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 = 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 {@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 href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
     *
     * @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:	1.65
Committed:	Thu Aug 8 20:12:10 2013 UTC (10 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.64:	+2 -6 lines
Log Message:	refactor definitions of "weakly consistent" into package-info.java
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import static java.util.concurrent.TimeUnit.NANOSECONDS;
9	import java.util.concurrent.locks.Condition;
10	import java.util.concurrent.locks.ReentrantLock;
11	import java.util.*;
12
13	/**
14	* An unbounded {@linkplain BlockingQueue blocking queue} of
15	* {@code Delayed} elements, in which an element can only be taken
16	* when its delay has expired. The <em>head</em> of the queue is that
17	* {@code Delayed} element whose delay expired furthest in the
18	* past. If no delay has expired there is no head and {@code poll}
19	* will return {@code null}. Expiration occurs when an element's
20	* {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
21	* than or equal to zero. Even though unexpired elements cannot be
22	* removed using {@code take} or {@code poll}, they are otherwise
23	* treated as normal elements. For example, the {@code size} method
24	* returns the count of both expired and unexpired elements.
25	* This queue does not permit null elements.
26	*
27	* <p>This class and its iterator implement all of the
28	* <em>optional</em> methods of the {@link Collection} and {@link
29	* Iterator} interfaces. The Iterator provided in method {@link
30	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
31	* the DelayQueue in any particular order.
32	*
33	* <p>This class is a member of the
34	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
35	* Java Collections Framework</a>.
36	*
37	* @since 1.5
38	* @author Doug Lea
39	* @param <E> the type of elements held in this collection
40	*/
41	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
42	implements BlockingQueue<E> {
43
44	private final transient ReentrantLock lock = new ReentrantLock();
45	private final PriorityQueue<E> q = new PriorityQueue<E>();
46
47	/**
48	* Thread designated to wait for the element at the head of
49	* the queue. This variant of the Leader-Follower pattern
50	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
51	* minimize unnecessary timed waiting. When a thread becomes
52	* the leader, it waits only for the next delay to elapse, but
53	* other threads await indefinitely. The leader thread must
54	* signal some other thread before returning from take() or
55	* poll(...), unless some other thread becomes leader in the
56	* interim. Whenever the head of the queue is replaced with
57	* an element with an earlier expiration time, the leader
58	* field is invalidated by being reset to null, and some
59	* waiting thread, but not necessarily the current leader, is
60	* signalled. So waiting threads must be prepared to acquire
61	* and lose leadership while waiting.
62	*/
63	private Thread leader = null;
64
65	/**
66	* Condition signalled when a newer element becomes available
67	* at the head of the queue or a new thread may need to
68	* become leader.
69	*/
70	private final Condition available = lock.newCondition();
71
72	/**
73	* Creates a new {@code DelayQueue} that is initially empty.
74	*/
75	public DelayQueue() {}
76
77	/**
78	* Creates a {@code DelayQueue} initially containing the elements of the
79	* given collection of {@link Delayed} instances.
80	*
81	* @param c the collection of elements to initially contain
82	* @throws NullPointerException if the specified collection or any
83	* of its elements are null
84	*/
85	public DelayQueue(Collection<? extends E> c) {
86	this.addAll(c);
87	}
88
89	/**
90	* Inserts the specified element into this delay queue.
91	*
92	* @param e the element to add
93	* @return {@code true} (as specified by {@link Collection#add})
94	* @throws NullPointerException if the specified element is null
95	*/
96	public boolean add(E e) {
97	return offer(e);
98	}
99
100	/**
101	* Inserts the specified element into this delay queue.
102	*
103	* @param e the element to add
104	* @return {@code true}
105	* @throws NullPointerException if the specified element is null
106	*/
107	public boolean offer(E e) {
108	final ReentrantLock lock = this.lock;
109	lock.lock();
110	try {
111	q.offer(e);
112	if (q.peek() == e) {
113	leader = null;
114	available.signal();
115	}
116	return true;
117	} finally {
118	lock.unlock();
119	}
120	}
121
122	/**
123	* Inserts the specified element into this delay queue. As the queue is
124	* unbounded this method will never block.
