util/concurrent/CopyOnWriteArraySet.java

/*
 * 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.*;

/**
 * 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> {@code
 * 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">
 * 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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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> {@code 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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;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;
    }

    /**
     * Tests for equality, coping with nulls.
     */
    private static boolean eq(Object o1, Object o2) {
        return (o1 == null) ? o2 == null : o1.equals(o2);
    }
}
Revision:	1.41
Committed:	Tue Feb 21 01:54:03 2012 UTC (12 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.40:	+1 -1 lines
Log Message:	use third person in javadoc first sentence
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import java.util.*;
9
10	/**
11	* A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
12	* for all of its operations. Thus, it shares the same basic properties:
13	* <ul>
14	* <li>It is best suited for applications in which set sizes generally
15	* stay small, read-only operations
16	* vastly outnumber mutative operations, and you need
17	* to prevent interference among threads during traversal.
18	* <li>It is thread-safe.
19	* <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.)
20	* are expensive since they usually entail copying the entire underlying
21	* array.
22	* <li>Iterators do not support the mutative <tt>remove</tt> operation.
23	* <li>Traversal via iterators is fast and cannot encounter
24	* interference from other threads. Iterators rely on
25	* unchanging snapshots of the array at the time the iterators were
26	* constructed.
27	* </ul>
28	*
29	* <p> <b>Sample Usage.</b> The following code sketch uses a
30	* copy-on-write set to maintain a set of Handler objects that
31	* perform some action upon state updates.
32	*
33	* <pre> {@code
34	* class Handler { void handle(); ... }
35	*
36	* class X {
37	* private final CopyOnWriteArraySet<Handler> handlers
38	* = new CopyOnWriteArraySet<Handler>();
39	* public void addHandler(Handler h) { handlers.add(h); }
40	*
41	* private long internalState;
42	* private synchronized void changeState() { internalState = ...; }
43	*
44	* public void update() {
45	* changeState();
46	* for (Handler handler : handlers)
47	* handler.handle();
48	* }
49	* }}</pre>
50	*
51	* <p>This class is a member of the
52	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
53	* Java Collections Framework</a>.
54	*
55	* @see CopyOnWriteArrayList
56	* @since 1.5
57	* @author Doug Lea
58	* @param <E> the type of elements held in this collection
59	*/
60	public class CopyOnWriteArraySet<E> extends AbstractSet<E>
61	implements java.io.Serializable {
62	private static final long serialVersionUID = 5457747651344034263L;
63
64	private final CopyOnWriteArrayList<E> al;
65
66	/**
67	* Creates an empty set.
68	*/
69	public CopyOnWriteArraySet() {
70	al = new CopyOnWriteArrayList<E>();
71	}
72
73	/**
74	* Creates a set containing all of the elements of the specified
75	* collection.
76	*
77	* @param c the collection of elements to initially contain
78	* @throws NullPointerException if the specified collection is null
79	*/
80	public CopyOnWriteArraySet(Collection<? extends E> c) {
81	al = new CopyOnWriteArrayList<E>();
82	al.addAllAbsent(c);
83	}
84
85	/**
86	* Returns the number of elements in this set.
87	*
88	* @return the number of elements in this set
89	*/
90	public int size() {
91	return al.size();
92	}
93
94	/**
95	* Returns <tt>true</tt> if this set contains no elements.
96	*
97	* @return <tt>true</tt> if this set contains no elements
98	*/
99	public boolean isEmpty() {
100	return al.isEmpty();
101	}
102
103	/**
104	* Returns <tt>true</tt> if this set contains the specified element.
105	* More formally, returns <tt>true</tt> if and only if this set
106	* contains an element <tt>e</tt> such that
107	* <tt>(o==null ? e==null : o.equals(e))</tt>.
108	*
109	* @param o element whose presence in this set is to be tested
110	* @return <tt>true</tt> if this set contains the specified element
111	*/
112	public boolean contains(Object o) {
113	return al.contains(o);
114	}
115
116	/**
117	* Returns an array containing all of the elements in this set.
118	* If this set makes any guarantees as to what order its elements
119	* are returned by its iterator, this method must return the
120	* elements in the same order.
121	*
122	* <p>The returned array will be "safe" in that no references to it
123	* are maintained by this set. (In other words, this method must
124	* allocate a new array even if this set is backed by an array).
