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/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import static java.util.concurrent.TimeUnit.NANOSECONDS;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.*;

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} of
 * {@code Delayed} elements, in which an element can only be taken
 * when its delay has expired.  The <em>head</em> of the queue is that
 * {@code Delayed} element whose delay expired furthest in the
 * past.  If no delay has expired there is no head and {@code poll}
 * will return {@code null}. Expiration occurs when an element's
 * {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
 * than or equal to zero.  Even though unexpired elements cannot be
 * removed using {@code take} or {@code poll}, they are otherwise
 * treated as normal elements. For example, the {@code size} 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.  The Iterator provided in method {@link
 * #iterator()} is <em>not</em> guaranteed to traverse the elements of
 * the DelayQueue in any particular order.
 *
 * <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 final transient 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;

    /**
     * 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 {@code DelayQueue} that is initially empty.
     */
    public DelayQueue() {}

    /**
     * Creates a {@code DelayQueue} 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 {@code true} (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 {@code true}
     * @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 {@code true}
     * @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 {@code null}
     * if this queue has no elements with an expired delay.
     *
     * @return the head of this queue, or {@code null} 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(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(NANOSECONDS);
                    if (delay <= 0)
                        return q.poll();
                    first = null; // don't retain ref while waiting
                    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 {@code null} 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(NANOSECONDS);
                    if (delay <= 0)
                        return q.poll();
                    if (nanos <= 0)
                        return null;
                    first = null; // don't retain ref while waiting
                    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 {@code null} if this queue is empty.  Unlike
     * {@code poll}, 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 {@code null} 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();
        }
    }

    /**
     * Returns first element only if it is expired.
     * Used only by drainTo.  Call only when holding lock.
     */
    private E peekExpired() {
        // assert lock.isHeldByCurrentThread();
        E first = q.peek();
        return (first == null || first.getDelay(NANOSECONDS) > 0) ?
            null : first;
    }

    /**
     * @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 e; (e = peekExpired()) != null;) {
                c.add(e);       // In this order, in case add() throws.
                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;
            for (E e; n < maxElements && (e = peekExpired()) != null;) {
                c.add(e);       // In this order, in case add() throws.
                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 {@code Integer.MAX_VALUE} because
     * a {@code DelayQueue} is not capacity constrained.
     *
     * @return {@code Integer.MAX_VALUE}
     */
    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
     * {@code null}.
     *
     * <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 {@code Delayed}:
     *
     * <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @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();
        }
    }

    /**
     * Identity-based version for use in Itr.remove
     */
    void removeEQ(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
                if (o == it.next()) {
                    it.remove();
                    break;
                }
            }
        } 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.
     *
     * <p>The returned iterator is a "weakly consistent" iterator that
     * will never throw {@link java.util.ConcurrentModificationException
     * 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();
            removeEQ(array[lastRet]);
            lastRet = -1;
        }
    }