125	*
126	* @param e the element to add
127	* @throws NullPointerException {@inheritDoc}
128	*/
129	public void put(E e) {
130	offer(e);
131	}
132
133	/**
134	* Inserts the specified element into this delay queue. As the queue is
135	* unbounded this method will never block.
136	*
137	* @param e the element to add
138	* @param timeout This parameter is ignored as the method never blocks
139	* @param unit This parameter is ignored as the method never blocks
140	* @return {@code true}
141	* @throws NullPointerException {@inheritDoc}
142	*/
143	public boolean offer(E e, long timeout, TimeUnit unit) {
144	return offer(e);
145	}
146
147	/**
148	* Retrieves and removes the head of this queue, or returns {@code null}
149	* if this queue has no elements with an expired delay.
150	*
151	* @return the head of this queue, or {@code null} if this
152	* queue has no elements with an expired delay
153	*/
154	public E poll() {
155	final ReentrantLock lock = this.lock;
156	lock.lock();
157	try {
158	E first = q.peek();
159	if (first == null \|\| first.getDelay(NANOSECONDS) > 0)
160	return null;
161	else
162	return q.poll();
163	} finally {
164	lock.unlock();
165	}
166	}
167
168	/**
169	* Retrieves and removes the head of this queue, waiting if necessary
170	* until an element with an expired delay is available on this queue.
171	*
172	* @return the head of this queue
173	* @throws InterruptedException {@inheritDoc}
174	*/
175	public E take() throws InterruptedException {
176	final ReentrantLock lock = this.lock;
177	lock.lockInterruptibly();
178	try {
179	for (;;) {
180	E first = q.peek();
181	if (first == null)
182	available.await();
183	else {
184	long delay = first.getDelay(NANOSECONDS);
185	if (delay <= 0)
186	return q.poll();
187	first = null; // don't retain ref while waiting
188	if (leader != null)
189	available.await();
190	else {
191	Thread thisThread = Thread.currentThread();
192	leader = thisThread;
193	try {
194	available.awaitNanos(delay);
195	} finally {
196	if (leader == thisThread)
197	leader = null;
198	}
199	}
200	}
201	}
202	} finally {
203	if (leader == null && q.peek() != null)
204	available.signal();
205	lock.unlock();
206	}
207	}
208
209	/**
210	* Retrieves and removes the head of this queue, waiting if necessary
211	* until an element with an expired delay is available on this queue,
212	* or the specified wait time expires.
213	*
214	* @return the head of this queue, or {@code null} if the
215	* specified waiting time elapses before an element with
216	* an expired delay becomes available
217	* @throws InterruptedException {@inheritDoc}
218	*/
219	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
220	long nanos = unit.toNanos(timeout);
221	final ReentrantLock lock = this.lock;
222	lock.lockInterruptibly();
223	try {
224	for (;;) {
225	E first = q.peek();
226	if (first == null) {
227	if (nanos <= 0)
228	return null;
229	else
230	nanos = available.awaitNanos(nanos);
231	} else {
232	long delay = first.getDelay(NANOSECONDS);
233	if (delay <= 0)
234	return q.poll();
235	if (nanos <= 0)
236	return null;
237	first = null; // don't retain ref while waiting
238	if (nanos < delay \|\| leader != null)
239	nanos = available.awaitNanos(nanos);
240	else {
241	Thread thisThread = Thread.currentThread();
242	leader = thisThread;
243	try {
244	long timeLeft = available.awaitNanos(delay);
245	nanos -= delay - timeLeft;
246	} finally {
247	if (leader == thisThread)
248	leader = null;
249	}
250	}
251	}
252	}
253	} finally {
254	if (leader == null && q.peek() != null)
255	available.signal();
256	lock.unlock();
257	}
258	}
259
260	/**
261	* Retrieves, but does not remove, the head of this queue, or
262	* returns {@code null} if this queue is empty. Unlike
263	* {@code poll}, if no expired elements are available in the queue,
264	* this method returns the element that will expire next,
265	* if one exists.
