util/concurrent/DelayQueue.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.concurrent.locks.*;
import java.util.*;

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} of
 * <tt>Delayed</tt> elements, in which an element can only be taken
 * when its delay has expired.  The <em>head</em> of the queue is that
 * <tt>Delayed</tt> element whose delay expired furthest in the
 * past.  If no delay has expired there is no head and <tt>poll</tt>
 * will return <tt>null</tt>. Expiration occurs when an element's
 * <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
 * than or equal to zero.  Even though unexpired elements cannot be
 * removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
 * treated as normal elements. For example, the <tt>size</tt> method
 * returns the count of both expired and unexpired elements.
 * This queue does not permit null elements.
 *
 * <p>This class and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 */

public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
    implements BlockingQueue<E> {

    private transient final ReentrantLock lock = new ReentrantLock();
    private final PriorityQueue<E> q = new PriorityQueue<E>();

    /**
     * Thread designated to wait for the element at the head of
     * the queue.  This variant of the Leader-Follower pattern
     * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
     * minimize unnecessary timed waiting.  When a thread becomes
     * the leader, it waits only for the next delay to elapse, but
     * other threads await indefinitely.  The leader thread must
     * signal some other thread before returning from take() or
     * poll(...), unless some other thread becomes leader in the
     * interim.  Whenever the head of the queue is replaced with
     * an element with an earlier expiration time, the leader
     * field is invalidated by being reset to null, and some
     * waiting thread, but not necessarily the current leader, is
     * signalled.  So waiting threads must be prepared to acquire
     * and lose leadership while waiting.
     */
    private Thread leader = null;

    /**
     * Condition signalled when a newer element becomes available
     * at the head of the queue or a new thread may need to
     * become leader.
     */
    private final Condition available = lock.newCondition();

    /**
     * Creates a new <tt>DelayQueue</tt> that is initially empty.
     */
    public DelayQueue() {}

    /**
     * Creates a <tt>DelayQueue</tt> initially containing the elements of the
     * given collection of {@link Delayed} instances.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public DelayQueue(Collection<? extends E> c) {
        this.addAll(c);
    }

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element into this delay queue.
     *
     * @param e the element to add
     * @return <tt>true</tt>
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.offer(e);
            if (q.peek() == e) {
                leader = null;
                available.signal();
            }
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Inserts the specified element into this delay queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) {
        offer(e);
    }

    /**
     * Inserts the specified element into this delay queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @param timeout This parameter is ignored as the method never blocks
     * @param unit This parameter is ignored as the method never blocks
     * @return <tt>true</tt>
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit) {
        return offer(e);
    }

    /**
     * Retrieves and removes the head of this queue, or returns <tt>null</tt>
     * if this queue has no elements with an expired delay.
     *
     * @return the head of this queue, or <tt>null</tt> if this
     *         queue has no elements with an expired delay
     */
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            E first = q.peek();
            if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
                return null;
            else
                return q.poll();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * until an element with an expired delay is available on this queue.
     *
     * @return the head of this queue
     * @throws InterruptedException {@inheritDoc}
     */
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E first = q.peek();
                if (first == null)
                    available.await();
                else {
                    long delay = first.getDelay(TimeUnit.NANOSECONDS);
                    if (delay <= 0)
                        return q.poll();
                    else if (leader != null)
                        available.await();
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            available.awaitNanos(delay);
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && q.peek() != null)
                available.signal();
            lock.unlock();
        }
    }