}
Revision:	1.7
Committed:	Sun Jan 18 20:17:32 2015 UTC (9 years, 5 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.6:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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
9	import static java.util.concurrent.TimeUnit.NANOSECONDS;
10	import java.util.concurrent.locks.Condition;
11	import java.util.concurrent.locks.ReentrantLock;
12	import java.util.*;
13
14	/**
15	* An unbounded {@linkplain BlockingQueue blocking queue} of
16	* {@code Delayed} elements, in which an element can only be taken
17	* when its delay has expired. The <em>head</em> of the queue is that
18	* {@code Delayed} element whose delay expired furthest in the
19	* past. If no delay has expired there is no head and {@code poll}
20	* will return {@code null}. Expiration occurs when an element's
21	* {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
22	* than or equal to zero. Even though unexpired elements cannot be
23	* removed using {@code take} or {@code poll}, they are otherwise
24	* treated as normal elements. For example, the {@code size} method
25	* returns the count of both expired and unexpired elements.
26	* This queue does not permit null elements.
27	*
28	* <p>This class and its iterator implement all of the
29	* <em>optional</em> methods of the {@link Collection} and {@link
30	* Iterator} interfaces. The Iterator provided in method {@link
31	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
32	* the DelayQueue in any particular order.
33	*
34	* <p>This class is a member of the
35	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
36	* Java Collections Framework</a>.
37	*
38	* @since 1.5
39	* @author Doug Lea
40	* @param <E> the type of elements held in this collection
41	*/
42	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
43	implements BlockingQueue<E> {
44
45	private final transient ReentrantLock lock = new ReentrantLock();
46	private final PriorityQueue<E> q = new PriorityQueue<E>();
47
48	/**
49	* Thread designated to wait for the element at the head of
50	* the queue. This variant of the Leader-Follower pattern
51	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
52	* minimize unnecessary timed waiting. When a thread becomes
53	* the leader, it waits only for the next delay to elapse, but
54	* other threads await indefinitely. The leader thread must
55	* signal some other thread before returning from take() or
56	* poll(...), unless some other thread becomes leader in the
57	* interim. Whenever the head of the queue is replaced with
58	* an element with an earlier expiration time, the leader
59	* field is invalidated by being reset to null, and some
60	* waiting thread, but not necessarily the current leader, is
61	* signalled. So waiting threads must be prepared to acquire
62	* and lose leadership while waiting.
63	*/
64	private Thread leader;
65
66	/**
67	* Condition signalled when a newer element becomes available
68	* at the head of the queue or a new thread may need to
69	* become leader.
70	*/
71	private final Condition available = lock.newCondition();
72
73	/**
74	* Creates a new {@code DelayQueue} that is initially empty.
75	*/
76	public DelayQueue() {}
77
78	/**
79	* Creates a {@code DelayQueue} initially containing the elements of the
80	* given collection of {@link Delayed} instances.
81	*
82	* @param c the collection of elements to initially contain
83	* @throws NullPointerException if the specified collection or any
84	* of its elements are null
85	*/
86	public DelayQueue(Collection<? extends E> c) {
87	this.addAll(c);
88	}
89
90	/**
91	* Inserts the specified element into this delay queue.
92	*
93	* @param e the element to add
94	* @return {@code true} (as specified by {@link Collection#add})
95	* @throws NullPointerException if the specified element is null
96	*/
97	public boolean add(E e) {
98	return offer(e);
99	}
100
101	/**
102	* Inserts the specified element into this delay queue.
103	*
104	* @param e the element to add
105	* @return {@code true}
106	* @throws NullPointerException if the specified element is null
107	*/
108	public boolean offer(E e) {
109	final ReentrantLock lock = this.lock;
110	lock.lock();
111	try {
112	q.offer(e);
113	if (q.peek() == e) {
114	leader = null;
115	available.signal();
116	}
117	return true;
118	} finally {
119	lock.unlock();
120	}
121	}
122
123	/**
124	* Inserts the specified element into this delay queue. As the queue is
125	* unbounded this method will never block.
126	*
127	* @param e the element to add
128	* @throws NullPointerException {@inheritDoc}
129	*/
130	public void put(E e) {
131	offer(e);
132	}
133
134	/**
135	* Inserts the specified element into this delay queue. As the queue is
136	* unbounded this method will never block.
137	*
138	* @param e the element to add
139	* @param timeout This parameter is ignored as the method never blocks
140	* @param unit This parameter is ignored as the method never blocks
141	* @return {@code true}
142	* @throws NullPointerException {@inheritDoc}
143	*/
144	public boolean offer(E e, long timeout, TimeUnit unit) {
145	return offer(e);
146	}
147
148	/**
149	* Retrieves and removes the head of this queue, or returns {@code null}
150	* if this queue has no elements with an expired delay.
151	*
152	* @return the head of this queue, or {@code null} if this
