util/concurrent/ArrayBlockingQueue.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain. Use, modify, and
 * redistribute this code in any way without acknowledgement.
 */

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

/**
 * A bounded {@link BlockingQueue blocking queue} backed by an array.
 * This queue orders elements FIFO (first-in-first-out).
 * The <em>head</em> of the queue is that element that has been on the
 * queue the longest time.
 * The <em>tail</em> of the queue is that element that has been on the
 * queue the shortest time.
 *
 * <p>This is a classic &quot;bounded buffer&quot;, in which a fixed-sized
 * array holds
 * elements inserted by producers and extracted by consumers.  Once
 * created, the capacity can not be increased.  Attempts to offer an
 * element to a full queue will result in the offer operation
 * blocking; attempts to retrieve an element from an empty queue will
 * similarly block.
 *
 * <p> This class supports an optional fairness policy for ordering
 * threads blocked on an insertion or removal.  By default, this
 * ordering is not guaranteed. However, an <tt>ArrayBlockingQueue</tt>
 * constructed with fairness set to <tt>true</tt> grants blocked
 * threads access in FIFO order. Fairness generally substantially
 * decreases throughput but reduces variablility and avoids
 * starvation.
 *
 * @since 1.5
 * @author Doug Lea
 */
public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /** The queued items */
    private transient final E[] items;
    /** items index for next take, poll or remove */
    private transient int takeIndex;
    /** items index for next put, offer, or add. */
    private transient int putIndex;
    /** Number of items in the queue */
    private int count;

    /**
     * An array used only during deserialization, to hold
     * items read back in from the stream, and then used
     * as "items" by readResolve via the private constructor.
     */
    private transient E[] deserializedItems;

    /*
     * Concurrency control via the classic two-condition algorithm
     * found in any textbook.
     */

    /** Main lock guarding all access */
    private final ReentrantLock lock;
    /** Condition for waiting takes */
    private final Condition notEmpty;
    /** Condition for wiating puts */
    private final Condition notFull;

    // Internal helper methods

    /**
     * Circularly increment i.
     */
    int inc(int i) {
        return (++i == items.length)? 0 : i;
    }

    /**
     * Insert element at current put position, advance, and signal.
     * Call only when holding lock.
     */
    private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
        notEmpty.signal();
    }

    /**
     * Extract element at current take position, advance, and signal.
     * Call only when holding lock.
     */
    private E extract() {
        E x = items[takeIndex];
        items[takeIndex] = null;
        takeIndex = inc(takeIndex);
        --count;
        notFull.signal();
        return x;
    }

    /**
     * Utility for remove and iterator.remove: Delete item at position i.
     * Call only when holding lock.
     */
    void removeAt(int i) {
        // if removing front item, just advance
        if (i == takeIndex) {
            items[takeIndex] = null;
            takeIndex = inc(takeIndex);
        }
        else {
            // slide over all others up through putIndex.
            for (;;) {
                int nexti = inc(i);
                if (nexti != putIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                }
                else {
                    items[i] = null;
                    putIndex = i;
                    break;
                }
            }
        }
        --count;
        notFull.signal();
    }

    /**
     * Internal constructor also used by readResolve.
     * Sets all final fields, plus count.
     * @param cap the capacity
     * @param array the array to use or null if should create new one
     * @param count the number of items in the array, where indices 0
     * to count-1 hold items.
     */
    private ArrayBlockingQueue(int cap, E[] array, int count,
                               ReentrantLock lk) {
        if (cap <= 0)
            throw new IllegalArgumentException();
        if (array == null)
            this.items = (E[]) new Object[cap];
        else
            this.items = array;
        this.putIndex = count;
        this.count = count;
        lock = lk;
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

    /**
     * Create an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity and default access policy.
     * @param capacity the capacity of this queue
     */
    public ArrayBlockingQueue(int capacity) {
        this(capacity, null, 0, new ReentrantLock());
    }

