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/licenses/publicdomain
 */

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&lt;Handler&gt; handlers
 *       = new CopyOnWriteArraySet&lt;Handler&gt;();
 *    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>
     *     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;
    }

    /**
     * Test for equality, coping with nulls.
     */
    private static boolean eq(Object o1, Object o2) {
        return (o1 == null ? o2 == null : o1.equals(o2));
    }
}
Revision:	1.35
Committed:	Sun May 20 08:15:53 2007 UTC (17 years ago) by jsr166
Branch:	MAIN
Changes since 1.34:	+2 -2 lines
Log Message:	legal notice
#	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/licenses/publicdomain
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>
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	* }
50	* </pre>
51	*
52	* <p>This class is a member of the
53	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
54	* Java Collections Framework</a>.
55	*
56	* @see CopyOnWriteArrayList
57	* @since 1.5
58	* @author Doug Lea
59	* @param <E> the type of elements held in this collection
60	*/
61	public class CopyOnWriteArraySet<E> extends AbstractSet<E>
62	implements java.io.Serializable {
63	private static final long serialVersionUID = 5457747651344034263L;
64
65	private final CopyOnWriteArrayList<E> al;
66
67	/**
68	* Creates an empty set.
69	*/
70	public CopyOnWriteArraySet() {
71	al = new CopyOnWriteArrayList<E>();
72	}
73
74	/**
75	* Creates a set containing all of the elements of the specified
76	* collection.
77	*
78	* @param c the collection of elements to initially contain
79	* @throws NullPointerException if the specified collection is null
80	*/
81	public CopyOnWriteArraySet(Collection<? extends E> c) {
82	al = new CopyOnWriteArrayList<E>();
83	al.addAllAbsent(c);
84	}
85
86	/**
87	* Returns the number of elements in this set.
88	*
89	* @return the number of elements in this set
90	*/
91	public int size() {
92	return al.size();
93	}
94
95	/**
96	* Returns <tt>true</tt> if this set contains no elements.
97	*
98	* @return <tt>true</tt> if this set contains no elements
99	*/
100	public boolean isEmpty() {
101	return al.isEmpty();
102	}
103
104	/**
105	* Returns <tt>true</tt> if this set contains the specified element.
106	* More formally, returns <tt>true</tt> if and only if this set
107	* contains an element <tt>e</tt> such that
108	* <tt>(o==null ? e==null : o.equals(e))</tt>.
109	*
110	* @param o element whose presence in this set is to be tested
111	* @return <tt>true</tt> if this set contains the specified element
112	*/
113	public boolean contains(Object o) {
114	return al.contains(o);
115	}
116
117	/**
118	* Returns an array containing all of the elements in this set.
119	* If this set makes any guarantees as to what order its elements
120	* are returned by its iterator, this method must return the
121	* elements in the same order.
122	*
123	* <p>The returned array will be "safe" in that no references to it
124	* are maintained by this set. (In other words, this method must
125	* allocate a new array even if this set is backed by an array).
126	* The caller is thus free to modify the returned array.
127	*
128	* <p>This method acts as bridge between array-based and collection-based
129	* APIs.
130	*
131	* @return an array containing all the elements in this set
132	*/
133	public Object[] toArray() {
134	return al.toArray();
135	}
136
137	/**
138	* Returns an array containing all of the elements in this set; the
139	* runtime type of the returned array is that of the specified array.
140	* If the set fits in the specified array, it is returned therein.
141	* Otherwise, a new array is allocated with the runtime type of the
142	* specified array and the size of this set.
143	*
144	* <p>If this set fits in the specified array with room to spare
145	* (i.e., the array has more elements than this set), the element in
146	* the array immediately following the end of the set is set to
147	* <tt>null</tt>. (This is useful in determining the length of this
148	* set <i>only</i> if the caller knows that this set does not contain
149	* any null elements.)
150	*
151	* <p>If this set makes any guarantees as to what order its elements
152	* are returned by its iterator, this method must return the elements
153	* in the same order.
154	*
155	* <p>Like the {@link #toArray()} method, this method acts as bridge between
156	* array-based and collection-based APIs. Further, this method allows
157	* precise control over the runtime type of the output array, and may,
158	* under certain circumstances, be used to save allocation costs.
159	*
160	* <p>Suppose <tt>x</tt> is a set known to contain only strings.
161	* The following code can be used to dump the set into a newly allocated
162	* array of <tt>String</tt>:
163	*
164	* <pre>
165	* String[] y = x.toArray(new String[0]);</pre>
166	*
167	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
168	* <tt>toArray()</tt>.
169	*
170	* @param a the array into which the elements of this set are to be
171	* stored, if it is big enough; otherwise, a new array of the same
172	* runtime type is allocated for this purpose.
173	* @return an array containing all the elements in this set
174	* @throws ArrayStoreException if the runtime type of the specified array
175	* is not a supertype of the runtime type of every element in this
176	* set
177	* @throws NullPointerException if the specified array is null
178	*/
179	public <T> T[] toArray(T[] a) {
180	return al.toArray(a);
181	}
182
183	/**
184	* Removes all of the elements from this set.
185	* The set will be empty after this call returns.
186	*/
187	public void clear() {
188	al.clear();
189	}
190
191	/**
192	* Removes the specified element from this set if it is present.
193	* More formally, removes an element <tt>e</tt> such that
194	* <tt>(o==null ? e==null : o.equals(e))</tt>,
195	* if this set contains such an element. Returns <tt>true</tt> if
196	* this set contained the element (or equivalently, if this set
197	* changed as a result of the call). (This set will not contain the
198	* element once the call returns.)
