util/concurrent/PriorityBlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;

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

/**
 * An unbounded {@linkplain BlockingQueue blocking queue} that uses
 * the same ordering rules as class {@link PriorityQueue} and supplies
 * blocking retrieval operations.  While this queue is logically
 * unbounded, attempted additions may fail due to resource exhaustion
 * (causing <tt>OutOfMemoryError</tt>). This class does not permit
 * <tt>null</tt> elements.  A priority queue relying on {@linkplain
 * Comparable natural ordering} also does not permit insertion of
 * non-comparable objects (doing so results in
 * <tt>ClassCastException</tt>).
 *
 * <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 PriorityBlockingQueue in any particular order. If you need
 * ordered traversal, consider using
 * <tt>Arrays.sort(pq.toArray())</tt>.  Also, method <tt>drainTo</tt>
 * can be used to <em>remove</em> some or all elements in priority
 * order and place them in another collection.
 *
 * <p>Operations on this class make no guarantees about the ordering
 * of elements with equal priority. If you need to enforce an
 * ordering, you can define custom classes or comparators that use a
 * secondary key to break ties in primary priority values.  For
 * example, here is a class that applies first-in-first-out
 * tie-breaking to comparable elements. To use it, you would insert a
 * <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.
 *
 *  <pre> {@code
 * class FIFOEntry<E extends Comparable<? super E>>
 *     implements Comparable<FIFOEntry<E>> {
 *   final static AtomicLong seq = new AtomicLong();
 *   final long seqNum;
 *   final E entry;
 *   public FIFOEntry(E entry) {
 *     seqNum = seq.getAndIncrement();
 *     this.entry = entry;
 *   }
 *   public E getEntry() { return entry; }
 *   public int compareTo(FIFOEntry<E> other) {
 *     int res = entry.compareTo(other.entry);
 *     if (res == 0 && other.entry != this.entry)
 *       res = (seqNum < other.seqNum ? -1 : 1);
 *     return res;
 *   }
 * }}</pre>
 *
 * <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 PriorityBlockingQueue<E> extends AbstractQueue<E>
    implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = 5595510919245408276L;

    final PriorityQueue<E> q;
    final ReentrantLock lock = new ReentrantLock(true);
    private final Condition notEmpty = lock.newCondition();

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the default
     * initial capacity (11) that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public PriorityBlockingQueue() {
        q = new PriorityQueue<E>();
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the specified
     * initial capacity that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity) {
        q = new PriorityQueue<E>(initialCapacity, null);
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> with the specified initial
     * capacity that orders its elements according to the specified
     * comparator.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @param  comparator the comparator that will be used to order this
     *         priority queue.  If {@code null}, the {@linkplain Comparable
     *         natural ordering} of the elements will be used.
     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity,
                                 Comparator<? super E> comparator) {
        q = new PriorityQueue<E>(initialCapacity, comparator);
    }

    /**
     * Creates a <tt>PriorityBlockingQueue</tt> containing the elements
     * in the specified collection.  If the specified collection is a
     * {@link SortedSet} or a {@link PriorityQueue},  this
     * priority queue will be ordered according to the same ordering.
     * Otherwise, this priority queue will be ordered according to the
     * {@linkplain Comparable natural ordering} of its elements.
     *
     * @param  c the collection whose elements are to be placed
     *         into this priority queue
     * @throws ClassCastException if elements of the specified collection
     *         cannot be compared to one another according to the priority
     *         queue's ordering
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public PriorityBlockingQueue(Collection<? extends E> c) {
        q = new PriorityQueue<E>(c);
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element into this priority queue.
     *
     * @param e the element to add
     * @return <tt>true</tt> (as specified by {@link Queue#offer})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            boolean ok = q.offer(e);
            assert ok;
            notEmpty.signal();
            return true;
        } finally {
            lock.unlock();
        }
    }

    /**
     * Inserts the specified element into this priority queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public void put(E e) {
        offer(e); // never need to block
    }