    /**
     * Create an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity and the specified access policy.
     * @param capacity the capacity of this queue
     * @param fair if <tt>true</tt> then queue accesses for threads blocked
     * on insertion or removal, are processed in FIFO order; if <tt>false</tt>
     * the access order is unspecified.
     */
    public ArrayBlockingQueue(int capacity, boolean fair) {
        this(capacity, null, 0, new ReentrantLock(fair));
    }


    // Have to override just to update the javadoc for @throws

    /**
     * @throws IllegalStateException {@inheritDoc}
     */
    public boolean add(E x) {
        return super.add(x);
    }

    /**
     * @throws IllegalStateException {@inheritDoc}
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }

   /**
    * Add the specified element to the tail of this queue if possible,
    * returning immediately if this queue is full.
    *
    * @throws NullPointerException {@inheritDoc}
    */
    public boolean offer(E x) {
        if (x == null) throw new NullPointerException();
        lock.lock();
        try {
            if (count == items.length)
                return false;
            else {
                insert(x);
                return true;
            }
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Add the specified element to the tail of this queue, waiting if
     * necessary up to the specified wait time for space to become available.
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E x, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (x == null) throw new NullPointerException();

        lock.lockInterruptibly();
        try {
            long nanos = unit.toNanos(timeout);
            for (;;) {
                if (count != items.length) {
                    insert(x);
                    return true;
                }
                if (nanos <= 0)
                    return false;
                try {
                    nanos = notFull.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notFull.signal(); // propagate to non-interrupted thread
                    throw ie;
                }
            }
        }
        finally {
            lock.unlock();
        }
    }


    public E poll() {
        lock.lock();
        try {
            if (count == 0)
                return null;
            E x = extract();
            return x;
        }
        finally {
            lock.unlock();
        }
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        lock.lockInterruptibly();
        try {
            long nanos = unit.toNanos(timeout);
            for (;;) {
                if (count != 0) {
                    E x = extract();
                    return x;
                }
                if (nanos <= 0)
                    return null;
                try {
                    nanos = notEmpty.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to non-interrupted thread
                    throw ie;
                }

            }
        }
        finally {
            lock.unlock();
        }
    }


    public boolean remove(Object x) {
        if (x == null) return false;
        lock.lock();
        try {
            int i = takeIndex;
            int k = 0;
            for (;;) {
                if (k++ >= count)
                    return false;
                if (x.equals(items[i])) {
                    removeAt(i);
                    return true;
                }
                i = inc(i);
            }

        }
        finally {
            lock.unlock();
        }
    }

    public E peek() {
        lock.lock();
        try {
            return (count == 0) ? null : items[takeIndex];
        }
        finally {
            lock.unlock();
        }
    }

    public E take() throws InterruptedException {
        lock.lockInterruptibly();
        try {
            try {
                while (count == 0)
                    notEmpty.await();
            }
            catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            E x = extract();
            return x;
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Add the specified element to the tail of this queue, waiting if
     * necessary for space to become available.
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E x) throws InterruptedException {

        if (x == null) throw new NullPointerException();

        lock.lockInterruptibly();
        try {
            try {
                while (count == items.length)
                    notFull.await();
            }
            catch (InterruptedException ie) {
                notFull.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            insert(x);
        }
        finally {
            lock.unlock();
        }
    }

    // this doc comment is overridden to remove the reference to collections
    // greater in size than Integer.MAX_VALUE
    /**
     * Return the number of elements in this collection.
     */
    public int size() {
        lock.lock();
        try {
            return count;
        }
        finally {
            lock.unlock();
        }
    }

