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>
     *     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:	1.38
Committed:	Tue Mar 15 19:47:03 2011 UTC (13 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	release-1_7_0
Changes since 1.37:	+1 -1 lines
Log Message:	Update Creative Commons license URL in legal notices
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	jsr166	1.35	* Expert Group and released to the public domain, as explained at
4	jsr166	1.38	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8			import java.util.*;
9
10			/**
11	jsr166	1.26	* 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	tim	1.1	* <ul>
14	dholmes	1.7	* <li>It is best suited for applications in which set sizes generally
15	tim	1.1	* stay small, read-only operations
16			* vastly outnumber mutative operations, and you need
17			* to prevent interference among threads during traversal.
18	dl	1.16	* <li>It is thread-safe.
19	jsr166	1.26	* <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	dl	1.18	* <li>Traversal via iterators is fast and cannot encounter
24	tim	1.1	* interference from other threads. Iterators rely on
25			* unchanging snapshots of the array at the time the iterators were
26	jsr166	1.26	* constructed.
27	tim	1.1	* </ul>
28	dl	1.18	*
29			* <p> <b>Sample Usage.</b> The following code sketch uses a
30	jsr166	1.21	* copy-on-write set to maintain a set of Handler objects that
31			* perform some action upon state updates.
32	dl	1.18	*
33	jsr166	1.37	* <pre> {@code
34	tim	1.6	* class Handler { void handle(); ... }
35	tim	1.1	*
36			* class X {
37	jsr166	1.37	* private final CopyOnWriteArraySet<Handler> handlers
38			* = new CopyOnWriteArraySet<Handler>();
39			* public void addHandler(Handler h) { handlers.add(h); }
40	tim	1.1	*
41	jsr166	1.37	* private long internalState;
42			* private synchronized void changeState() { internalState = ...; }
43	tim	1.1	*
44	jsr166	1.37	* public void update() {
45			* changeState();
46			* for (Handler handler : handlers)
47			* handler.handle();
48			* }
49			* }}</pre>
50	dl	1.14	*
51			* <p>This class is a member of the
52	jsr166	1.34	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
53	dl	1.14	* Java Collections Framework</a>.
54			*
55	dl	1.16	* @see CopyOnWriteArrayList
56	dl	1.4	* @since 1.5
57			* @author Doug Lea
58	dl	1.11	* @param <E> the type of elements held in this collection
59	dl	1.4	*/
60	tim	1.1	public class CopyOnWriteArraySet<E> extends AbstractSet<E>
61	dl	1.18	implements java.io.Serializable {
62	dl	1.8	private static final long serialVersionUID = 5457747651344034263L;
63	tim	1.1
64			private final CopyOnWriteArrayList<E> al;
65
66			/**
67	dl	1.15	* Creates an empty set.
68	tim	1.1	*/
69			public CopyOnWriteArraySet() {
70			al = new CopyOnWriteArrayList<E>();
71			}
72
73			/**
74	dl	1.15	* Creates a set containing all of the elements of the specified
75	jsr166	1.23	* collection.
76	jsr166	1.25	*
77	jsr166	1.23	* @param c the collection of elements to initially contain
78			* @throws NullPointerException if the specified collection is null
79	tim	1.1	*/
80	tim	1.12	public CopyOnWriteArraySet(Collection<? extends E> c) {
81	tim	1.1	al = new CopyOnWriteArrayList<E>();
82			al.addAllAbsent(c);
83			}
84
85	jsr166	1.26	/**
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	jsr166	1.36	return al.size();
92	jsr166	1.26	}
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	jsr166	1.36	return al.isEmpty();
101	jsr166	1.26	}
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	jsr166	1.28	* contains an element <tt>e</tt> such that
107	jsr166	1.26	* <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	jsr166	1.36	return al.contains(o);
114	jsr166	1.26	}
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	jsr166	1.36	return al.toArray();
134	jsr166	1.26	}
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>
164			* String[] y = x.toArray(new String[0]);</pre>
165			*
166			* Note that <tt>toArray(new Object[0])</tt> is identical in function to
167			* <tt>toArray()</tt>.