    /**
     * Inserts the specified element into this priority queue. As the queue is
     * unbounded this method will never block.
     *
     * @param e the element to add
     * @param timeout This parameter is ignored as the method never blocks
     * @param unit This parameter is ignored as the method never blocks
     * @return <tt>true</tt>
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e, long timeout, TimeUnit unit) {
        return offer(e); // never need to block
    }

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

    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            E x;
            while ( (x = q.poll()) == null)
                notEmpty.await();
            return x;
        } finally {
            lock.unlock();
        }
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            E x;
            while ( (x = q.poll()) == null) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            return x;
        } finally {
            lock.unlock();
        }
    }

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

    /**
     * Returns the comparator used to order the elements in this queue,
     * or <tt>null</tt> if this queue uses the {@linkplain Comparable
     * natural ordering} of its elements.
     *
     * @return the comparator used to order the elements in this queue,
     *         or <tt>null</tt> if this queue uses the natural
     *         ordering of its elements
     */
    public Comparator<? super E> comparator() {
        return q.comparator();
    }

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

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

    /**
     * Removes a single instance of the specified element from this queue,
     * if it is present.  More formally, removes an element {@code e} such
     * that {@code o.equals(e)}, if this queue contains one or more such
     * elements.  Returns {@code true} if and only if this queue contained
     * the specified element (or equivalently, if this queue changed as a
     * result of the call).
     *
     * @param o element to be removed from this queue, if present
     * @return <tt>true</tt> if this queue changed as a result of the call
     */
    public boolean remove(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.remove(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns {@code true} if this queue contains the specified element.
     * More formally, returns {@code true} if and only if this queue contains
     * at least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this queue
     * @return <tt>true</tt> if this queue contains the specified element
     */
    public boolean contains(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return q.contains(o);
        } finally {
            lock.unlock();
        }
    }

    /**
     * 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();
        }
    }


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

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

    /**
     * Atomically removes all of the elements from this queue.
     * The queue will be empty after this call returns.
     */
    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            q.clear();
        } finally {
            lock.unlock();
        }
    }

    /**
     * Returns an array containing all of the elements in this queue; the
     * runtime type of the returned array is that of the specified array.
     * The returned array elements are in no particular order.
     * If the queue fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this queue.
     *
     * <p>If this queue fits in the specified array with room to spare
     * (i.e., the array has more elements than this queue), the element in
     * the array immediately following the end of the queue is set to
     * <tt>null</tt>.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
     * The following code can be used to dump the queue into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of 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();
        }
    }

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

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

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

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

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

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

    /**
     * Saves the state to a stream (that is, serializes it).  This
     * merely wraps default serialization within lock.  The
     * serialization strategy for items is left to underlying
     * Queue. Note that locking is not needed on deserialization, so
     * readObject is not defined, just relying on default.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        lock.lock();
        try {
            s.defaultWriteObject();
        } finally {
            lock.unlock();
        }
    }