    // this doc comment is a modified copy of the inherited doc comment,
    // without the reference to unlimited queues.
    /**
     * Return the number of elements that this queue can ideally (in
     * the absence of memory or resource constraints) accept without
     * blocking. This is always equal to the initial capacity of this queue
     * less the current <tt>size</tt> of this queue.
     * <p>Note that you <em>cannot</em> always tell if
     * an attempt to <tt>add</tt> an element will succeed by
     * inspecting <tt>remainingCapacity</tt> because it may be the
     * case that a waiting consumer is ready to <tt>take</tt> an
     * element out of an otherwise full queue.
     */
    public int remainingCapacity() {
        lock.lock();
        try {
            return items.length - count;
        }
        finally {
            lock.unlock();
        }
    }


    public boolean contains(Object x) {
        if (x == null) return false;
        lock.lock();
        try {
            int i = takeIndex;
            int k = 0;
            while (k++ < count) {
                if (x.equals(items[i]))
                    return true;
                i = inc(i);
            }
            return false;
        }
        finally {
            lock.unlock();
        }
    }

    public Object[] toArray() {
        lock.lock();
        try {
            E[] a = (E[]) new Object[count];
            int k = 0;
            int i = takeIndex;
            while (k < count) {
                a[k++] = items[i];
                i = inc(i);
            }
            return a;
        }
        finally {
            lock.unlock();
        }
    }

    public <T> T[] toArray(T[] a) {
        lock.lock();
        try {
            if (a.length < count)
                a = (T[])java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), count);

            int k = 0;
            int i = takeIndex;
            while (k < count) {
                a[k++] = (T)items[i];
                i = inc(i);
            }
            if (a.length > count)
                a[count] = null;
            return a;
        }
        finally {
            lock.unlock();
        }
    }

    public String toString() {
        lock.lock();
        try {
            return super.toString();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     *
     * @return an iterator over the elements in this queue in proper sequence.
     */
    public Iterator<E> iterator() {
        lock.lock();
        try {
            return new Itr();
        }
        finally {
            lock.unlock();
        }
    }

    /**
     * Iterator for ArrayBlockingQueue
     */
    private class Itr implements Iterator<E> {
        /**
         * Index of element to be returned by next,
         * or a negative number if no such.
         */
        private int nextIndex;

        /**
         * nextItem holds on to item fields because once we claim
         * that an element exists in hasNext(), we must return it in
         * the following next() call even if it was in the process of
         * being removed when hasNext() was called.
         **/
        private E nextItem;

        /**
         * Index of element returned by most recent call to next.
         * Reset to -1 if this element is deleted by a call to remove.
         */
        private int lastRet;

        Itr() {
            lastRet = -1;
            if (count == 0)
                nextIndex = -1;
            else {
                nextIndex = takeIndex;
                nextItem = items[takeIndex];
            }
        }

        public boolean hasNext() {
            /*
             * No sync. We can return true by mistake here
             * only if this iterator passed across threads,
             * which we don't support anyway.
             */
            return nextIndex >= 0;
        }

        /**
         * Check whether nextIndex is valid; if so setting nextItem.
         * Stops iterator when either hits putIndex or sees null item.
         */
        private void checkNext() {
            if (nextIndex == putIndex) {
                nextIndex = -1;
                nextItem = null;
            }
            else {
                nextItem = items[nextIndex];
                if (nextItem == null)
                    nextIndex = -1;
            }
        }

        public E next() {
            lock.lock();
            try {
                if (nextIndex < 0)
                    throw new NoSuchElementException();
                lastRet = nextIndex;
                E x = nextItem;
                nextIndex = inc(nextIndex);
                checkNext();
                return x;
            }
            finally {
                lock.unlock();
            }
        }

        public void remove() {
            lock.lock();
            try {
                int i = lastRet;
                if (i == -1)
                    throw new IllegalStateException();
                lastRet = -1;

                int ti = takeIndex;
                removeAt(i);
                // back up cursor (reset to front if was first element)
                nextIndex = (i == ti) ? takeIndex : i;
                checkNext();
            }
            finally {
                lock.unlock();
            }
        }
    }

    /**
     * Save the state to a stream (that is, serialize it).
     *
     * @serialData The maximumSize is emitted (int), followed by all of
     * its elements (each an <tt>E</tt>) in the proper order.
     * @param s the stream
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

