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.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.PriorityQueue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * 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 queue
 */
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();
            return (first == null || first.getDelay(NANOSECONDS) > 0)
                ? null
                : 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 <= 0L)
                        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 <= 0L)
                        return null;
                    else
                        nanos = available.awaitNanos(nanos);
                } else {
                    long delay = first.getDelay(NANOSECONDS);
                    if (delay <= 0L)
                        return q.poll();
                    if (nanos <= 0L)
                        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) {
        Objects.requireNonNull(c);
        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) {
        Objects.requireNonNull(c);
        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();
            return (E)array[lastRet = cursor++];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            removeEQ(array[lastRet]);
            lastRet = -1;
        }
    }

}
Revision:	1.75
Committed:	Thu Dec 29 22:37:31 2016 UTC (7 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.74:	+5 -5 lines
Log Message:	whitespace
#	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
11	import java.util.AbstractQueue;
12	import java.util.Collection;
13	import java.util.Iterator;
14	import java.util.NoSuchElementException;
15	import java.util.Objects;
16	import java.util.PriorityQueue;
17	import java.util.concurrent.locks.Condition;
18	import java.util.concurrent.locks.ReentrantLock;
19
20	/**
21	* An unbounded {@linkplain BlockingQueue blocking queue} of
22	* {@code Delayed} elements, in which an element can only be taken
23	* when its delay has expired. The <em>head</em> of the queue is that
24	* {@code Delayed} element whose delay expired furthest in the
25	* past. If no delay has expired there is no head and {@code poll}
26	* will return {@code null}. Expiration occurs when an element's
27	* {@code getDelay(TimeUnit.NANOSECONDS)} method returns a value less
28	* than or equal to zero. Even though unexpired elements cannot be
29	* removed using {@code take} or {@code poll}, they are otherwise
30	* treated as normal elements. For example, the {@code size} method
31	* returns the count of both expired and unexpired elements.
32	* This queue does not permit null elements.
33	*
34	* <p>This class and its iterator implement all of the <em>optional</em>
35	* methods of the {@link Collection} and {@link Iterator} interfaces.
36	* The Iterator provided in method {@link #iterator()} is <em>not</em>
37	* guaranteed to traverse the elements of the DelayQueue in any
38	* particular order.
39	*
40	* <p>This class is a member of the
41	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
42	* Java Collections Framework</a>.
43	*
44	* @since 1.5
45	* @author Doug Lea
46	* @param <E> the type of elements held in this queue
47	*/
48	public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
49	implements BlockingQueue<E> {
50
51	private final transient ReentrantLock lock = new ReentrantLock();
52	private final PriorityQueue<E> q = new PriorityQueue<E>();
53
54	/**
55	* Thread designated to wait for the element at the head of
56	* the queue. This variant of the Leader-Follower pattern
57	* (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to
58	* minimize unnecessary timed waiting. When a thread becomes
59	* the leader, it waits only for the next delay to elapse, but
60	* other threads await indefinitely. The leader thread must
61	* signal some other thread before returning from take() or
62	* poll(...), unless some other thread becomes leader in the
63	* interim. Whenever the head of the queue is replaced with
64	* an element with an earlier expiration time, the leader
65	* field is invalidated by being reset to null, and some
66	* waiting thread, but not necessarily the current leader, is
67	* signalled. So waiting threads must be prepared to acquire
68	* and lose leadership while waiting.
69	*/
70	private Thread leader;
71
72	/**
73	* Condition signalled when a newer element becomes available
74	* at the head of the queue or a new thread may need to
75	* become leader.
76	*/
77	private final Condition available = lock.newCondition();
78
79	/**
80	* Creates a new {@code DelayQueue} that is initially empty.
81	*/
82	public DelayQueue() {}
83
84	/**
85	* Creates a {@code DelayQueue} initially containing the elements of the
86	* given collection of {@link Delayed} instances.
87	*
88	* @param c the collection of elements to initially contain
89	* @throws NullPointerException if the specified collection or any
90	* of its elements are null
91	*/
92	public DelayQueue(Collection<? extends E> c) {
93	this.addAll(c);
94	}
95
96	/**
97	* Inserts the specified element into this delay queue.