}
Revision:	1.56
Committed:	Mon Sep 20 20:23:52 2010 UTC (13 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.55:	+7 -7 lines
Log Message:	Fix javadoc code samples to use <pre> {@code so that we can replace < with <, etc.
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.concurrent.locks.*;
10	import java.util.*;
11
12	/**
13	* An unbounded {@linkplain BlockingQueue blocking queue} that uses
14	* the same ordering rules as class {@link PriorityQueue} and supplies
15	* blocking retrieval operations. While this queue is logically
16	* unbounded, attempted additions may fail due to resource exhaustion
17	* (causing <tt>OutOfMemoryError</tt>). This class does not permit
18	* <tt>null</tt> elements. A priority queue relying on {@linkplain
19	* Comparable natural ordering} also does not permit insertion of
20	* non-comparable objects (doing so results in
21	* <tt>ClassCastException</tt>).
22	*
23	* <p>This class and its iterator implement all of the
24	* <em>optional</em> methods of the {@link Collection} and {@link
25	* Iterator} interfaces. The Iterator provided in method {@link
26	* #iterator()} is <em>not</em> guaranteed to traverse the elements of
27	* the PriorityBlockingQueue in any particular order. If you need
28	* ordered traversal, consider using
29	* <tt>Arrays.sort(pq.toArray())</tt>. Also, method <tt>drainTo</tt>
30	* can be used to <em>remove</em> some or all elements in priority
31	* order and place them in another collection.
32	*
33	* <p>Operations on this class make no guarantees about the ordering
34	* of elements with equal priority. If you need to enforce an
35	* ordering, you can define custom classes or comparators that use a
36	* secondary key to break ties in primary priority values. For
37	* example, here is a class that applies first-in-first-out
38	* tie-breaking to comparable elements. To use it, you would insert a
39	* <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.
40	*
41	* <pre> {@code
42	* class FIFOEntry<E extends Comparable<? super E>>
43	* implements Comparable<FIFOEntry<E>> {
44	* final static AtomicLong seq = new AtomicLong();
45	* final long seqNum;
46	* final E entry;
47	* public FIFOEntry(E entry) {
48	* seqNum = seq.getAndIncrement();
49	* this.entry = entry;
50	* }
51	* public E getEntry() { return entry; }
52	* public int compareTo(FIFOEntry<E> other) {
53	* int res = entry.compareTo(other.entry);
54	* if (res == 0 && other.entry != this.entry)
55	* res = (seqNum < other.seqNum ? -1 : 1);
56	* return res;
57	* }
58	* }}</pre>
59	*
60	* <p>This class is a member of the
61	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
62	* Java Collections Framework</a>.
63	*
64	* @since 1.5
65	* @author Doug Lea
66	* @param <E> the type of elements held in this collection
67	*/
68	public class PriorityBlockingQueue<E> extends AbstractQueue<E>
69	implements BlockingQueue<E>, java.io.Serializable {
70	private static final long serialVersionUID = 5595510919245408276L;
71
72	final PriorityQueue<E> q;
73	final ReentrantLock lock = new ReentrantLock(true);
74	private final Condition notEmpty = lock.newCondition();
75
76	/**
77	* Creates a <tt>PriorityBlockingQueue</tt> with the default
78	* initial capacity (11) that orders its elements according to
79	* their {@linkplain Comparable natural ordering}.
80	*/
81	public PriorityBlockingQueue() {
82	q = new PriorityQueue<E>();
83	}
84
85	/**
86	* Creates a <tt>PriorityBlockingQueue</tt> with the specified
87	* initial capacity that orders its elements according to their
88	* {@linkplain Comparable natural ordering}.
89	*
90	* @param initialCapacity the initial capacity for this priority queue
91	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
92	* than 1
93	*/
94	public PriorityBlockingQueue(int initialCapacity) {
95	q = new PriorityQueue<E>(initialCapacity, null);
96	}
97
98	/**
99	* Creates a <tt>PriorityBlockingQueue</tt> with the specified initial
100	* capacity that orders its elements according to the specified
101	* comparator.
102	*
103	* @param initialCapacity the initial capacity for this priority queue
104	* @param comparator the comparator that will be used to order this
105	* priority queue. If {@code null}, the {@linkplain Comparable
106	* natural ordering} of the elements will be used.
107	* @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
108	* than 1
109	*/
110	public PriorityBlockingQueue(int initialCapacity,
