CopyOnWriteArraySet

/* * @(#)CopyOnWriteArraySet.java 1.11 06/04/21 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util.concurrent; import java.util.*; /** * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList} * for all of its operations. Thus, it shares the same basic properties: * <ul> * <li>It is best suited for applications in which set sizes generally * stay small, read-only operations * vastly outnumber mutative operations, and you need * to prevent interference among threads during traversal. * <li>It is thread-safe. * <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.) * are expensive since they usually entail copying the entire underlying * array. * <li>Iterators do not support the mutative <tt>remove</tt> operation. * <li>Traversal via iterators is fast and cannot encounter * interference from other threads. Iterators rely on * unchanging snapshots of the array at the time the iterators were * constructed. * </ul> * * <p> <b>Sample Usage.</b> The following code sketch uses a * copy-on-write set to maintain a set of Handler objects that * perform some action upon state updates. * * <pre> * class Handler { void handle(); ... } * * class X { * private final CopyOnWriteArraySet<Handler> handlers * = new CopyOnWriteArraySet<Handler>(); * public void addHandler(Handler h) { handlers.add(h); } * * private long internalState; * private synchronized void changeState() { internalState = ...; } * * public void update() { * changeState(); * for (Handler handler : handlers) * handler.handle(); * } * } * </pre> * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html" mce_href="technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @see CopyOnWriteArrayList * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this collection */ public class CopyOnWriteArraySet<E> extends AbstractSet<E> implements java.io.Serializable { private static final long serialVersionUID = 5457747651344034263L; private final CopyOnWriteArrayList<E> al; /** * Creates an empty set. */ public CopyOnWriteArraySet() { al = new CopyOnWriteArrayList<E>(); } /** * Creates a set containing all of the elements of the specified * collection. * * @param c the collection of elements to initially contain * @throws NullPointerException if the specified collection is null */ public CopyOnWriteArraySet(Collection<? extends E> c) { al = new CopyOnWriteArrayList<E>(); al.addAllAbsent(c); } /** * Returns the number of elements in this set. * * @return the number of elements in this set */ public int size() { return al.size(); } /** * Returns <tt>true</tt> if this set contains no elements. * * @return <tt>true</tt> if this set contains no elements */ public boolean isEmpty() { return al.isEmpty(); } /** * Returns <tt>true</tt> if this set contains the specified element. * More formally, returns <tt>true</tt> if and only if this set * contains an element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this set is to be tested * @return <tt>true</tt> if this set contains the specified element */ public boolean contains(Object o) { return al.contains(o); } /** * 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. * * <p>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. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all the elements in this set */ public Object[] toArray() { return al.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. * * <p>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 * <tt>null</tt>. (This is useful in determining the length of this * set <i>only</i> if the caller knows that this set does not contain * any null elements.) * * <p>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. * * <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 set known to contain only strings. * The following code can be used to dump the set 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 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 */ public <T> T[] toArray(T[] a) { return al.toArray(a); } /** * Removes all of the elements from this set. * The set will be empty after this call returns. */ public void clear() { al.clear(); } /** * Removes the specified element from this set if it is present. * More formally, removes an element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>, * if this set contains such an element. Returns <tt>true</tt> 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 <tt>true</tt> if this set contained the specified element */ public boolean remove(Object o) { return al.remove(o); } /** * Adds the specified element to this set if it is not already present. * More formally, adds the specified element <tt>e</tt> to this set if * the set contains no element <tt>e2</tt> such that * <tt>(e==null ? e2==null : e.equals(e2))</tt>. * If this set already contains the element, the call leaves the set * unchanged and returns <tt>false</tt>. * * @param e element to be added to this set * @return <tt>true</tt> if this set did not already contain the specified * element */ public boolean add(E e) { return al.addIfAbsent(e); } /** * Returns <tt>true</tt> if this set contains all of the elements of the * specified collection. If the specified collection is also a set, this * method returns <tt>true</tt> if it is a <i>subset</i> of this set. * * @param c collection to be checked for containment in this set * @return <tt>true</tt> if this set contains all of the elements of the * specified collection * @throws NullPointerException if the specified collection is null * @see #contains(Object) */ public boolean containsAll(Collection<?> c) { return al.containsAll(c); } /** * Adds all of the elements in the specified collection to this set if * they're not already present. If the specified collection is also a * set, the <tt>addAll</tt> operation effectively modifies this set so * that its value is the <i>union</i> 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 <tt>true</tt> if this set changed as a result of the call * @throws NullPointerException if the specified collection is null * @see #add(Object) */ public boolean addAll(Collection<? extends E> c) { return al.addAllAbsent(c) > 0; } /** * Removes from this set all of its elements that are 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 * <i>asymmetric set difference</i> of the two sets. * * @param c collection containing elements to be removed from this set * @return <tt>true</tt> if this set changed as a result of the call * @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) */ public boolean removeAll(Collection<?> c) { return al.removeAll(c); } /** * Retains only the elements in this set that are contained in the * specified collection. 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 <i>intersection</i> of the * two sets. * * @param c collection containing elements to be retained in this set * @return <tt>true</tt> if this set changed as a result of the call * @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) */ public boolean retainAll(Collection<?> c) { return al.retainAll(c); } /** * Returns an iterator over the elements contained in this set * in the order in which these elements were added. * * <p>The returned iterator provides a snapshot of the state of the set * when the iterator was constructed. No synchronization is needed while * traversing the iterator. The iterator does <em>NOT</em> support the * <tt>remove</tt> method. * * @return an iterator over the elements in this set */ public Iterator<E> iterator() { return al.iterator(); } /** * Compares the specified object with this set for equality. * Returns {@code true} if the specified object is the same object * as this object, or if it is also a {@link Set} and the elements * returned by an {@linkplain List#iterator() iterator} over the * specified set are the same as the elements returned by an * iterator over this set. More formally, the two iterators are * considered to return the same elements if they return the same * number of elements and for every element {@code e1} returned by * the iterator over the specified set, there is an element * {@code e2} returned by the iterator over this set such that * {@code (e1==null ? e2==null : e1.equals(e2))}. * * @param o object to be compared for equality with this set * @return {@code true} if the specified object is equal to this set */ public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof Set)) return false; Set<?> set = (Set<?>)(o); Iterator<?> it = set.iterator(); // Uses O(n^2) algorithm that is only appropriate // for small sets, which CopyOnWriteArraySets should be. // Use a single snapshot of underlying array Object[] elements = al.getArray(); int len = elements.length; // Mark matched elements to avoid re-checking boolean[] matched = new boolean[len]; int k = 0; outer: while (it.hasNext()) { if (++k > len) return false; Object x = it.next(); for (int i = 0; i < len; ++i) { if (!matched[i] && eq(x, elements[i])) { matched[i] = true; continue outer; } } return false; } return k == len; } /** * Test for equality, coping with nulls. */ private static boolean eq(Object o1, Object o2) { return (o1 == null ? o2 == null : o1.equals(o2)); } }