    /**
     * Retrieves and removes the head of this queue, waiting if necessary
     * until an element with an expired delay is available on this queue,
     * or the specified wait time expires.
     *
     * @return the head of this queue, or <tt>null</tt> if the
     *         specified waiting time elapses before an element with
     *         an expired delay becomes available
     * @throws InterruptedException {@inheritDoc}
     */
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            for (;;) {
                E first = q.peek();
                if (first == null) {
                    if (nanos <= 0)
                        return null;
                    else
                        nanos = available.awaitNanos(nanos);
                } else {
                    long delay = first.getDelay(TimeUnit.NANOSECONDS);
                    if (delay <= 0)
                        return q.poll();
                    if (nanos <= 0)
                        return null;
                    if (nanos < delay || leader != null)
                        nanos = available.awaitNanos(nanos);
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            long timeLeft = available.awaitNanos(delay);
                            nanos -= delay - timeLeft;
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && q.peek() != null)
                available.signal();
            lock.unlock();
        }
    }

    /**
     * Retrieves, but does not remove, the head of this queue, or
     * returns <tt>null</tt> if this queue is empty.  Unlike
     * <tt>poll</tt>, if no expired elements are available in the queue,
     * this method returns the element that will expire next,
     * if one exists.
     *
     * @return the head of this queue, or <tt>null</tt> if this
     *         queue is empty.
     */
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.peek();
        } finally {
            lock.unlock();
        }
    }

    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.size();
        } finally {
            lock.unlock();
        }
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = 0;
            for (;;) {
                E first = q.peek();
                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
                    break;
                c.add(q.poll());
                ++n;
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = 0;
            while (n < maxElements) {
                E first = q.peek();
                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
                    break;
                c.add(q.poll());
                ++n;
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Atomically removes all of the elements from this delay queue.
     * The queue will be empty after this call returns.
     * Elements with an unexpired delay are not waited for; they are
     * simply discarded from the queue.
     */
    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.clear();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Always returns <tt>Integer.MAX_VALUE</tt> because
     * a <tt>DelayQueue</tt> is not capacity constrained.
     *
     * @return <tt>Integer.MAX_VALUE</tt>
     */
    public int remainingCapacity() {
        return Integer.MAX_VALUE;
    }

    /**
     * Returns an array containing all of the elements in this queue.
     * The returned array elements are in no particular order.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this queue.  (In other words, this method must allocate
     * a new 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 of the elements in this queue
     */
    public Object[] toArray() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.toArray();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns an array containing all of the elements in this queue; the
     * runtime type of the returned array is that of the specified array.
     * The returned array elements are in no particular order.
     * If the queue 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 queue.
     *
     * <p>If this queue fits in the specified array with room to spare
     * (i.e., the array has more elements than this queue), the element in
     * the array immediately following the end of the queue is set to
     * <tt>null</tt>.
     *
     * <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>The following code can be used to dump a delay queue into a newly
     * allocated array of <tt>Delayed</tt>:
     *
     * <pre>
     *     Delayed[] a = q.toArray(new Delayed[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 the queue 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 of the elements in this queue
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this queue
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.toArray(a);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Removes a single instance of the specified element from this
     * queue, if it is present, whether or not it has expired.
     */
    public boolean remove(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.remove(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns an iterator over all the elements (both expired and
     * unexpired) in this queue. The iterator does not return the
     * elements in any particular order.  The returned
     * <tt>Iterator</tt> is a "weakly consistent" iterator that will
     * never throw {@link ConcurrentModificationException}, and
     * guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed
     * to) reflect any modifications subsequent to construction.
     *
     * @return an iterator over the elements in this queue
     */
    public Iterator<E> iterator() {
        return new Itr(toArray());
    }

    /**
     * Snapshot iterator that works off copy of underlying q array.
     */
    private class Itr implements Iterator<E> {
        final Object[] array; // Array of all elements
        int cursor;           // index of next element to return;
        int lastRet;          // index of last element, or -1 if no such

        Itr(Object[] array) {
            lastRet = -1;
            this.array = array;
        }

        public boolean hasNext() {
            return cursor < array.length;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (cursor >= array.length)
                throw new NoSuchElementException();
            lastRet = cursor;
            return (E)array[cursor++];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            Object x = array[lastRet];
            lastRet = -1;
            // Traverse underlying queue to find == element,
            // not just a .equals element.
            lock.lock();
            try {
                for (Iterator it = q.iterator(); it.hasNext(); ) {
                    if (it.next() == x) {
                        it.remove();
                        return;
                    }
                }
            } finally {
                lock.unlock();
            }
        }
    }