111	Comparator<? super E> comparator) {
112	q = new PriorityQueue<E>(initialCapacity, comparator);
113	}
114
115	/**
116	* Creates a <tt>PriorityBlockingQueue</tt> containing the elements
117	* in the specified collection. If the specified collection is a
118	* {@link SortedSet} or a {@link PriorityQueue}, this
119	* priority queue will be ordered according to the same ordering.
120	* Otherwise, this priority queue will be ordered according to the
121	* {@linkplain Comparable natural ordering} of its elements.
122	*
123	* @param c the collection whose elements are to be placed
124	* into this priority queue
125	* @throws ClassCastException if elements of the specified collection
126	* cannot be compared to one another according to the priority
127	* queue's ordering
128	* @throws NullPointerException if the specified collection or any
129	* of its elements are null
130	*/
131	public PriorityBlockingQueue(Collection<? extends E> c) {
132	q = new PriorityQueue<E>(c);
133	}
134
135	/**
136	* Inserts the specified element into this priority queue.
137	*
138	* @param e the element to add
139	* @return <tt>true</tt> (as specified by {@link Collection#add})
140	* @throws ClassCastException if the specified element cannot be compared
141	* with elements currently in the priority queue according to the
142	* priority queue's ordering
143	* @throws NullPointerException if the specified element is null
144	*/
145	public boolean add(E e) {
146	return offer(e);
147	}
148
149	/**
150	* Inserts the specified element into this priority queue.
151	*
152	* @param e the element to add
153	* @return <tt>true</tt> (as specified by {@link Queue#offer})
154	* @throws ClassCastException if the specified element cannot be compared
155	* with elements currently in the priority queue according to the
156	* priority queue's ordering
157	* @throws NullPointerException if the specified element is null
158	*/
159	public boolean offer(E e) {
160	final ReentrantLock lock = this.lock;
161	lock.lock();
162	try {
163	boolean ok = q.offer(e);
164	assert ok;
165	notEmpty.signal();
166	return true;
167	} finally {
168	lock.unlock();
169	}
170	}
171
172	/**
173	* Inserts the specified element into this priority queue. As the queue is
174	* unbounded this method will never block.
175	*
176	* @param e the element to add
177	* @throws ClassCastException if the specified element cannot be compared
178	* with elements currently in the priority queue according to the
179	* priority queue's ordering
180	* @throws NullPointerException if the specified element is null
181	*/
182	public void put(E e) {
183	offer(e); // never need to block
184	}
185
186	/**
187	* Inserts the specified element into this priority queue. As the queue is
188	* unbounded this method will never block.
189	*
190	* @param e the element to add
191	* @param timeout This parameter is ignored as the method never blocks
192	* @param unit This parameter is ignored as the method never blocks
193	* @return <tt>true</tt>
194	* @throws ClassCastException if the specified element cannot be compared
195	* with elements currently in the priority queue according to the
196	* priority queue's ordering
197	* @throws NullPointerException if the specified element is null
198	*/
199	public boolean offer(E e, long timeout, TimeUnit unit) {
200	return offer(e); // never need to block
201	}
202
203	public E poll() {
204	final ReentrantLock lock = this.lock;
205	lock.lock();
206	try {
207	return q.poll();
208	} finally {
209	lock.unlock();
210	}
211	}
212
213	public E take() throws InterruptedException {
214	final ReentrantLock lock = this.lock;
215	lock.lockInterruptibly();
216	try {
217	E x;
218	while ( (x = q.poll()) == null)
219	notEmpty.await();
220	return x;
221	} finally {
222	lock.unlock();
223	}
224	}
225
226	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
227	long nanos = unit.toNanos(timeout);
228	final ReentrantLock lock = this.lock;
229	lock.lockInterruptibly();
230	try {
231	E x;
232	while ( (x = q.poll()) == null) {
233	if (nanos <= 0)
234	return null;
235	nanos = notEmpty.awaitNanos(nanos);
236	}
237	return x;
238	} finally {
239	lock.unlock();
240	}
241	}
242
243	public E peek() {
244	final ReentrantLock lock = this.lock;
245	lock.lock();
246	try {
247	return q.peek();
248	} finally {
249	lock.unlock();
250	}
251	}
252
253	/**