168			*
169			* @param a the array into which the elements of this set are to be
170			* stored, if it is big enough; otherwise, a new array of the same
171			* runtime type is allocated for this purpose.
172			* @return an array containing all the elements in this set
173			* @throws ArrayStoreException if the runtime type of the specified array
174			* is not a supertype of the runtime type of every element in this
175			* set
176			* @throws NullPointerException if the specified array is null
177			*/
178			public <T> T[] toArray(T[] a) {
179	jsr166	1.36	return al.toArray(a);
180	jsr166	1.26	}
181
182			/**
183			* Removes all of the elements from this set.
184	jsr166	1.27	* The set will be empty after this call returns.
185	jsr166	1.26	*/
186			public void clear() {
187			al.clear();
188			}
189
190			/**
191			* Removes the specified element from this set if it is present.
192			* More formally, removes an element <tt>e</tt> such that
193	jsr166	1.28	* <tt>(o==null ? e==null : o.equals(e))</tt>,
194			* if this set contains such an element. Returns <tt>true</tt> if
195			* this set contained the element (or equivalently, if this set
196			* changed as a result of the call). (This set will not contain the
197	jsr166	1.26	* element once the call returns.)
198			*
199			* @param o object to be removed from this set, if present
200			* @return <tt>true</tt> if this set contained the specified element
201			*/
202			public boolean remove(Object o) {
203	jsr166	1.36	return al.remove(o);
204	jsr166	1.26	}
205
206			/**
207			* Adds the specified element to this set if it is not already present.
208	jsr166	1.28	* More formally, adds the specified element <tt>e</tt> to this set if
209			* the set contains no element <tt>e2</tt> such that
210			* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
211			* If this set already contains the element, the call leaves the set
212			* unchanged and returns <tt>false</tt>.
213	jsr166	1.26	*
214			* @param e element to be added to this set
215			* @return <tt>true</tt> if this set did not already contain the specified
216			* element
217			*/
218			public boolean add(E e) {
219	jsr166	1.36	return al.addIfAbsent(e);
220	jsr166	1.26	}
221	tim	1.1
222	jsr166	1.26	/**
223			* Returns <tt>true</tt> if this set contains all of the elements of the
224			* specified collection. If the specified collection is also a set, this
225			* method returns <tt>true</tt> if it is a <i>subset</i> of this set.
226			*
227			* @param c collection to be checked for containment in this set
228			* @return <tt>true</tt> if this set contains all of the elements of the
229	jsr166	1.36	* specified collection
230	jsr166	1.26	* @throws NullPointerException if the specified collection is null
231			* @see #contains(Object)
232			*/
233			public boolean containsAll(Collection<?> c) {
234	jsr166	1.36	return al.containsAll(c);
235	jsr166	1.26	}
236
237			/**
238			* Adds all of the elements in the specified collection to this set if
239			* they're not already present. If the specified collection is also a
240			* set, the <tt>addAll</tt> operation effectively modifies this set so
241			* that its value is the <i>union</i> of the two sets. The behavior of
242	jsr166	1.28	* this operation is undefined if the specified collection is modified
243	jsr166	1.26	* while the operation is in progress.
244			*
245			* @param c collection containing elements to be added to this set
246			* @return <tt>true</tt> if this set changed as a result of the call
247			* @throws NullPointerException if the specified collection is null
248			* @see #add(Object)
249			*/
250			public boolean addAll(Collection<? extends E> c) {
251	jsr166	1.36	return al.addAllAbsent(c) > 0;
252	jsr166	1.26	}
253
254			/**
255			* Removes from this set all of its elements that are contained in the
256			* specified collection. If the specified collection is also a set,
257			* this operation effectively modifies this set so that its value is the
258			* <i>asymmetric set difference</i> of the two sets.