98	*
99	* @param e the element to add
100	* @return {@code true} (as specified by {@link Collection#add})
101	* @throws NullPointerException if the specified element is null
102	*/
103	public boolean add(E e) {
104	return offer(e);
105	}
106
107	/**
108	* Inserts the specified element into this delay queue.
109	*
110	* @param e the element to add
111	* @return {@code true}
112	* @throws NullPointerException if the specified element is null
113	*/
114	public boolean offer(E e) {
115	final ReentrantLock lock = this.lock;
116	lock.lock();
117	try {
118	q.offer(e);
119	if (q.peek() == e) {
120	leader = null;
121	available.signal();
122	}
123	return true;
124	} finally {
125	lock.unlock();
126	}
127	}
128
129	/**
130	* Inserts the specified element into this delay queue. As the queue is
131	* unbounded this method will never block.
132	*
133	* @param e the element to add
134	* @throws NullPointerException {@inheritDoc}
135	*/
136	public void put(E e) {
137	offer(e);
138	}
139
140	/**
141	* Inserts the specified element into this delay queue. As the queue is
142	* unbounded this method will never block.
143	*
144	* @param e the element to add
145	* @param timeout This parameter is ignored as the method never blocks
146	* @param unit This parameter is ignored as the method never blocks
147	* @return {@code true}
148	* @throws NullPointerException {@inheritDoc}
149	*/
150	public boolean offer(E e, long timeout, TimeUnit unit) {
151	return offer(e);
152	}
153
154	/**
155	* Retrieves and removes the head of this queue, or returns {@code null}
156	* if this queue has no elements with an expired delay.
157	*
158	* @return the head of this queue, or {@code null} if this
159	* queue has no elements with an expired delay
160	*/
161	public E poll() {
162	final ReentrantLock lock = this.lock;
163	lock.lock();
164	try {
165	E first = q.peek();
166	return (first == null \|\| first.getDelay(NANOSECONDS) > 0)
167	? null
168	: q.poll();
169	} finally {
170	lock.unlock();
171	}
172	}
173
174	/**
175	* Retrieves and removes the head of this queue, waiting if necessary
176	* until an element with an expired delay is available on this queue.
177	*
178	* @return the head of this queue
179	* @throws InterruptedException {@inheritDoc}
180	*/
181	public E take() throws InterruptedException {
182	final ReentrantLock lock = this.lock;
183	lock.lockInterruptibly();
184	try {
185	for (;;) {
186	E first = q.peek();
187	if (first == null)
188	available.await();
189	else {
190	long delay = first.getDelay(NANOSECONDS);
191	if (delay <= 0L)
192	return q.poll();
193	first = null; // don't retain ref while waiting
194	if (leader != null)
195	available.await();
196	else {
197	Thread thisThread = Thread.currentThread();
198	leader = thisThread;
199	try {
200	available.awaitNanos(delay);
201	} finally {
202	if (leader == thisThread)
203	leader = null;
204	}
205	}
206	}
207	}
208	} finally {
209	if (leader == null && q.peek() != null)
210	available.signal();
211	lock.unlock();
212	}
213	}
214
215	/**
216	* Retrieves and removes the head of this queue, waiting if necessary
217	* until an element with an expired delay is available on this queue,
218	* or the specified wait time expires.
219	*
220	* @return the head of this queue, or {@code null} if the
221	* specified waiting time elapses before an element with
222	* an expired delay becomes available
223	* @throws InterruptedException {@inheritDoc}
224	*/
225	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
226	long nanos = unit.toNanos(timeout);
227	final ReentrantLock lock = this.lock;
228	lock.lockInterruptibly();
229	try {
230	for (;;) {
231	E first = q.peek();
232	if (first == null) {
233	if (nanos <= 0L)
234	return null;
235	else
236	nanos = available.awaitNanos(nanos);
237	} else {
238	long delay = first.getDelay(NANOSECONDS);
239	if (delay <= 0L)
240	return q.poll();
241	if (nanos <= 0L)
242	return null;
243	first = null; // don't retain ref while waiting
244	if (nanos < delay \|\| leader != null)
245	nanos = available.awaitNanos(nanos);
246	else {
247	Thread thisThread = Thread.currentThread();
248	leader = thisThread;
249	try {
250	long timeLeft = available.awaitNanos(delay);
251	nanos -= delay - timeLeft;
252	} finally {
253	if (leader == thisThread)
254	leader = null;
255	}
256	}
257	}
258	}
259	} finally {
260	if (leader == null && q.peek() != null)
261	available.signal();
262	lock.unlock();
263	}
264	}
265
266	/**
267	* Retrieves, but does not remove, the head of this queue, or
268	* returns {@code null} if this queue is empty. Unlike
269	* {@code poll}, if no expired elements are available in the queue,
270	* this method returns the element that will expire next,
271	* if one exists.