}
Revision:	1.49
Committed:	Sun May 18 23:47:56 2008 UTC (16 years ago) by jsr166
Branch:	MAIN
Changes since 1.48:	+41 -41 lines
Log Message:	Sync with OpenJDK; untabify
#	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.concurrent.locks.*;
9	import java.util.*;
10
11	/**
12	* An unbounded {@linkplain BlockingQueue blocking queue} of
13	* <tt>Delayed</tt> elements, in which an element can only be taken
14	* when its delay has expired. The <em>head</em> of the queue is that
15	* <tt>Delayed</tt> element whose delay expired furthest in the
16	* past. If no delay has expired there is no head and <tt>poll</tt>
17	* will return <tt>null</tt>. Expiration occurs when an element's
18	* <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
19	* than or equal to zero. Even though unexpired elements cannot be
20	* removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
21	* treated as normal elements. For example, the <tt>size</tt> method
22	* returns the count of both expired and unexpired elements.
23	* This queue does not permit null elements.
24	*
25	* <p>This class and its iterator implement all of the
26	* <em>optional</em> methods of the {@link Collection} and {@link
27	* Iterator} interfaces.
28	*
29	* <p>This class is a member of the
30	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
31	* Java Collections Framework</a>.
32	*
33	* @since 1.5
34	* @author Doug Lea
35	* @param <E> the type of elements held in this collection
36	*/
37
38	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
39	implements BlockingQueue<E> {
40
41	private transient final ReentrantLock lock = new ReentrantLock();
42	private final PriorityQueue<E> q = new PriorityQueue<E>();
43
44	/**
45	* Thread designated to wait for the element at the head of
46	* the queue. This variant of the Leader-Follower pattern
47	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
48	* minimize unnecessary timed waiting. When a thread becomes
49	* the leader, it waits only for the next delay to elapse, but
50	* other threads await indefinitely. The leader thread must
51	* signal some other thread before returning from take() or
52	* poll(...), unless some other thread becomes leader in the
53	* interim. Whenever the head of the queue is replaced with
54	* an element with an earlier expiration time, the leader
55	* field is invalidated by being reset to null, and some
56	* waiting thread, but not necessarily the current leader, is
57	* signalled. So waiting threads must be prepared to acquire
58	* and lose leadership while waiting.
59	*/
60	private Thread leader = null;
61
62	/**
63	* Condition signalled when a newer element becomes available
64	* at the head of the queue or a new thread may need to
65	* become leader.
66	*/
67	private final Condition available = lock.newCondition();
68
69	/**
70	* Creates a new <tt>DelayQueue</tt> that is initially empty.
71	*/
72	public DelayQueue() {}
73
74	/**
75	* Creates a <tt>DelayQueue</tt> initially containing the elements of the
76	* given collection of {@link Delayed} instances.
77	*
78	* @param c the collection of elements to initially contain
79	* @throws NullPointerException if the specified collection or any
80	* of its elements are null
81	*/
82	public DelayQueue(Collection<? extends E> c) {
83	this.addAll(c);
84	}
85
86	/**
87	* Inserts the specified element into this delay queue.
88	*
89	* @param e the element to add
90	* @return <tt>true</tt> (as specified by {@link Collection#add})
91	* @throws NullPointerException if the specified element is null
92	*/
93	public boolean add(E e) {
94	return offer(e);
95	}
96
97	/**
98	* Inserts the specified element into this delay queue.
99	*
100	* @param e the element to add
101	* @return <tt>true</tt>
102	* @throws NullPointerException if the specified element is null
103	*/