254	* Returns the comparator used to order the elements in this queue,
255	* or <tt>null</tt> if this queue uses the {@linkplain Comparable
256	* natural ordering} of its elements.
257	*
258	* @return the comparator used to order the elements in this queue,
259	* or <tt>null</tt> if this queue uses the natural
260	* ordering of its elements
261	*/
262	public Comparator<? super E> comparator() {
263	return q.comparator();
264	}
265
266	public int size() {
267	final ReentrantLock lock = this.lock;
268	lock.lock();
269	try {
270	return q.size();
271	} finally {
272	lock.unlock();
273	}
274	}
275
276	/**
277	* Always returns <tt>Integer.MAX_VALUE</tt> because
278	* a <tt>PriorityBlockingQueue</tt> is not capacity constrained.
279	* @return <tt>Integer.MAX_VALUE</tt>
280	*/
281	public int remainingCapacity() {
282	return Integer.MAX_VALUE;
283	}
284
285	/**
286	* Removes a single instance of the specified element from this queue,
287	* if it is present. More formally, removes an element {@code e} such
288	* that {@code o.equals(e)}, if this queue contains one or more such
289	* elements. Returns {@code true} if and only if this queue contained
290	* the specified element (or equivalently, if this queue changed as a
291	* result of the call).
292	*
293	* @param o element to be removed from this queue, if present
294	* @return <tt>true</tt> if this queue changed as a result of the call
295	*/
296	public boolean remove(Object o) {
297	final ReentrantLock lock = this.lock;
298	lock.lock();
299	try {
300	return q.remove(o);
301	} finally {
302	lock.unlock();
303	}
304	}
305
306	/**
307	* Returns {@code true} if this queue contains the specified element.
308	* More formally, returns {@code true} if and only if this queue contains
309	* at least one element {@code e} such that {@code o.equals(e)}.
310	*
311	* @param o object to be checked for containment in this queue
312	* @return <tt>true</tt> if this queue contains the specified element
313	*/
314	public boolean contains(Object o) {
315	final ReentrantLock lock = this.lock;
316	lock.lock();
317	try {
318	return q.contains(o);
319	} finally {
320	lock.unlock();
321	}
322	}
323
324	/**
325	* Returns an array containing all of the elements in this queue.
326	* The returned array elements are in no particular order.
327	*
328	* <p>The returned array will be "safe" in that no references to it are
329	* maintained by this queue. (In other words, this method must allocate
330	* a new array). The caller is thus free to modify the returned array.
331	*
332	* <p>This method acts as bridge between array-based and collection-based
333	* APIs.
334	*
335	* @return an array containing all of the elements in this queue
336	*/
337	public Object[] toArray() {
338	final ReentrantLock lock = this.lock;
339	lock.lock();
340	try {
341	return q.toArray();
342	} finally {
343	lock.unlock();
344	}
345	}
346
347
348	public String toString() {
349	final ReentrantLock lock = this.lock;
350	lock.lock();
351	try {
352	return q.toString();
353	} finally {
354	lock.unlock();
355	}
356	}
357
358	/**
359	* @throws UnsupportedOperationException {@inheritDoc}
360	* @throws ClassCastException {@inheritDoc}
361	* @throws NullPointerException {@inheritDoc}
362	* @throws IllegalArgumentException {@inheritDoc}
363	*/
364	public int drainTo(Collection<? super E> c) {
365	if (c == null)
366	throw new NullPointerException();
367	if (c == this)
368	throw new IllegalArgumentException();
369	final ReentrantLock lock = this.lock;
370	lock.lock();
371	try {
372	int n = 0;
373	E e;
374	while ( (e = q.poll()) != null) {
375	c.add(e);
376	++n;
377	}
378	return n;
379	} finally {
380	lock.unlock();
381	}
382	}
383
384	/**
385	* @throws UnsupportedOperationException {@inheritDoc}
386	* @throws ClassCastException {@inheritDoc}
387	* @throws NullPointerException {@inheritDoc}
388	* @throws IllegalArgumentException {@inheritDoc}
389	*/
390	public int drainTo(Collection<? super E> c, int maxElements) {
391	if (c == null)
392	throw new NullPointerException();
393	if (c == this)
394	throw new IllegalArgumentException();
395	if (maxElements <= 0)
396	return 0;
397	final ReentrantLock lock = this.lock;
398	lock.lock();
399	try {
400	int n = 0;
401	E e;
402	while (n < maxElements && (e = q.poll()) != null) {
403	c.add(e);
404	++n;
405	}
406	return n;
407	} finally {
408	lock.unlock();
409	}
410	}
411
412	/**
413	* Atomically removes all of the elements from this queue.