153	* queue has no elements with an expired delay
154	*/
155	public E poll() {
156	final ReentrantLock lock = this.lock;
157	lock.lock();
158	try {
159	E first = q.peek();
160	if (first == null \|\| first.getDelay(NANOSECONDS) > 0)
161	return null;
162	else
163	return q.poll();
164	} finally {
165	lock.unlock();
166	}
167	}
168
169	/**
170	* Retrieves and removes the head of this queue, waiting if necessary
171	* until an element with an expired delay is available on this queue.
172	*
173	* @return the head of this queue
174	* @throws InterruptedException {@inheritDoc}
175	*/
176	public E take() throws InterruptedException {
177	final ReentrantLock lock = this.lock;
178	lock.lockInterruptibly();
179	try {
180	for (;;) {
181	E first = q.peek();
182	if (first == null)
183	available.await();
184	else {
185	long delay = first.getDelay(NANOSECONDS);
186	if (delay <= 0)
187	return q.poll();
188	first = null; // don't retain ref while waiting
189	if (leader != null)
190	available.await();
191	else {
192	Thread thisThread = Thread.currentThread();
193	leader = thisThread;
194	try {
195	available.awaitNanos(delay);
196	} finally {
197	if (leader == thisThread)
198	leader = null;
199	}
200	}
201	}
202	}
203	} finally {
204	if (leader == null && q.peek() != null)
205	available.signal();
206	lock.unlock();
207	}
208	}
209
210	/**
211	* Retrieves and removes the head of this queue, waiting if necessary
212	* until an element with an expired delay is available on this queue,
213	* or the specified wait time expires.
214	*
215	* @return the head of this queue, or {@code null} if the
216	* specified waiting time elapses before an element with
217	* an expired delay becomes available
218	* @throws InterruptedException {@inheritDoc}
219	*/
220	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
221	long nanos = unit.toNanos(timeout);
222	final ReentrantLock lock = this.lock;
223	lock.lockInterruptibly();
224	try {
225	for (;;) {
226	E first = q.peek();
227	if (first == null) {
228	if (nanos <= 0)
229	return null;
230	else
231	nanos = available.awaitNanos(nanos);
232	} else {
233	long delay = first.getDelay(NANOSECONDS);
234	if (delay <= 0)
235	return q.poll();
236	if (nanos <= 0)
237	return null;
238	first = null; // don't retain ref while waiting
239	if (nanos < delay \|\| leader != null)
240	nanos = available.awaitNanos(nanos);
241	else {
242	Thread thisThread = Thread.currentThread();
243	leader = thisThread;
244	try {
245	long timeLeft = available.awaitNanos(delay);
246	nanos -= delay - timeLeft;
247	} finally {
248	if (leader == thisThread)
249	leader = null;
250	}
251	}
252	}
253	}
254	} finally {
255	if (leader == null && q.peek() != null)
256	available.signal();
257	lock.unlock();
258	}
259	}
260
261	/**
262	* Retrieves, but does not remove, the head of this queue, or
263	* returns {@code null} if this queue is empty. Unlike
264	* {@code poll}, if no expired elements are available in the queue,
265	* this method returns the element that will expire next,
266	* if one exists.
267	*
268	* @return the head of this queue, or {@code null} if this
269	* queue is empty
270	*/
271	public E peek() {
272	final ReentrantLock lock = this.lock;
273	lock.lock();
274	try {
275	return q.peek();
276	} finally {
277	lock.unlock();
278	}
279	}
280
281	public int size() {
282	final ReentrantLock lock = this.lock;
283	lock.lock();
284	try {
285	return q.size();
286	} finally {
287	lock.unlock();
288	}
289	}
290
291	/**
292	* Returns first element only if it is expired.
293	* Used only by drainTo. Call only when holding lock.
294	*/
295	private E peekExpired() {
296	// assert lock.isHeldByCurrentThread();
297	E first = q.peek();
298	return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
299	null : first;
300	}
301
302	/**
303	* @throws UnsupportedOperationException {@inheritDoc}
304	* @throws ClassCastException {@inheritDoc}
305	* @throws NullPointerException {@inheritDoc}
306	* @throws IllegalArgumentException {@inheritDoc}
307	*/
308	public int drainTo(Collection<? super E> c) {
309	if (c == null)
310	throw new NullPointerException();
311	if (c == this)
312	throw new IllegalArgumentException();
313	final ReentrantLock lock = this.lock;
314	lock.lock();
315	try {
316	int n = 0;
317	for (E e; (e = peekExpired()) != null;) {
318	c.add(e); // In this order, in case add() throws.
319	q.poll();
320	++n;
321	}
322	return n;
323	} finally {
324	lock.unlock();
325	}
326	}
327
328	/**
329	* @throws UnsupportedOperationException {@inheritDoc}
330	* @throws ClassCastException {@inheritDoc}
331	* @throws NullPointerException {@inheritDoc}
332	* @throws IllegalArgumentException {@inheritDoc}
333	*/
334	public int drainTo(Collection<? super E> c, int maxElements) {
335	if (c == null)
336	throw new NullPointerException();
337	if (c == this)
338	throw new IllegalArgumentException();
339	if (maxElements <= 0)
340	return 0;
341	final ReentrantLock lock = this.lock;
342	lock.lock();
343	try {
344	int n = 0;
345	for (E e; n < maxElements && (e = peekExpired()) != null;) {
346	c.add(e); // In this order, in case add() throws.
347	q.poll();
348	++n;
349	}
350	return n;
351	} finally {
352	lock.unlock();
353	}
354	}
355
356	/**
357	* Atomically removes all of the elements from this delay queue.
358	* The queue will be empty after this call returns.