104	public boolean offer(E e) {
105	final ReentrantLock lock = this.lock;
106	lock.lock();
107	try {
108	q.offer(e);
109	if (q.peek() == e) {
110	leader = null;
111	available.signal();
112	}
113	return true;
114	} finally {
115	lock.unlock();
116	}
117	}
118
119	/**
120	* Inserts the specified element into this delay queue. As the queue is
121	* unbounded this method will never block.
122	*
123	* @param e the element to add
124	* @throws NullPointerException {@inheritDoc}
125	*/
126	public void put(E e) {
127	offer(e);
128	}
129
130	/**
131	* Inserts the specified element into this delay queue. As the queue is
132	* unbounded this method will never block.
133	*
134	* @param e the element to add
135	* @param timeout This parameter is ignored as the method never blocks
136	* @param unit This parameter is ignored as the method never blocks
137	* @return <tt>true</tt>
138	* @throws NullPointerException {@inheritDoc}
139	*/
140	public boolean offer(E e, long timeout, TimeUnit unit) {
141	return offer(e);
142	}
143
144	/**
145	* Retrieves and removes the head of this queue, or returns <tt>null</tt>
146	* if this queue has no elements with an expired delay.
147	*
148	* @return the head of this queue, or <tt>null</tt> if this
149	* queue has no elements with an expired delay
150	*/
151	public E poll() {
152	final ReentrantLock lock = this.lock;
153	lock.lock();
154	try {
155	E first = q.peek();
156	if (first == null \|\| first.getDelay(TimeUnit.NANOSECONDS) > 0)
157	return null;
158	else
159	return q.poll();
160	} finally {
161	lock.unlock();
162	}
163	}
164
165	/**
166	* Retrieves and removes the head of this queue, waiting if necessary
167	* until an element with an expired delay is available on this queue.
168	*
169	* @return the head of this queue
170	* @throws InterruptedException {@inheritDoc}
171	*/
172	public E take() throws InterruptedException {
173	final ReentrantLock lock = this.lock;
174	lock.lockInterruptibly();
175	try {
176	for (;;) {
177	E first = q.peek();
178	if (first == null)
179	available.await();
180	else {
181	long delay = first.getDelay(TimeUnit.NANOSECONDS);
182	if (delay <= 0)
183	return q.poll();
184	else if (leader != null)
185	available.await();
186	else {
187	Thread thisThread = Thread.currentThread();
188	leader = thisThread;
189	try {
190	available.awaitNanos(delay);
191	} finally {
192	if (leader == thisThread)
193	leader = null;
194	}
195	}
196	}
197	}
198	} finally {
199	if (leader == null && q.peek() != null)
200	available.signal();
201	lock.unlock();
202	}
203	}
204
205	/**
206	* Retrieves and removes the head of this queue, waiting if necessary
207	* until an element with an expired delay is available on this queue,
208	* or the specified wait time expires.
209	*
210	* @return the head of this queue, or <tt>null</tt> if the
211	* specified waiting time elapses before an element with
212	* an expired delay becomes available
213	* @throws InterruptedException {@inheritDoc}
214	*/
215	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
216	long nanos = unit.toNanos(timeout);
217	final ReentrantLock lock = this.lock;
218	lock.lockInterruptibly();
219	try {
220	for (;;) {
221	E first = q.peek();
222	if (first == null) {
223	if (nanos <= 0)
224	return null;
225	else
226	nanos = available.awaitNanos(nanos);
227	} else {
228	long delay = first.getDelay(TimeUnit.NANOSECONDS);
229	if (delay <= 0)
230	return q.poll();
231	if (nanos <= 0)
232	return null;
233	if (nanos < delay \|\| leader != null)
234	nanos = available.awaitNanos(nanos);
235	else {
236	Thread thisThread = Thread.currentThread();
237	leader = thisThread;
238	try {
239	long timeLeft = available.awaitNanos(delay);
240	nanos -= delay - timeLeft;
241	} finally {
242	if (leader == thisThread)
243	leader = null;
244	}
245	}
246	}
247	}
248	} finally {