414	* The queue will be empty after this call returns.
415	*/
416	public void clear() {
417	final ReentrantLock lock = this.lock;
418	lock.lock();
419	try {
420	q.clear();
421	} finally {
422	lock.unlock();
423	}
424	}
425
426	/**
427	* Returns an array containing all of the elements in this queue; the
428	* runtime type of the returned array is that of the specified array.
429	* The returned array elements are in no particular order.
430	* If the queue fits in the specified array, it is returned therein.
431	* Otherwise, a new array is allocated with the runtime type of the
432	* specified array and the size of this queue.
433	*
434	* <p>If this queue fits in the specified array with room to spare
435	* (i.e., the array has more elements than this queue), the element in
436	* the array immediately following the end of the queue is set to
437	* <tt>null</tt>.
438	*
439	* <p>Like the {@link #toArray()} method, this method acts as bridge between
440	* array-based and collection-based APIs. Further, this method allows
441	* precise control over the runtime type of the output array, and may,
442	* under certain circumstances, be used to save allocation costs.
443	*
444	* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
445	* The following code can be used to dump the queue into a newly
446	* allocated array of <tt>String</tt>:
447	*
448	* <pre>
449	* String[] y = x.toArray(new String[0]);</pre>
450	*
451	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
452	* <tt>toArray()</tt>.
453	*
454	* @param a the array into which the elements of the queue are to
455	* be stored, if it is big enough; otherwise, a new array of the
456	* same runtime type is allocated for this purpose
457	* @return an array containing all of the elements in this queue
458	* @throws ArrayStoreException if the runtime type of the specified array
459	* is not a supertype of the runtime type of every element in
460	* this queue
461	* @throws NullPointerException if the specified array is null
462	*/
463	public <T> T[] toArray(T[] a) {
464	final ReentrantLock lock = this.lock;
465	lock.lock();
466	try {
467	return q.toArray(a);
468	} finally {
469	lock.unlock();
470	}
471	}
472
473	/**
474	* Returns an iterator over the elements in this queue. The
475	* iterator does not return the elements in any particular order.
476	* The returned <tt>Iterator</tt> is a "weakly consistent"
477	* iterator that will never throw {@link
478	* ConcurrentModificationException}, and guarantees to traverse
479	* elements as they existed upon construction of the iterator, and
480	* may (but is not guaranteed to) reflect any modifications
481	* subsequent to construction.
482	*
483	* @return an iterator over the elements in this queue
484	*/
485	public Iterator<E> iterator() {
486	return new Itr(toArray());
487	}
488
489	/**
490	* Snapshot iterator that works off copy of underlying q array.
491	*/
492	private class Itr implements Iterator<E> {
493	final Object[] array; // Array of all elements
494	int cursor; // index of next element to return;
495	int lastRet; // index of last element, or -1 if no such
496
497	Itr(Object[] array) {
498	lastRet = -1;
499	this.array = array;
500	}
501
502	public boolean hasNext() {
503	return cursor < array.length;
504	}
505
506	public E next() {
507	if (cursor >= array.length)
508	throw new NoSuchElementException();
509	lastRet = cursor;
510	return (E)array[cursor++];
511	}
512
513	public void remove() {
514	if (lastRet < 0)
515	throw new IllegalStateException();
516	Object x = array[lastRet];
517	lastRet = -1;
518	// Traverse underlying queue to find == element,
519	// not just a .equals element.
520	lock.lock();
521	try {
522	for (Iterator it = q.iterator(); it.hasNext(); ) {
523	if (it.next() == x) {
524	it.remove();
525	return;
526	}
527	}
528	} finally {
529	lock.unlock();
530	}
531	}
532	}
533
534	/**
535	* Saves the state to a stream (that is, serializes it). This
536	* merely wraps default serialization within lock. The
537	* serialization strategy for items is left to underlying
538	* Queue. Note that locking is not needed on deserialization, so
539	* readObject is not defined, just relying on default.
540	*/
541	private void writeObject(java.io.ObjectOutputStream s)
542	throws java.io.IOException {
543	lock.lock();
544	try {
545	s.defaultWriteObject();
546	} finally {
547	lock.unlock();
548	}
549	}
550
551	}