        // Write out element count, and any hidden stuff
        s.defaultWriteObject();
        // Write out maximumSize == items length
        s.writeInt(items.length);

        // Write out all elements in the proper order.
        int i = takeIndex;
        int k = 0;
        while (k++ < count) {
            s.writeObject(items[i]);
            i = inc(i);
        }
    }

    /**
     * Reconstitute this queue instance from a stream (that is,
     * deserialize it).
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in size, and any hidden stuff
        s.defaultReadObject();
        int size = count;

        // Read in array length and allocate array
        int arrayLength = s.readInt();

        // We use deserializedItems here because "items" is final
        deserializedItems = (E[]) new Object[arrayLength];

        // Read in all elements in the proper order into deserializedItems
        for (int i = 0; i < size; i++)
            deserializedItems[i] = (E)s.readObject();
    }

    /**
     * Throw away the object created with readObject, and replace it
     * with a usable ArrayBlockingQueue.
     * @return the ArrayBlockingQueue
     */
    private Object readResolve() throws java.io.ObjectStreamException {
        E[] array = deserializedItems;
        deserializedItems = null;
        return new ArrayBlockingQueue(array.length, array, count, lock);
    }
}
Revision:	1.15
Committed:	Mon Jul 28 16:00:19 2003 UTC (20 years, 10 months ago) by tim
Branch:	MAIN
Changes since 1.14:	+21 -21 lines
Log Message:	Added addAll() back in.
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain. Use, modify, and
4	* redistribute this code in any way without acknowledgement.
5	*/
6
7	package java.util.concurrent;
8	import java.util.concurrent.locks.*;
9	import java.util.*;
10
11	/**
12	* A bounded {@link BlockingQueue blocking queue} backed by an array.
13	* This queue orders elements FIFO (first-in-first-out).
14	* The <em>head</em> of the queue is that element that has been on the
15	* queue the longest time.
16	* The <em>tail</em> of the queue is that element that has been on the
17	* queue the shortest time.
18	*
19	* <p>This is a classic "bounded buffer", in which a fixed-sized
20	* array holds
21	* elements inserted by producers and extracted by consumers. Once
22	* created, the capacity can not be increased. Attempts to offer an
23	* element to a full queue will result in the offer operation
24	* blocking; attempts to retrieve an element from an empty queue will
25	* similarly block.
26	*
27	* <p> This class supports an optional fairness policy for ordering
28	* threads blocked on an insertion or removal. By default, this
29	* ordering is not guaranteed. However, an <tt>ArrayBlockingQueue</tt>
30	* constructed with fairness set to <tt>true</tt> grants blocked
31	* threads access in FIFO order. Fairness generally substantially
32	* decreases throughput but reduces variablility and avoids
33	* starvation.
34	*
35	* @since 1.5
36	* @author Doug Lea
37	*/
38	public class ArrayBlockingQueue<E> extends AbstractQueue<E>
39	implements BlockingQueue<E>, java.io.Serializable {
40
41	/** The queued items */
42	private transient final E[] items;
43	/** items index for next take, poll or remove */
44	private transient int takeIndex;
45	/** items index for next put, offer, or add. */
46	private transient int putIndex;
47	/** Number of items in the queue */
48	private int count;
49
50	/**
51	* An array used only during deserialization, to hold
52	* items read back in from the stream, and then used
53	* as "items" by readResolve via the private constructor.
54	*/
55	private transient E[] deserializedItems;