249	if (leader == null && q.peek() != null)
250	available.signal();
251	lock.unlock();
252	}
253	}
254
255	/**
256	* Retrieves, but does not remove, the head of this queue, or
257	* returns <tt>null</tt> if this queue is empty. Unlike
258	* <tt>poll</tt>, if no expired elements are available in the queue,
259	* this method returns the element that will expire next,
260	* if one exists.
261	*
262	* @return the head of this queue, or <tt>null</tt> if this
263	* queue is empty.
264	*/
265	public E peek() {
266	final ReentrantLock lock = this.lock;
267	lock.lock();
268	try {
269	return q.peek();
270	} finally {
271	lock.unlock();
272	}
273	}
274
275	public int size() {
276	final ReentrantLock lock = this.lock;
277	lock.lock();
278	try {
279	return q.size();
280	} finally {
281	lock.unlock();
282	}
283	}
284
285	/**
286	* @throws UnsupportedOperationException {@inheritDoc}
287	* @throws ClassCastException {@inheritDoc}
288	* @throws NullPointerException {@inheritDoc}
289	* @throws IllegalArgumentException {@inheritDoc}
290	*/
291	public int drainTo(Collection<? super E> c) {
292	if (c == null)
293	throw new NullPointerException();
294	if (c == this)
295	throw new IllegalArgumentException();
296	final ReentrantLock lock = this.lock;
297	lock.lock();
298	try {
299	int n = 0;
300	for (;;) {
301	E first = q.peek();
302	if (first == null \|\| first.getDelay(TimeUnit.NANOSECONDS) > 0)
303	break;
304	c.add(q.poll());
305	++n;
306	}
307	return n;
308	} finally {
309	lock.unlock();
310	}
311	}
312
313	/**
314	* @throws UnsupportedOperationException {@inheritDoc}
315	* @throws ClassCastException {@inheritDoc}
316	* @throws NullPointerException {@inheritDoc}
317	* @throws IllegalArgumentException {@inheritDoc}
318	*/
319	public int drainTo(Collection<? super E> c, int maxElements) {
320	if (c == null)
321	throw new NullPointerException();
322	if (c == this)
323	throw new IllegalArgumentException();
324	if (maxElements <= 0)
325	return 0;
326	final ReentrantLock lock = this.lock;
327	lock.lock();
328	try {
329	int n = 0;
330	while (n < maxElements) {
331	E first = q.peek();
332	if (first == null \|\| first.getDelay(TimeUnit.NANOSECONDS) > 0)
333	break;
334	c.add(q.poll());
335	++n;
336	}
337	return n;
338	} finally {
339	lock.unlock();
340	}
341	}
342
343	/**
344	* Atomically removes all of the elements from this delay queue.
345	* The queue will be empty after this call returns.
346	* Elements with an unexpired delay are not waited for; they are
347	* simply discarded from the queue.
348	*/
349	public void clear() {
350	final ReentrantLock lock = this.lock;
351	lock.lock();
352	try {
353	q.clear();
354	} finally {
355	lock.unlock();
356	}
357	}
358
359	/**
360	* Always returns <tt>Integer.MAX_VALUE</tt> because
361	* a <tt>DelayQueue</tt> is not capacity constrained.
362	*
363	* @return <tt>Integer.MAX_VALUE</tt>
364	*/
365	public int remainingCapacity() {
366	return Integer.MAX_VALUE;
367	}
368
369	/**
370	* Returns an array containing all of the elements in this queue.
371	* The returned array elements are in no particular order.
372	*
373	* <p>The returned array will be "safe" in that no references to it are
374	* maintained by this queue. (In other words, this method must allocate
375	* a new array). The caller is thus free to modify the returned array.
376	*
377	* <p>This method acts as bridge between array-based and collection-based
378	* APIs.
379	*
380	* @return an array containing all of the elements in this queue
381	*/
382	public Object[] toArray() {
383	final ReentrantLock lock = this.lock;
384	lock.lock();
385	try {
386	return q.toArray();
387	} finally {
388	lock.unlock();
389	}
390	}
391
392	/**
393	* Returns an array containing all of the elements in this queue; the
394	* runtime type of the returned array is that of the specified array.
