/* * Copyright (c) 1997, 2006, 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; /** * A collection that contains no duplicate elements. More formally, sets * contain no pair of elements e1 and e2 such that * e1.equals(e2), and at most one null element. As implied by * its name, this interface models the mathematical set abstraction. * *

The Set interface places additional stipulations, beyond those * inherited from the Collection interface, on the contracts of all * constructors and on the contracts of the add, equals and * hashCode methods. Declarations for other inherited methods are * also included here for convenience. (The specifications accompanying these * declarations have been tailored to the Set interface, but they do * not contain any additional stipulations.) * *

The additional stipulation on constructors is, not surprisingly, * that all constructors must create a set that contains no duplicate elements * (as defined above). * *

Note: Great care must be exercised if mutable objects are used as set * elements. The behavior of a set is not specified if the value of an object * is changed in a manner that affects equals comparisons while the * object is an element in the set. A special case of this prohibition is * that it is not permissible for a set to contain itself as an element. * *

Some set 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 set 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. * *

This interface is a member of the * * Java Collections Framework. * * @param the type of elements maintained by this set * * @author Josh Bloch * @author Neal Gafter * @see Collection * @see List * @see SortedSet * @see HashSet * @see TreeSet * @see AbstractSet * @see Collections#singleton(java.lang.Object) * @see Collections#EMPTY_SET * @since 1.2 */ public interface Set extends Collection { // Query Operations /** * Returns the number of elements in this set (its cardinality). If this * set contains more than Integer.MAX_VALUE elements, returns * Integer.MAX_VALUE. * * @return the number of elements in this set (its cardinality) */ int size(); /** * Returns true if this set contains no elements. * * @return true if this set contains no elements */ boolean isEmpty(); /** * Returns true if this set contains the specified element. * More formally, returns true if and only if this set * contains an element e such that * (o==null ? e==null : o.equals(e)). * * @param o element whose presence in this set is to be tested * @return true if this set contains the specified element * @throws ClassCastException if the type of the specified element * is incompatible with this set * (optional) * @throws NullPointerException if the specified element is null and this * set does not permit null elements * (optional) */ boolean contains(Object o); /** * Returns an iterator over the elements in this set. The elements are * returned in no particular order (unless this set is an instance of some * class that provides a guarantee). * * @return an iterator over the elements in this set */ Iterator iterator(); /** * Returns an array containing all of the elements in this set. * If this set 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 set. (In other words, this method must * allocate a new array even if this set 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 the elements in this set */ Object[] toArray(); /** * Returns an array containing all of the elements in this set; the * runtime type of the returned array is that of the specified array. * If the set 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 set. * *

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

If this set 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 set known to contain only strings. * The following code can be used to dump the set 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 set 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 the elements in this set * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in this * set * @throws NullPointerException if the specified array is null */ T[] toArray(T[] a); // Modification Operations /** * Adds the specified element to this set if it is not already present * (optional operation). More formally, adds the specified element * e to this set if the set contains no element e2 * such that * (e==null ? e2==null : e.equals(e2)). * If this set already contains the element, the call leaves the set * unchanged and returns false. In combination with the * restriction on constructors, this ensures that sets never contain * duplicate elements. * *