199	*
200	* @param o object to be removed from this set, if present
201	* @return <tt>true</tt> if this set contained the specified element
202	*/
203	public boolean remove(Object o) {
204	return al.remove(o);
205	}
206
207	/**
208	* Adds the specified element to this set if it is not already present.
209	* More formally, adds the specified element <tt>e</tt> to this set if
210	* the set contains no element <tt>e2</tt> such that
211	* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
212	* If this set already contains the element, the call leaves the set
213	* unchanged and returns <tt>false</tt>.
214	*
215	* @param e element to be added to this set
216	* @return <tt>true</tt> if this set did not already contain the specified
217	* element
218	*/
219	public boolean add(E e) {
220	return al.addIfAbsent(e);
221	}
222
223	/**
224	* Returns <tt>true</tt> if this set contains all of the elements of the
225	* specified collection. If the specified collection is also a set, this
226	* method returns <tt>true</tt> if it is a <i>subset</i> of this set.
227	*
228	* @param c collection to be checked for containment in this set
229	* @return <tt>true</tt> if this set contains all of the elements of the
230	* specified collection
231	* @throws NullPointerException if the specified collection is null
232	* @see #contains(Object)
233	*/
234	public boolean containsAll(Collection<?> c) {
235	return al.containsAll(c);
236	}
237
238	/**
239	* Adds all of the elements in the specified collection to this set if
240	* they're not already present. If the specified collection is also a
241	* set, the <tt>addAll</tt> operation effectively modifies this set so
242	* that its value is the <i>union</i> of the two sets. The behavior of
243	* this operation is undefined if the specified collection is modified
244	* while the operation is in progress.
245	*
246	* @param c collection containing elements to be added to this set
247	* @return <tt>true</tt> if this set changed as a result of the call
248	* @throws NullPointerException if the specified collection is null
249	* @see #add(Object)
250	*/
251	public boolean addAll(Collection<? extends E> c) {
252	return al.addAllAbsent(c) > 0;
253	}
254
255	/**
256	* Removes from this set all of its elements that are contained in the
257	* specified collection. If the specified collection is also a set,
258	* this operation effectively modifies this set so that its value is the
259	* <i>asymmetric set difference</i> of the two sets.
260	*
261	* @param c collection containing elements to be removed from this set
262	* @return <tt>true</tt> if this set changed as a result of the call
263	* @throws ClassCastException if the class of an element of this set
264	* is incompatible with the specified collection (optional)
265	* @throws NullPointerException if this set contains a null element and the
266	* specified collection does not permit null elements (optional),
267	* or if the specified collection is null
268	* @see #remove(Object)
269	*/
270	public boolean removeAll(Collection<?> c) {
271	return al.removeAll(c);
272	}
273
274	/**
275	* Retains only the elements in this set that are contained in the
276	* specified collection. In other words, removes from this set all of
277	* its elements that are not contained in the specified collection. If
278	* the specified collection is also a set, this operation effectively
279	* modifies this set so that its value is the <i>intersection</i> of the
280	* two sets.
281	*
282	* @param c collection containing elements to be retained in this set
283	* @return <tt>true</tt> if this set changed as a result of the call
284	* @throws ClassCastException if the class of an element of this set
285	* is incompatible with the specified collection (optional)
286	* @throws NullPointerException if this set contains a null element and the
287	* specified collection does not permit null elements (optional),
288	* or if the specified collection is null
289	* @see #remove(Object)
290	*/
291	public boolean retainAll(Collection<?> c) {
292	return al.retainAll(c);
293	}
294
295	/**
296	* Returns an iterator over the elements contained in this set
297	* in the order in which these elements were added.
298	*
299	* <p>The returned iterator provides a snapshot of the state of the set
300	* when the iterator was constructed. No synchronization is needed while
301	* traversing the iterator. The iterator does <em>NOT</em> support the
302	* <tt>remove</tt> method.
303	*
304	* @return an iterator over the elements in this set
305	*/
306	public Iterator<E> iterator() {
307	return al.iterator();
308	}
309
310	/**
311	* Compares the specified object with this set for equality.
312	* Returns {@code true} if the specified object is the same object
313	* as this object, or if it is also a {@link Set} and the elements
314	* returned by an {@linkplain List#iterator() iterator} over the
315	* specified set are the same as the elements returned by an
316	* iterator over this set. More formally, the two iterators are
317	* considered to return the same elements if they return the same
318	* number of elements and for every element {@code e1} returned by
319	* the iterator over the specified set, there is an element
320	* {@code e2} returned by the iterator over this set such that
321	* {@code (e1==null ? e2==null : e1.equals(e2))}.
322	*
323	* @param o object to be compared for equality with this set
324	* @return {@code true} if the specified object is equal to this set
325	*/
326	public boolean equals(Object o) {
327	if (o == this)
328	return true;
329	if (!(o instanceof Set))
330	return false;
331	Set<?> set = (Set<?>)(o);
332	Iterator<?> it = set.iterator();
333
334	// Uses O(n^2) algorithm that is only appropriate
335	// for small sets, which CopyOnWriteArraySets should be.
336
337	// Use a single snapshot of underlying array
338	Object[] elements = al.getArray();
339	int len = elements.length;
340	// Mark matched elements to avoid re-checking
341	boolean[] matched = new boolean[len];
342	int k = 0;
343	outer: while (it.hasNext()) {
344	if (++k > len)
345	return false;
346	Object x = it.next();
347	for (int i = 0; i < len; ++i) {
348	if (!matched[i] && eq(x, elements[i])) {
349	matched[i] = true;
350	continue outer;
351	}
352	}
353	return false;
354	}
355	return k == len;
356	}
357
358	/**
359	* Test for equality, coping with nulls.
360	*/
361	private static boolean eq(Object o1, Object o2) {
362	return (o1 == null ? o2 == null : o1.equals(o2));
363	}
364	}