395	* The returned array elements are in no particular order.
396	* If the queue fits in the specified array, it is returned therein.
397	* Otherwise, a new array is allocated with the runtime type of the
398	* specified array and the size of this queue.
399	*
400	* <p>If this queue fits in the specified array with room to spare
401	* (i.e., the array has more elements than this queue), the element in
402	* the array immediately following the end of the queue is set to
403	* <tt>null</tt>.
404	*
405	* <p>Like the {@link #toArray()} method, this method acts as bridge between
406	* array-based and collection-based APIs. Further, this method allows
407	* precise control over the runtime type of the output array, and may,
408	* under certain circumstances, be used to save allocation costs.
409	*
410	* <p>The following code can be used to dump a delay queue into a newly
411	* allocated array of <tt>Delayed</tt>:
412	*
413	* <pre>
414	* Delayed[] a = q.toArray(new Delayed[0]);</pre>
415	*
416	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
417	* <tt>toArray()</tt>.
418	*
419	* @param a the array into which the elements of the queue are to
420	* be stored, if it is big enough; otherwise, a new array of the
421	* same runtime type is allocated for this purpose
422	* @return an array containing all of the elements in this queue
423	* @throws ArrayStoreException if the runtime type of the specified array
424	* is not a supertype of the runtime type of every element in
425	* this queue
426	* @throws NullPointerException if the specified array is null
427	*/
428	public <T> T[] toArray(T[] a) {
429	final ReentrantLock lock = this.lock;
430	lock.lock();
431	try {
432	return q.toArray(a);
433	} finally {
434	lock.unlock();
435	}
436	}
437
438	/**
439	* Removes a single instance of the specified element from this
440	* queue, if it is present, whether or not it has expired.
441	*/
442	public boolean remove(Object o) {
443	final ReentrantLock lock = this.lock;
444	lock.lock();
445	try {
446	return q.remove(o);
447	} finally {
448	lock.unlock();
449	}
450	}
451
452	/**
453	* Returns an iterator over all the elements (both expired and
454	* unexpired) in this queue. The iterator does not return the
455	* elements in any particular order. The returned
456	* <tt>Iterator</tt> is a "weakly consistent" iterator that will
457	* never throw {@link ConcurrentModificationException}, and
458	* guarantees to traverse elements as they existed upon
459	* construction of the iterator, and may (but is not guaranteed
460	* to) reflect any modifications subsequent to construction.
461	*
462	* @return an iterator over the elements in this queue
463	*/
464	public Iterator<E> iterator() {
465	return new Itr(toArray());
466	}
467
468	/**
469	* Snapshot iterator that works off copy of underlying q array.
470	*/
471	private class Itr implements Iterator<E> {
472	final Object[] array; // Array of all elements
473	int cursor; // index of next element to return;
474	int lastRet; // index of last element, or -1 if no such
475
476	Itr(Object[] array) {
477	lastRet = -1;
478	this.array = array;
479	}
480
481	public boolean hasNext() {
482	return cursor < array.length;
483	}
484
485	@SuppressWarnings("unchecked")
486	public E next() {
487	if (cursor >= array.length)
488	throw new NoSuchElementException();
489	lastRet = cursor;
490	return (E)array[cursor++];
491	}
492
493	public void remove() {
494	if (lastRet < 0)
495	throw new IllegalStateException();
496	Object x = array[lastRet];
497	lastRet = -1;
498	// Traverse underlying queue to find == element,
499	// not just a .equals element.
500	lock.lock();
501	try {
502	for (Iterator it = q.iterator(); it.hasNext(); ) {
503	if (it.next() == x) {
504	it.remove();
505	return;
506	}
507	}
508	} finally {
509	lock.unlock();
510	}
511	}
512	}
513
514	}