The stipulation above does not imply that sets must accept all * elements; sets may refuse to add any particular element, including * null, and throw an exception, as described in the * specification for {@link Collection#add Collection.add}. * Individual set implementations should clearly document any * restrictions on the elements that they may contain. * * @param e element to be added to this set * @return true if this set did not already contain the specified * element * @throws UnsupportedOperationException if the add operation * is not supported by this set * @throws ClassCastException if the class of the specified element * prevents it from being added to this set * @throws NullPointerException if the specified element is null and this * set does not permit null elements * @throws IllegalArgumentException if some property of the specified element * prevents it from being added to this set */ boolean add(E e); /** * Removes the specified element from this set if it is present * (optional operation). More formally, removes an element e * such that * (o==null ? e==null : o.equals(e)), if * this set contains such an element. Returns true if this set * contained the element (or equivalently, if this set changed as a * result of the call). (This set will not contain the element once the * call returns.) * * @param o object to be removed from this set, if present * @return true if this set contained the specified element * @throws ClassCastException if the type of the specified element * is incompatible with this set * (optional) * @throws NullPointerException if the specified element is null and this * set does not permit null elements * (optional) * @throws UnsupportedOperationException if the remove operation * is not supported by this set */ boolean remove(Object o); // Bulk Operations /** * Returns true if this set contains all of the elements of the * specified collection. If the specified collection is also a set, this * method returns true if it is a subset of this set. * * @param c collection to be checked for containment in this set * @return true if this set contains all of the elements of the * specified collection * @throws ClassCastException if the types of one or more elements * in the specified collection are incompatible with this * set * (optional) * @throws NullPointerException if the specified collection contains one * or more null elements and this set 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 set if * they're not already present (optional operation). If the specified * collection is also a set, the addAll operation effectively * modifies this set so that its value is the union of the two * sets. The behavior of this operation is undefined if the specified * collection is modified while the operation is in progress. * * @param c collection containing elements to be added to this set * @return true if this set changed as a result of the call * * @throws UnsupportedOperationException if the addAll operation * is not supported by this set * @throws ClassCastException if the class of an element of the * specified collection prevents it from being added to this set * @throws NullPointerException if the specified collection contains one * or more null elements and this set 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 set * @see #add(Object) */ boolean addAll(Collection c); /** * Retains only the elements in this set that are contained in the * specified collection (optional operation). In other words, removes * from this set all of its elements that are not contained in the * specified collection. If the specified collection is also a set, this * operation effectively modifies this set so that its value is the * intersection of the two sets. * * @param c collection containing elements to be retained in this set * @return true if this set changed as a result of the call * @throws UnsupportedOperationException if the retainAll operation * is not supported by this set * @throws ClassCastException if the class of an element of this set * is incompatible with the specified collection * (optional) * @throws NullPointerException if this set contains a null element and the * specified collection does not permit null elements * (optional), * or if the specified collection is null * @see #remove(Object) */ boolean retainAll(Collection c); /** * Removes from this set all of its elements that are contained in the * specified collection (optional operation). If the specified * collection is also a set, this operation effectively modifies this * set so that its value is the asymmetric set difference of * the two sets. * * @param c collection containing elements to be removed from this set * @return true if this set changed as a result of the call * @throws UnsupportedOperationException if the removeAll operation * is not supported by this set * @throws ClassCastException if the class of an element of this set * is incompatible with the specified collection * (optional) * @throws NullPointerException if this set contains a null element and the * specified collection does not permit null elements * (optional), * or if the specified collection is null * @see #remove(Object) * @see #contains(Object) */ boolean removeAll(Collection c); /** * Removes all of the elements from this set (optional operation). * The set will be empty after this call returns. * * @throws UnsupportedOperationException if the clear method * is not supported by this set */ void clear(); // Comparison and hashing /** * Compares the specified object with this set for equality. Returns * true if the specified object is also a set, the two sets * have the same size, and every member of the specified set is * contained in this set (or equivalently, every member of this set is * contained in the specified set). This definition ensures that the * equals method works properly across different implementations of the * set interface. * * @param o object to be compared for equality with this set * @return true if the specified object is equal to this set */ boolean equals(Object o); /** * Returns the hash code value for this set. The hash code of a set is * defined to be the sum of the hash codes of the elements in the set, * where the hash code of a null element is defined to be zero. * This ensures that s1.equals(s2) implies that * s1.hashCode()==s2.hashCode() for any two sets s1 * and s2, as required by the general contract of * {@link Object#hashCode}. * * @return the hash code value for this set * @see Object#equals(Object) * @see Set#equals(Object) */ int hashCode(); }