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/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.*;

// BEGIN android-note
// removed link to collections framework docs
// END android-note

/**
 * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
 * for all of its operations.  Thus, it shares the same basic properties:
 * 
    *
  • 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. *
  • It is thread-safe. *
  • Mutative operations ({@code add}, {@code set}, {@code remove}, etc.) * are expensive since they usually entail copying the entire underlying * array. *
  • Iterators do not support the mutative {@code remove} operation. *
  • 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. *
* *

Sample Usage. The following code sketch uses a * copy-on-write set to maintain a set of Handler objects that * perform some action upon state updates. * *

 {@code
 * class Handler { void handle(); ... }
 *
 * class X {
 *   private final CopyOnWriteArraySet handlers
 *     = new CopyOnWriteArraySet();
 *   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();
 *   }
 * }}
* * @see CopyOnWriteArrayList * @since 1.5 * @author Doug Lea * @param the type of elements held in this collection */ public class CopyOnWriteArraySet extends AbstractSet implements java.io.Serializable { private final CopyOnWriteArrayList al; /** * Creates an empty set. */ public CopyOnWriteArraySet() { al = new CopyOnWriteArrayList(); } /** * 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 c) { al = new CopyOnWriteArrayList(); 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 {@code true} if this set contains no elements. * * @return {@code true} if this set contains no elements */ public boolean isEmpty() { return al.isEmpty(); } /** * Returns {@code true} if this set contains the specified element. * More formally, returns {@code true} if and only if this set * contains an element {@code e} such that * (o==null ? e==null : o.equals(e)). * * @param o element whose presence in this set is to be tested * @return {@code true} 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. * *

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 */ 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. * *

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 * {@code 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 {@code 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 {@code String}: * *

 {@code String[] y = x.toArray(new String[0]);}
* * Note that {@code toArray(new Object[0])} is identical in function to * {@code 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 */ public 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 {@code e} such that * (o==null ? e==null : o.equals(e)), * if this set contains such an element. Returns {@code 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 {@code true} 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 {@code e} to this set if * the set contains no element {@code 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 {@code false}. * * @param e element to be added to this set * @return {@code true} if this set did not already contain the specified * element */ public boolean add(E e) { return al.addIfAbsent(e); } /** * Returns {@code true} if this set contains all of the elements of the * specified collection. If the specified collection is also a set, this * method returns {@code true} if it is a subset of this set. * * @param c collection to be checked for containment in this set * @return {@code true} 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 {@code 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 {@code true} 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 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 * asymmetric set difference of the two sets. * * @param c collection containing elements to be removed from this set * @return {@code true} 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 intersection of the * two sets. * * @param c collection containing elements to be retained in this set * @return {@code true} 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. * *

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 NOT support the * {@code remove} method. * * @return an iterator over the elements in this set */ public Iterator 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; } /** * Tests for equality, coping with nulls. */ private static boolean eq(Object o1, Object o2) { return (o1 == null) ? o2 == null : o1.equals(o2); } }





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