125	* The caller is thus free to modify the returned array.
126	*
127	* <p>This method acts as bridge between array-based and collection-based
128	* APIs.
129	*
130	* @return an array containing all the elements in this set
131	*/
132	public Object[] toArray() {
133	return al.toArray();
134	}
135
136	/**
137	* Returns an array containing all of the elements in this set; the
138	* runtime type of the returned array is that of the specified array.
139	* If the set fits in the specified array, it is returned therein.
140	* Otherwise, a new array is allocated with the runtime type of the
141	* specified array and the size of this set.
142	*
143	* <p>If this set fits in the specified array with room to spare
144	* (i.e., the array has more elements than this set), the element in
145	* the array immediately following the end of the set is set to
146	* <tt>null</tt>. (This is useful in determining the length of this
147	* set <i>only</i> if the caller knows that this set does not contain
148	* any null elements.)
149	*
150	* <p>If this set makes any guarantees as to what order its elements
151	* are returned by its iterator, this method must return the elements
152	* in the same order.
153	*
154	* <p>Like the {@link #toArray()} method, this method acts as bridge between
155	* array-based and collection-based APIs. Further, this method allows
156	* precise control over the runtime type of the output array, and may,
157	* under certain circumstances, be used to save allocation costs.
158	*
159	* <p>Suppose <tt>x</tt> is a set known to contain only strings.
160	* The following code can be used to dump the set into a newly allocated
161	* array of <tt>String</tt>:
162	*
163	* <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
164	*
165	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
166	* <tt>toArray()</tt>.
167	*
168	* @param a the array into which the elements of this set are to be
169	* stored, if it is big enough; otherwise, a new array of the same
170	* runtime type is allocated for this purpose.
171	* @return an array containing all the elements in this set
172	* @throws ArrayStoreException if the runtime type of the specified array
173	* is not a supertype of the runtime type of every element in this
174	* set
175	* @throws NullPointerException if the specified array is null
176	*/
177	public <T> T[] toArray(T[] a) {
178	return al.toArray(a);
179	}
180
181	/**
182	* Removes all of the elements from this set.
183	* The set will be empty after this call returns.
184	*/
185	public void clear() {
186	al.clear();
187	}
188
189	/**
190	* Removes the specified element from this set if it is present.
191	* More formally, removes an element <tt>e</tt> such that
192	* <tt>(o==null ? e==null : o.equals(e))</tt>,
193	* if this set contains such an element. Returns <tt>true</tt> if
194	* this set contained the element (or equivalently, if this set
195	* changed as a result of the call). (This set will not contain the
196	* element once the call returns.)
197	*
198	* @param o object to be removed from this set, if present
199	* @return <tt>true</tt> if this set contained the specified element
200	*/
201	public boolean remove(Object o) {
202	return al.remove(o);
203	}
204
205	/**
206	* Adds the specified element to this set if it is not already present.
207	* More formally, adds the specified element <tt>e</tt> to this set if
208	* the set contains no element <tt>e2</tt> such that
209	* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
210	* If this set already contains the element, the call leaves the set
211	* unchanged and returns <tt>false</tt>.
212	*
213	* @param e element to be added to this set
214	* @return <tt>true</tt> if this set did not already contain the specified
215	* element
216	*/
217	public boolean add(E e) {
218	return al.addIfAbsent(e);
219	}
220
221	/**
222	* Returns <tt>true</tt> if this set contains all of the elements of the
223	* specified collection. If the specified collection is also a set, this
224	* method returns <tt>true</tt> if it is a <i>subset</i> of this set.
225	*
226	* @param c collection to be checked for containment in this set
227	* @return <tt>true</tt> if this set contains all of the elements of the
228	* specified collection
229	* @throws NullPointerException if the specified collection is null
230	* @see #contains(Object)
231	*/
232	public boolean containsAll(Collection<?> c) {
233	return al.containsAll(c);
234	}
235
236	/**
237	* Adds all of the elements in the specified collection to this set if
238	* they're not already present. If the specified collection is also a
239	* set, the <tt>addAll</tt> operation effectively modifies this set so
240	* that its value is the <i>union</i> of the two sets. The behavior of
241	* this operation is undefined if the specified collection is modified
242	* while the operation is in progress.