56
57	/*
58	* Concurrency control via the classic two-condition algorithm
59	* found in any textbook.
60	*/
61
62	/** Main lock guarding all access */
63	private final ReentrantLock lock;
64	/** Condition for waiting takes */
65	private final Condition notEmpty;
66	/** Condition for wiating puts */
67	private final Condition notFull;
68
69	// Internal helper methods
70
71	/**
72	* Circularly increment i.
73	*/
74	int inc(int i) {
75	return (++i == items.length)? 0 : i;
76	}
77
78	/**
79	* Insert element at current put position, advance, and signal.
80	* Call only when holding lock.
81	*/
82	private void insert(E x) {
83	items[putIndex] = x;
84	putIndex = inc(putIndex);
85	++count;
86	notEmpty.signal();
87	}
88
89	/**
90	* Extract element at current take position, advance, and signal.
91	* Call only when holding lock.
92	*/
93	private E extract() {
94	E x = items[takeIndex];
95	items[takeIndex] = null;
96	takeIndex = inc(takeIndex);
97	--count;
98	notFull.signal();
99	return x;
100	}
101
102	/**
103	* Utility for remove and iterator.remove: Delete item at position i.
104	* Call only when holding lock.
105	*/
106	void removeAt(int i) {
107	// if removing front item, just advance
108	if (i == takeIndex) {
109	items[takeIndex] = null;
110	takeIndex = inc(takeIndex);
111	}
112	else {
113	// slide over all others up through putIndex.
114	for (;;) {
115	int nexti = inc(i);
116	if (nexti != putIndex) {
117	items[i] = items[nexti];
118	i = nexti;
119	}
120	else {
121	items[i] = null;
122	putIndex = i;
123	break;
124	}
125	}
126	}
127	--count;
128	notFull.signal();
129	}
130
131	/**
132	* Internal constructor also used by readResolve.
133	* Sets all final fields, plus count.
134	* @param cap the capacity
135	* @param array the array to use or null if should create new one
136	* @param count the number of items in the array, where indices 0
137	* to count-1 hold items.
138	*/
139	private ArrayBlockingQueue(int cap, E[] array, int count,
140	ReentrantLock lk) {
141	if (cap <= 0)
142	throw new IllegalArgumentException();
143	if (array == null)
144	this.items = (E[]) new Object[cap];
145	else
146	this.items = array;
147	this.putIndex = count;
148	this.count = count;
149	lock = lk;
150	notEmpty = lock.newCondition();
151	notFull = lock.newCondition();
152	}
153
154	/**
155	* Create an <tt>ArrayBlockingQueue</tt> with the given (fixed)
156	* capacity and default access policy.
157	* @param capacity the capacity of this queue
158	*/
159	public ArrayBlockingQueue(int capacity) {
160	this(capacity, null, 0, new ReentrantLock());
161	}
162
163	/**
164	* Create an <tt>ArrayBlockingQueue</tt> with the given (fixed)
165	* capacity and the specified access policy.
166	* @param capacity the capacity of this queue
167	* @param fair if <tt>true</tt> then queue accesses for threads blocked
168	* on insertion or removal, are processed in FIFO order; if <tt>false</tt>
169	* the access order is unspecified.
170	*/
171	public ArrayBlockingQueue(int capacity, boolean fair) {
172	this(capacity, null, 0, new ReentrantLock(fair));
173	}
174
175
176	// Have to override just to update the javadoc for @throws
177
178	/**
179	* @throws IllegalStateException {@inheritDoc}
180	*/
181	public boolean add(E x) {
182	return super.add(x);
183	}
184
185	/**
186	* @throws IllegalStateException {@inheritDoc}
187	*/
188	public boolean addAll(Collection<? extends E> c) {
189	return super.addAll(c);
190	}
191
192	/**
193	* Add the specified element to the tail of this queue if possible,
194	* returning immediately if this queue is full.
195	*
196	* @throws NullPointerException {@inheritDoc}
197	*/
198	public boolean offer(E x) {