266	*
267	* @return the head of this queue, or {@code null} if this
268	* queue is empty
269	*/
270	public E peek() {
271	final ReentrantLock lock = this.lock;
272	lock.lock();
273	try {
274	return q.peek();
275	} finally {
276	lock.unlock();
277	}
278	}
279
280	public int size() {
281	final ReentrantLock lock = this.lock;
282	lock.lock();
283	try {
284	return q.size();
285	} finally {
286	lock.unlock();
287	}
288	}
289
290	/**
291	* Returns first element only if it is expired.
292	* Used only by drainTo. Call only when holding lock.
293	*/
294	private E peekExpired() {
295	// assert lock.isHeldByCurrentThread();
296	E first = q.peek();
297	return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
298	null : first;
299	}
300
301	/**
302	* @throws UnsupportedOperationException {@inheritDoc}
303	* @throws ClassCastException {@inheritDoc}
304	* @throws NullPointerException {@inheritDoc}
305	* @throws IllegalArgumentException {@inheritDoc}
306	*/
307	public int drainTo(Collection<? super E> c) {
308	if (c == null)
309	throw new NullPointerException();
310	if (c == this)
311	throw new IllegalArgumentException();
312	final ReentrantLock lock = this.lock;
313	lock.lock();
314	try {
315	int n = 0;
316	for (E e; (e = peekExpired()) != null;) {
317	c.add(e); // In this order, in case add() throws.
318	q.poll();
319	++n;
320	}
321	return n;
322	} finally {
323	lock.unlock();
324	}
325	}
326
327	/**
328	* @throws UnsupportedOperationException {@inheritDoc}
329	* @throws ClassCastException {@inheritDoc}
330	* @throws NullPointerException {@inheritDoc}
331	* @throws IllegalArgumentException {@inheritDoc}
332	*/
333	public int drainTo(Collection<? super E> c, int maxElements) {
334	if (c == null)
335	throw new NullPointerException();
336	if (c == this)
337	throw new IllegalArgumentException();
338	if (maxElements <= 0)
339	return 0;
340	final ReentrantLock lock = this.lock;
341	lock.lock();
342	try {
343	int n = 0;
344	for (E e; n < maxElements && (e = peekExpired()) != null;) {
345	c.add(e); // In this order, in case add() throws.
346	q.poll();
347	++n;
348	}
349	return n;
350	} finally {
351	lock.unlock();
352	}
353	}
354
355	/**
356	* Atomically removes all of the elements from this delay queue.
357	* The queue will be empty after this call returns.
358	* Elements with an unexpired delay are not waited for; they are
359	* simply discarded from the queue.
360	*/
361	public void clear() {
362	final ReentrantLock lock = this.lock;
363	lock.lock();
364	try {
365	q.clear();
366	} finally {
367	lock.unlock();
368	}
369	}
370
371	/**
372	* Always returns {@code Integer.MAX_VALUE} because
373	* a {@code DelayQueue} is not capacity constrained.
374	*
375	* @return {@code Integer.MAX_VALUE}
376	*/
377	public int remainingCapacity() {
378	return Integer.MAX_VALUE;
379	}
380
381	/**
382	* Returns an array containing all of the elements in this queue.
383	* The returned array elements are in no particular order.
384	*
385	* <p>The returned array will be "safe" in that no references to it are
386	* maintained by this queue. (In other words, this method must allocate
387	* a new array). The caller is thus free to modify the returned array.
388	*
389	* <p>This method acts as bridge between array-based and collection-based
390	* APIs.
