no elements. * * @return true if this collection contains no elements */ boolean isEmpty(); /** * Returns true if this collection contains the specified element. * More formally, returns true if and only if this collection * contains at least one element e such that * (o==null ? e==null : o.equals(e)). * * @param o element whose presence in this collection is to be tested * @return true if this collection contains the specified * element * @throws ClassCastException if the type of the specified element * is incompatible with this collection * (optional) * @throws NullPointerException if the specified element is null and this * collection does not permit null elements * (optional) */ 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 Iterator over the elements in this collection */ Iterator 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 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. * *

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. * *

If this collection fits in the specified array with room to spare * (i.e., the array has more elements than this collection), the element * in the array immediately following the end of the collection is set to * null. (This is useful in determining the length of this * collection only if the caller knows that this collection does * not contain any null elements.) * *

If this collection makes any guarantees as to what order its elements * are returned by its iterator, this method must return the elements in * the same order. * *

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. * *

Suppose x is a collection known to contain only strings. * The following code can be used to dump the collection into a newly * allocated array of String: * * 

     *     String[] y = x.toArray(new String[0]);
* * Note that toArray(new Object[0]) is identical in function to * toArray(). * * @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[] toArray(T[] a); // Modification Operations /** * Ensures that this collection contains the specified element (optional * operation). Returns true if this collection changed as a * result of the call. (Returns false if this collection does * not permit duplicates and already contains the specified element.)

* * Collections that support this operation may place limitations on what * elements may be added to this collection. In particular, some * collections will refuse to add null elements, and others will * impose restrictions on the type of elements that may be added. * Collection classes should clearly specify in their documentation any * restrictions on what elements may be added.

* * If a collection refuses to add a particular element for any reason * other than that it already contains the element, it must throw * an exception (rather than returning false). This preserves * the invariant that a collection always contains the specified element * after this call returns. * * @param e element whose presence in this collection is to be ensured * @return true if this collection changed as a result of the * call * @throws UnsupportedOperationException if the add operation * is not supported by this collection * @throws ClassCastException if the class of the specified element * prevents it from being added to this collection * @throws NullPointerException if the specified element is null and this * collection does not permit null elements * @throws IllegalArgumentException if some property of the element * prevents it from being added to this collection * @throws IllegalStateException if the element cannot be added at this * time due to insertion restrictions */ boolean add(E e); /** * Removes a single instance of the specified element from this * collection, if it is present (optional operation). More formally, * removes an element e such that * (o==null ? e==null : o.equals(e)), if * this collection contains one or more such elements. Returns * true if this collection contained the specified element (or * equivalently, if this collection changed as a result of the call). * * @param o element to be removed from this collection, if present * @return 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 * (optional) * @throws NullPointerException if the specified element is null and this * collection does not permit null elements * (optional) * @throws UnsupportedOperationException if the remove operation * is not supported by this collection */ boolean remove(Object o); // Bulk Operations /** * Returns true if this collection contains all of the elements * in the specified collection. * * @param c collection to be checked for containment in this collection * @return 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 * (optional) * @throws NullPointerException if the specified collection contains one * or more null elements and this collection does not permit null * elements * (optional), * or if the specified collection is null. * @see #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 true if this collection changed as a result of the call * @throws UnsupportedOperationException if the 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 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 true if this collection changed as a result of the * call * @throws UnsupportedOperationException if the 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 * (optional) * @throws NullPointerException if this collection contains one or more * null elements and the specified collection does not support * null elements * (optional), * or if the specified collection is null * @see #remove(Object) * @see #contains(Object) */ boolean removeAll(Collection c); /** * 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 true if this collection changed as a result of the call * @throws UnsupportedOperationException if the 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 * (optional) * @throws NullPointerException if this collection contains one or more * null elements and the specified collection does not permit null * elements * (optional), * or if the specified collection is null * @see #remove(Object) * @see #contains(Object) */ boolean retainAll(Collection c); /** * Removes all of the elements from this collection (optional operation). * The collection will be empty after this method returns. * * @throws UnsupportedOperationException if the clear operation * is not supported by this collection */ void clear(); // Comparison and hashing /** * Compares the specified object with this collection for equality.

* * While the Collection interface adds no stipulations to the * general contract for the Object.equals, programmers who * implement the Collection interface "directly" (in other words, * create a class that is a Collection but is not a Set * or a List) must exercise care if they choose to override the * Object.equals. It is not necessary to do so, and the simplest * course of action is to rely on Object's implementation, but * the implementor may wish to implement a "value comparison" in place of * the default "reference comparison." (The List and * Set interfaces mandate such value comparisons.)

* * The general contract for the Object.equals method states that * equals must be symmetric (in other words, a.equals(b) if and * only if b.equals(a)). The contracts for List.equals * and Set.equals state that lists are only equal to other lists, * and sets to other sets. Thus, a custom equals method for a * collection class that implements neither the List nor * Set interface must return false when this collection * is compared to any list or set. (By the same logic, it is not possible * to write a class that correctly implements both the Set and * List interfaces.) * * @param o object to be compared for equality with this collection * @return true if the specified object is equal to this * collection * * @see Object#equals(Object) * @see Set#equals(Object) * @see List#equals(Object) */ boolean equals(Object o); /** * Returns the hash code value for this collection. While the * Collection interface adds no stipulations to the general * contract for the Object.hashCode method, programmers should * take note that any class that overrides the Object.equals * method must also override the Object.hashCode method in order * to satisfy the general contract for the Object.hashCode method. * In particular, c1.equals(c2) implies that * c1.hashCode()==c2.hashCode(). * * @return the hash code value for this collection * * @see Object#hashCode() * @see Object#equals(Object) */ int hashCode(); }