243	*
244	* @param c collection containing elements to be added to this set
245	* @return <tt>true</tt> if this set changed as a result of the call
246	* @throws NullPointerException if the specified collection is null
247	* @see #add(Object)
248	*/
249	public boolean addAll(Collection<? extends E> c) {
250	return al.addAllAbsent(c) > 0;
251	}
252
253	/**
254	* Removes from this set all of its elements that are contained in the
255	* specified collection. If the specified collection is also a set,
256	* this operation effectively modifies this set so that its value is the
257	* <i>asymmetric set difference</i> of the two sets.
258	*
259	* @param c collection containing elements to be removed from this set
260	* @return <tt>true</tt> if this set changed as a result of the call
261	* @throws ClassCastException if the class of an element of this set
262	* is incompatible with the specified collection (optional)
263	* @throws NullPointerException if this set contains a null element and the
264	* specified collection does not permit null elements (optional),
265	* or if the specified collection is null
266	* @see #remove(Object)
267	*/
268	public boolean removeAll(Collection<?> c) {
269	return al.removeAll(c);
270	}
271
272	/**
273	* Retains only the elements in this set that are contained in the
274	* specified collection. In other words, removes from this set all of
275	* its elements that are not contained in the specified collection. If
276	* the specified collection is also a set, this operation effectively
277	* modifies this set so that its value is the <i>intersection</i> of the
278	* two sets.
279	*
280	* @param c collection containing elements to be retained in this set
281	* @return <tt>true</tt> if this set changed as a result of the call
282	* @throws ClassCastException if the class of an element of this set
283	* is incompatible with the specified collection (optional)
284	* @throws NullPointerException if this set contains a null element and the
285	* specified collection does not permit null elements (optional),
286	* or if the specified collection is null
287	* @see #remove(Object)
288	*/
289	public boolean retainAll(Collection<?> c) {
290	return al.retainAll(c);
291	}
292
293	/**
294	* Returns an iterator over the elements contained in this set
295	* in the order in which these elements were added.
296	*
297	* <p>The returned iterator provides a snapshot of the state of the set
298	* when the iterator was constructed. No synchronization is needed while
299	* traversing the iterator. The iterator does <em>NOT</em> support the
300	* <tt>remove</tt> method.
301	*
302	* @return an iterator over the elements in this set
303	*/
304	public Iterator<E> iterator() {
305	return al.iterator();
306	}
307
308	/**
309	* Compares the specified object with this set for equality.
310	* Returns {@code true} if the specified object is the same object
311	* as this object, or if it is also a {@link Set} and the elements
312	* returned by an {@linkplain List#iterator() iterator} over the
313	* specified set are the same as the elements returned by an
314	* iterator over this set. More formally, the two iterators are
315	* considered to return the same elements if they return the same
316	* number of elements and for every element {@code e1} returned by
317	* the iterator over the specified set, there is an element
318	* {@code e2} returned by the iterator over this set such that
319	* {@code (e1==null ? e2==null : e1.equals(e2))}.
320	*
321	* @param o object to be compared for equality with this set
322	* @return {@code true} if the specified object is equal to this set
323	*/
324	public boolean equals(Object o) {
325	if (o == this)
326	return true;
327	if (!(o instanceof Set))
328	return false;
329	Set<?> set = (Set<?>)(o);
330	Iterator<?> it = set.iterator();
331
332	// Uses O(n^2) algorithm that is only appropriate
333	// for small sets, which CopyOnWriteArraySets should be.
334
335	// Use a single snapshot of underlying array
336	Object[] elements = al.getArray();
337	int len = elements.length;
338	// Mark matched elements to avoid re-checking
339	boolean[] matched = new boolean[len];
340	int k = 0;
341	outer: while (it.hasNext()) {
342	if (++k > len)
343	return false;
344	Object x = it.next();
345	for (int i = 0; i < len; ++i) {
346	if (!matched[i] && eq(x, elements[i])) {
347	matched[i] = true;
348	continue outer;
349	}
350	}
351	return false;
352	}
353	return k == len;
354	}
355
356	/**
357	* Tests for equality, coping with nulls.
358	*/
359	private static boolean eq(Object o1, Object o2) {
360	return (o1 == null) ? o2 == null : o1.equals(o2);
361	}
362	}