272	*
273	* @return the head of this queue, or {@code null} if this
274	* queue is empty
275	*/
276	public E peek() {
277	final ReentrantLock lock = this.lock;
278	lock.lock();
279	try {
280	return q.peek();
281	} finally {
282	lock.unlock();
283	}
284	}
285
286	public int size() {
287	final ReentrantLock lock = this.lock;
288	lock.lock();
289	try {
290	return q.size();
291	} finally {
292	lock.unlock();
293	}
294	}
295
296	/**
297	* Returns first element only if it is expired.
298	* Used only by drainTo. Call only when holding lock.
299	*/
300	private E peekExpired() {
301	// assert lock.isHeldByCurrentThread();
302	E first = q.peek();
303	return (first == null \|\| first.getDelay(NANOSECONDS) > 0) ?
304	null : first;
305	}
306
307	/**
308	* @throws UnsupportedOperationException {@inheritDoc}
309	* @throws ClassCastException {@inheritDoc}
310	* @throws NullPointerException {@inheritDoc}
311	* @throws IllegalArgumentException {@inheritDoc}
312	*/
313	public int drainTo(Collection<? super E> c) {
314	Objects.requireNonNull(c);
315	if (c == this)
316	throw new IllegalArgumentException();
317	final ReentrantLock lock = this.lock;
318	lock.lock();
319	try {
320	int n = 0;
321	for (E e; (e = peekExpired()) != null;) {
322	c.add(e); // In this order, in case add() throws.
323	q.poll();
324	++n;
325	}
326	return n;
327	} finally {
328	lock.unlock();
329	}
330	}
331
332	/**
333	* @throws UnsupportedOperationException {@inheritDoc}
334	* @throws ClassCastException {@inheritDoc}
335	* @throws NullPointerException {@inheritDoc}
336	* @throws IllegalArgumentException {@inheritDoc}
337	*/
338	public int drainTo(Collection<? super E> c, int maxElements) {
339	Objects.requireNonNull(c);
340	if (c == this)
341	throw new IllegalArgumentException();
342	if (maxElements <= 0)
343	return 0;
344	final ReentrantLock lock = this.lock;
345	lock.lock();
346	try {
347	int n = 0;
348	for (E e; n < maxElements && (e = peekExpired()) != null;) {
349	c.add(e); // In this order, in case add() throws.
350	q.poll();
351	++n;
352	}
353	return n;
354	} finally {
355	lock.unlock();
356	}
357	}
358
359	/**
360	* Atomically removes all of the elements from this delay queue.
361	* The queue will be empty after this call returns.
362	* Elements with an unexpired delay are not waited for; they are
363	* simply discarded from the queue.
364	*/
365	public void clear() {
366	final ReentrantLock lock = this.lock;
367	lock.lock();
368	try {
369	q.clear();
370	} finally {
371	lock.unlock();
372	}
373	}
374
375	/**
376	* Always returns {@code Integer.MAX_VALUE} because
377	* a {@code DelayQueue} is not capacity constrained.
378	*
379	* @return {@code Integer.MAX_VALUE}
380	*/
381	public int remainingCapacity() {
382	return Integer.MAX_VALUE;
383	}
384
385	/**
386	* Returns an array containing all of the elements in this queue.
387	* The returned array elements are in no particular order.
388	*
389	* <p>The returned array will be "safe" in that no references to it are
390	* maintained by this queue. (In other words, this method must allocate
391	* a new array). The caller is thus free to modify the returned array.