359	* Elements with an unexpired delay are not waited for; they are
360	* simply discarded from the queue.
361	*/
362	public void clear() {
363	final ReentrantLock lock = this.lock;
364	lock.lock();
365	try {
366	q.clear();
367	} finally {
368	lock.unlock();
369	}
370	}
371
372	/**
373	* Always returns {@code Integer.MAX_VALUE} because
374	* a {@code DelayQueue} is not capacity constrained.
375	*
376	* @return {@code Integer.MAX_VALUE}
377	*/
378	public int remainingCapacity() {
379	return Integer.MAX_VALUE;
380	}
381
382	/**
383	* Returns an array containing all of the elements in this queue.
384	* The returned array elements are in no particular order.
385	*
386	* <p>The returned array will be "safe" in that no references to it are
387	* maintained by this queue. (In other words, this method must allocate
388	* a new array). The caller is thus free to modify the returned array.
389	*
390	* <p>This method acts as bridge between array-based and collection-based
391	* APIs.
392	*
393	* @return an array containing all of the elements in this queue
394	*/
395	public Object[] toArray() {
396	final ReentrantLock lock = this.lock;
397	lock.lock();
398	try {
399	return q.toArray();
400	} finally {
401	lock.unlock();
402	}
403	}
404
405	/**
406	* Returns an array containing all of the elements in this queue; the
407	* runtime type of the returned array is that of the specified array.
408	* The returned array elements are in no particular order.
409	* If the queue fits in the specified array, it is returned therein.
410	* Otherwise, a new array is allocated with the runtime type of the
411	* specified array and the size of this queue.
412	*
413	* <p>If this queue fits in the specified array with room to spare
414	* (i.e., the array has more elements than this queue), the element in
415	* the array immediately following the end of the queue is set to
416	* {@code null}.
417	*
418	* <p>Like the {@link #toArray()} method, this method acts as bridge between
419	* array-based and collection-based APIs. Further, this method allows
420	* precise control over the runtime type of the output array, and may,
421	* under certain circumstances, be used to save allocation costs.
422	*
423	* <p>The following code can be used to dump a delay queue into a newly
424	* allocated array of {@code Delayed}:
425	*
426	* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
427	*
428	* Note that {@code toArray(new Object[0])} is identical in function to
429	* {@code toArray()}.
430	*
431	* @param a the array into which the elements of the queue are to
432	* be stored, if it is big enough; otherwise, a new array of the
433	* same runtime type is allocated for this purpose
434	* @return an array containing all of the elements in this queue
435	* @throws ArrayStoreException if the runtime type of the specified array
436	* is not a supertype of the runtime type of every element in
437	* this queue
438	* @throws NullPointerException if the specified array is null
439	*/
440	public <T> T[] toArray(T[] a) {
441	final ReentrantLock lock = this.lock;
442	lock.lock();
443	try {
444	return q.toArray(a);
445	} finally {
446	lock.unlock();
447	}
448	}
449
450	/**
451	* Removes a single instance of the specified element from this
452	* queue, if it is present, whether or not it has expired.
453	*/
454	public boolean remove(Object o) {
455	final ReentrantLock lock = this.lock;
456	lock.lock();
457	try {
458	return q.remove(o);
459	} finally {
460	lock.unlock();
461	}
462	}
463
464	/**
465	* Identity-based version for use in Itr.remove
466	*/
467	void removeEQ(Object o) {
468	final ReentrantLock lock = this.lock;
469	lock.lock();
470	try {
471	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
472	if (o == it.next()) {
473	it.remove();
474	break;
475	}
476	}
477	} finally {
478	lock.unlock();
479	}
480	}
481
482	/**
483	* Returns an iterator over all the elements (both expired and
484	* unexpired) in this queue. The iterator does not return the
485	* elements in any particular order.
486	*
487	* <p>The returned iterator is a "weakly consistent" iterator that
488	* will never throw {@link java.util.ConcurrentModificationException
489	* ConcurrentModificationException}, and guarantees to traverse
490	* elements as they existed upon construction of the iterator, and
491	* may (but is not guaranteed to) reflect any modifications
492	* subsequent to construction.
493	*
494	* @return an iterator over the elements in this queue
495	*/
496	public Iterator<E> iterator() {
497	return new Itr(toArray());
498	}
499
500	/**
501	* Snapshot iterator that works off copy of underlying q array.
502	*/
503	private class Itr implements Iterator<E> {
504	final Object[] array; // Array of all elements
505	int cursor; // index of next element to return
506	int lastRet; // index of last element, or -1 if no such
507
508	Itr(Object[] array) {
509	lastRet = -1;
510	this.array = array;
511	}
512
513	public boolean hasNext() {
514	return cursor < array.length;
515	}
516
517	@SuppressWarnings("unchecked")
518	public E next() {
519	if (cursor >= array.length)
520	throw new NoSuchElementException();
521	lastRet = cursor;
522	return (E)array[cursor++];
523	}
524
525	public void remove() {
526	if (lastRet < 0)
527	throw new IllegalStateException();
528	removeEQ(array[lastRet]);
529	lastRet = -1;
530	}
531	}
532
533	}