199	if (x == null) throw new NullPointerException();
200	lock.lock();
201	try {
202	if (count == items.length)
203	return false;
204	else {
205	insert(x);
206	return true;
207	}
208	}
209	finally {
210	lock.unlock();
211	}
212	}
213
214	/**
215	* Add the specified element to the tail of this queue, waiting if
216	* necessary up to the specified wait time for space to become available.
217	* @throws NullPointerException {@inheritDoc}
218	*/
219	public boolean offer(E x, long timeout, TimeUnit unit)
220	throws InterruptedException {
221
222	if (x == null) throw new NullPointerException();
223
224	lock.lockInterruptibly();
225	try {
226	long nanos = unit.toNanos(timeout);
227	for (;;) {
228	if (count != items.length) {
229	insert(x);
230	return true;
231	}
232	if (nanos <= 0)
233	return false;
234	try {
235	nanos = notFull.awaitNanos(nanos);
236	}
237	catch (InterruptedException ie) {
238	notFull.signal(); // propagate to non-interrupted thread
239	throw ie;
240	}
241	}
242	}
243	finally {
244	lock.unlock();
245	}
246	}
247
248
249	public E poll() {
250	lock.lock();
251	try {
252	if (count == 0)
253	return null;
254	E x = extract();
255	return x;
256	}
257	finally {
258	lock.unlock();
259	}
260	}
261
262	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
263	lock.lockInterruptibly();
264	try {
265	long nanos = unit.toNanos(timeout);
266	for (;;) {
267	if (count != 0) {
268	E x = extract();
269	return x;
270	}
271	if (nanos <= 0)
272	return null;
273	try {
274	nanos = notEmpty.awaitNanos(nanos);
275	}
276	catch (InterruptedException ie) {
277	notEmpty.signal(); // propagate to non-interrupted thread
278	throw ie;
279	}
280
281	}
282	}
283	finally {
284	lock.unlock();
285	}
286	}
287
288
289	public boolean remove(Object x) {
290	if (x == null) return false;
291	lock.lock();
292	try {
293	int i = takeIndex;
294	int k = 0;
295	for (;;) {
296	if (k++ >= count)
297	return false;
298	if (x.equals(items[i])) {
299	removeAt(i);
300	return true;
301	}
302	i = inc(i);
303	}
304
305	}
306	finally {
307	lock.unlock();
308	}
309	}
310
311	public E peek() {
312	lock.lock();
313	try {
314	return (count == 0) ? null : items[takeIndex];
315	}
316	finally {
317	lock.unlock();
318	}
319	}
320
321	public E take() throws InterruptedException {
322	lock.lockInterruptibly();
323	try {
324	try {
325	while (count == 0)
326	notEmpty.await();
327	}
328	catch (InterruptedException ie) {
329	notEmpty.signal(); // propagate to non-interrupted thread
330	throw ie;
331	}
332	E x = extract();
333	return x;
334	}
335	finally {
336	lock.unlock();
337	}
338	}
339
340	/**
341	* Add the specified element to the tail of this queue, waiting if
342	* necessary for space to become available.
343	* @throws NullPointerException {@inheritDoc}
344	*/
345	public void put(E x) throws InterruptedException {
346
347	if (x == null) throw new NullPointerException();
348
349	lock.lockInterruptibly();
350	try {
351	try {
352	while (count == items.length)
353	notFull.await();
354	}
355	catch (InterruptedException ie) {
356	notFull.signal(); // propagate to non-interrupted thread
357	throw ie;
358	}
359	insert(x);
360	}
361	finally {
362	lock.unlock();
363	}
364	}
365
366	// this doc comment is overridden to remove the reference to collections
367	// greater in size than Integer.MAX_VALUE
368	/**
369	* Return the number of elements in this collection.
370	*/
371	public int size() {
372	lock.lock();
373	try {
374	return count;
375	}
376	finally {
377	lock.unlock();
378	}
379	}
380
381	// this doc comment is a modified copy of the inherited doc comment,
382	// without the reference to unlimited queues.
383	/**
384	* Return the number of elements that this queue can ideally (in