391	*
392	* @return an array containing all of the elements in this queue
393	*/
394	public Object[] toArray() {
395	final ReentrantLock lock = this.lock;
396	lock.lock();
397	try {
398	return q.toArray();
399	} finally {
400	lock.unlock();
401	}
402	}
403
404	/**
405	* Returns an array containing all of the elements in this queue; the
406	* runtime type of the returned array is that of the specified array.
407	* The returned array elements are in no particular order.
408	* If the queue fits in the specified array, it is returned therein.
409	* Otherwise, a new array is allocated with the runtime type of the
410	* specified array and the size of this queue.
411	*
412	* <p>If this queue fits in the specified array with room to spare
413	* (i.e., the array has more elements than this queue), the element in
414	* the array immediately following the end of the queue is set to
415	* {@code null}.
416	*
417	* <p>Like the {@link #toArray()} method, this method acts as bridge between
418	* array-based and collection-based APIs. Further, this method allows
419	* precise control over the runtime type of the output array, and may,
420	* under certain circumstances, be used to save allocation costs.
421	*
422	* <p>The following code can be used to dump a delay queue into a newly
423	* allocated array of {@code Delayed}:
424	*
425	* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
426	*
427	* Note that {@code toArray(new Object[0])} is identical in function to
428	* {@code toArray()}.
429	*
430	* @param a the array into which the elements of the queue are to
431	* be stored, if it is big enough; otherwise, a new array of the
432	* same runtime type is allocated for this purpose
433	* @return an array containing all of the elements in this queue
434	* @throws ArrayStoreException if the runtime type of the specified array
435	* is not a supertype of the runtime type of every element in
436	* this queue
437	* @throws NullPointerException if the specified array is null
438	*/
439	public <T> T[] toArray(T[] a) {
440	final ReentrantLock lock = this.lock;
441	lock.lock();
442	try {
443	return q.toArray(a);
444	} finally {
445	lock.unlock();
446	}
447	}
448
449	/**
450	* Removes a single instance of the specified element from this
451	* queue, if it is present, whether or not it has expired.
452	*/
453	public boolean remove(Object o) {
454	final ReentrantLock lock = this.lock;
455	lock.lock();
456	try {
457	return q.remove(o);
458	} finally {
459	lock.unlock();
460	}
461	}
462
463	/**
464	* Identity-based version for use in Itr.remove
465	*/
466	void removeEQ(Object o) {
467	final ReentrantLock lock = this.lock;
468	lock.lock();
469	try {
470	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
471	if (o == it.next()) {
472	it.remove();
473	break;
474	}
475	}
476	} finally {
477	lock.unlock();
478	}
479	}
480
481	/**
482	* Returns an iterator over all the elements (both expired and
483	* unexpired) in this queue. The iterator does not return the
484	* elements in any particular order.
485	*
486	* <p>The returned iterator is
487	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
488	*
489	* @return an iterator over the elements in this queue
490	*/
491	public Iterator<E> iterator() {
492	return new Itr(toArray());
493	}
494
495	/**
496	* Snapshot iterator that works off copy of underlying q array.
497	*/
498	private class Itr implements Iterator<E> {
499	final Object[] array; // Array of all elements
500	int cursor; // index of next element to return
501	int lastRet; // index of last element, or -1 if no such
502
503	Itr(Object[] array) {
504	lastRet = -1;
505	this.array = array;
506	}
507
508	public boolean hasNext() {
509	return cursor < array.length;
510	}
511
512	@SuppressWarnings("unchecked")
513	public E next() {
514	if (cursor >= array.length)
515	throw new NoSuchElementException();
516	lastRet = cursor;
517	return (E)array[cursor++];
518	}
519
520	public void remove() {
521	if (lastRet < 0)
522	throw new IllegalStateException();
523	removeEQ(array[lastRet]);
524	lastRet = -1;
525	}
526	}
527
528	}