259			*
260			* @param c collection containing elements to be removed from this set
261			* @return <tt>true</tt> if this set changed as a result of the call
262			* @throws ClassCastException if the class of an element of this set
263			* is incompatible with the specified collection (optional)
264			* @throws NullPointerException if this set contains a null element and the
265			* specified collection does not permit null elements (optional),
266			* or if the specified collection is null
267			* @see #remove(Object)
268			*/
269			public boolean removeAll(Collection<?> c) {
270	jsr166	1.36	return al.removeAll(c);
271	jsr166	1.26	}
272
273			/**
274			* Retains only the elements in this set that are contained in the
275			* specified collection. In other words, removes from this set all of
276			* its elements that are not contained in the specified collection. If
277			* the specified collection is also a set, this operation effectively
278			* modifies this set so that its value is the <i>intersection</i> of the
279			* two sets.
280			*
281			* @param c collection containing elements to be retained in this set
282			* @return <tt>true</tt> if this set changed as a result of the call
283			* @throws ClassCastException if the class of an element of this set
284			* is incompatible with the specified collection (optional)
285			* @throws NullPointerException if this set contains a null element and the
286			* specified collection does not permit null elements (optional),
287			* or if the specified collection is null
288			* @see #remove(Object)
289			*/
290			public boolean retainAll(Collection<?> c) {
291	jsr166	1.36	return al.retainAll(c);
292	jsr166	1.26	}
293	tim	1.1
294	dl	1.20	/**
295	jsr166	1.24	* Returns an iterator over the elements contained in this set
296	dl	1.20	* in the order in which these elements were added.
297	jsr166	1.26	*
298			* <p>The returned iterator provides a snapshot of the state of the set
299			* when the iterator was constructed. No synchronization is needed while
300			* traversing the iterator. The iterator does <em>NOT</em> support the
301			* <tt>remove</tt> method.
302			*
303			* @return an iterator over the elements in this set
304	dl	1.20	*/
305	jsr166	1.26	public Iterator<E> iterator() {
306	jsr166	1.36	return al.iterator();
307	jsr166	1.26	}
308	dl	1.20
309	dl	1.30	/**
310			* Compares the specified object with this set for equality.
311	jsr166	1.33	* Returns {@code true} if the specified object is the same object
312			* as this object, or if it is also a {@link Set} and the elements
313			* returned by an {@linkplain List#iterator() iterator} over the
314			* specified set are the same as the elements returned by an
315			* iterator over this set. More formally, the two iterators are
316			* considered to return the same elements if they return the same
317			* number of elements and for every element {@code e1} returned by
318			* the iterator over the specified set, there is an element
319			* {@code e2} returned by the iterator over this set such that
320			* {@code (e1==null ? e2==null : e1.equals(e2))}.
321	dl	1.30	*
322			* @param o object to be compared for equality with this set
323	jsr166	1.33	* @return {@code true} if the specified object is equal to this set
324	dl	1.30	*/
325			public boolean equals(Object o) {
326			if (o == this)
327			return true;
328			if (!(o instanceof Set))
329			return false;
330			Set<?> set = (Set<?>)(o);
331	jsr166	1.36	Iterator<?> it = set.iterator();
332	dl	1.30
333			// Uses O(n^2) algorithm that is only appropriate
334			// for small sets, which CopyOnWriteArraySets should be.
335
336			// Use a single snapshot of underlying array
337	jsr166	1.36	Object[] elements = al.getArray();
338			int len = elements.length;
339	dl	1.30	// Mark matched elements to avoid re-checking
340			boolean[] matched = new boolean[len];
341			int k = 0;
342	jsr166	1.33	outer: while (it.hasNext()) {
343	dl	1.30	if (++k > len)
344			return false;
345	jsr166	1.33	Object x = it.next();
346	dl	1.30	for (int i = 0; i < len; ++i) {
347			if (!matched[i] && eq(x, elements[i])) {
348			matched[i] = true;
349	jsr166	1.36	continue outer;
350	dl	1.30	}
351			}
352	jsr166	1.36	return false;
353	dl	1.30	}
354			return k == len;
355			}
356	jsr166	1.32
357	dl	1.30	/**
358			* Test for equality, coping with nulls.
359			*/
360			private static boolean eq(Object o1, Object o2) {
361			return (o1 == null ? o2 == null : o1.equals(o2));
362			}
363	tim	1.1	}