392	*
393	* <p>This method acts as bridge between array-based and collection-based
394	* APIs.
395	*
396	* @return an array containing all of the elements in this queue
397	*/
398	public Object[] toArray() {
399	final ReentrantLock lock = this.lock;
400	lock.lock();
401	try {
402	return q.toArray();
403	} finally {
404	lock.unlock();
405	}
406	}
407
408	/**
409	* Returns an array containing all of the elements in this queue; the
410	* runtime type of the returned array is that of the specified array.
411	* The returned array elements are in no particular order.
412	* If the queue fits in the specified array, it is returned therein.
413	* Otherwise, a new array is allocated with the runtime type of the
414	* specified array and the size of this queue.
415	*
416	* <p>If this queue fits in the specified array with room to spare
417	* (i.e., the array has more elements than this queue), the element in
418	* the array immediately following the end of the queue is set to
419	* {@code null}.
420	*
421	* <p>Like the {@link #toArray()} method, this method acts as bridge between
422	* array-based and collection-based APIs. Further, this method allows
423	* precise control over the runtime type of the output array, and may,
424	* under certain circumstances, be used to save allocation costs.
425	*
426	* <p>The following code can be used to dump a delay queue into a newly
427	* allocated array of {@code Delayed}:
428	*
429	* <pre> {@code Delayed[] a = q.toArray(new Delayed[0]);}</pre>
430	*
431	* Note that {@code toArray(new Object[0])} is identical in function to
432	* {@code toArray()}.
433	*
434	* @param a the array into which the elements of the queue are to
435	* be stored, if it is big enough; otherwise, a new array of the
436	* same runtime type is allocated for this purpose
437	* @return an array containing all of the elements in this queue
438	* @throws ArrayStoreException if the runtime type of the specified array
439	* is not a supertype of the runtime type of every element in
440	* this queue
441	* @throws NullPointerException if the specified array is null
442	*/
443	public <T> T[] toArray(T[] a) {
444	final ReentrantLock lock = this.lock;
445	lock.lock();
446	try {
447	return q.toArray(a);
448	} finally {
449	lock.unlock();
450	}
451	}
452
453	/**
454	* Removes a single instance of the specified element from this
455	* queue, if it is present, whether or not it has expired.
456	*/
457	public boolean remove(Object o) {
458	final ReentrantLock lock = this.lock;
459	lock.lock();
460	try {
461	return q.remove(o);
462	} finally {
463	lock.unlock();
464	}
465	}
466
467	/**
468	* Identity-based version for use in Itr.remove.
469	*/
470	void removeEQ(Object o) {
471	final ReentrantLock lock = this.lock;
472	lock.lock();
473	try {
474	for (Iterator<E> it = q.iterator(); it.hasNext(); ) {
475	if (o == it.next()) {
476	it.remove();
477	break;
478	}
479	}
480	} finally {
481	lock.unlock();
482	}
483	}
484
485	/**
486	* Returns an iterator over all the elements (both expired and
487	* unexpired) in this queue. The iterator does not return the
488	* elements in any particular order.
489	*
490	* <p>The returned iterator is
491	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
492	*
493	* @return an iterator over the elements in this queue
494	*/
495	public Iterator<E> iterator() {
496	return new Itr(toArray());
497	}
498
499	/**
500	* Snapshot iterator that works off copy of underlying q array.
501	*/
502	private class Itr implements Iterator<E> {
503	final Object[] array; // Array of all elements
504	int cursor; // index of next element to return
505	int lastRet; // index of last element, or -1 if no such
506
507	Itr(Object[] array) {
508	lastRet = -1;
509	this.array = array;
510	}
511
512	public boolean hasNext() {
513	return cursor < array.length;
514	}
515
516	@SuppressWarnings("unchecked")
517	public E next() {
518	if (cursor >= array.length)
519	throw new NoSuchElementException();
520	return (E)array[lastRet = cursor++];
521	}
522
523	public void remove() {
524	if (lastRet < 0)
525	throw new IllegalStateException();
526	removeEQ(array[lastRet]);
527	lastRet = -1;
528	}
529	}
530
531	}