385	* the absence of memory or resource constraints) accept without
386	* blocking. This is always equal to the initial capacity of this queue
387	* less the current <tt>size</tt> of this queue.
388	* <p>Note that you <em>cannot</em> always tell if
389	* an attempt to <tt>add</tt> an element will succeed by
390	* inspecting <tt>remainingCapacity</tt> because it may be the
391	* case that a waiting consumer is ready to <tt>take</tt> an
392	* element out of an otherwise full queue.
393	*/
394	public int remainingCapacity() {
395	lock.lock();
396	try {
397	return items.length - count;
398	}
399	finally {
400	lock.unlock();
401	}
402	}
403
404
405	public boolean contains(Object x) {
406	if (x == null) return false;
407	lock.lock();
408	try {
409	int i = takeIndex;
410	int k = 0;
411	while (k++ < count) {
412	if (x.equals(items[i]))
413	return true;
414	i = inc(i);
415	}
416	return false;
417	}
418	finally {
419	lock.unlock();
420	}
421	}
422
423	public Object[] toArray() {
424	lock.lock();
425	try {
426	E[] a = (E[]) new Object[count];
427	int k = 0;
428	int i = takeIndex;
429	while (k < count) {
430	a[k++] = items[i];
431	i = inc(i);
432	}
433	return a;
434	}
435	finally {
436	lock.unlock();
437	}
438	}
439
440	public <T> T[] toArray(T[] a) {
441	lock.lock();
442	try {
443	if (a.length < count)
444	a = (T[])java.lang.reflect.Array.newInstance(a.getClass().getComponentType(), count);
445
446	int k = 0;
447	int i = takeIndex;
448	while (k < count) {
449	a[k++] = (T)items[i];
450	i = inc(i);
451	}
452	if (a.length > count)
453	a[count] = null;
454	return a;
455	}
456	finally {
457	lock.unlock();
458	}
459	}
460
461	public String toString() {
462	lock.lock();
463	try {
464	return super.toString();
465	}
466	finally {
467	lock.unlock();
468	}
469	}
470
471	/**
472	* Returns an iterator over the elements in this queue in proper sequence.
473	*
474	* @return an iterator over the elements in this queue in proper sequence.
475	*/
476	public Iterator<E> iterator() {
477	lock.lock();
478	try {
479	return new Itr();
480	}
481	finally {
482	lock.unlock();
483	}
484	}
485
486	/**
487	* Iterator for ArrayBlockingQueue
488	*/
489	private class Itr implements Iterator<E> {
490	/**
491	* Index of element to be returned by next,
492	* or a negative number if no such.
493	*/
494	private int nextIndex;
495
496	/**
497	* nextItem holds on to item fields because once we claim
498	* that an element exists in hasNext(), we must return it in
499	* the following next() call even if it was in the process of
500	* being removed when hasNext() was called.
501	**/
502	private E nextItem;
503
504	/**
505	* Index of element returned by most recent call to next.
506	* Reset to -1 if this element is deleted by a call to remove.
507	*/
508	private int lastRet;
509
510	Itr() {
511	lastRet = -1;
512	if (count == 0)
513	nextIndex = -1;
514	else {
515	nextIndex = takeIndex;
516	nextItem = items[takeIndex];
517	}
518	}
519
520	public boolean hasNext() {
521	/*
522	* No sync. We can return true by mistake here
523	* only if this iterator passed across threads,
524	* which we don't support anyway.
525	*/
526	return nextIndex >= 0;
527	}
528
529	/**
530	* Check whether nextIndex is valid; if so setting nextItem.
531	* Stops iterator when either hits putIndex or sees null item.
532	*/
533	private void checkNext() {
534	if (nextIndex == putIndex) {
535	nextIndex = -1;
536	nextItem = null;
537	}
538	else {
539	nextItem = items[nextIndex];
540	if (nextItem == null)
541	nextIndex = -1;
542	}
543	}
544
545	public E next() {
546	lock.lock();
547	try {
548	if (nextIndex < 0)
549	throw new NoSuchElementException();
550	lastRet = nextIndex;
551	E x = nextItem;
552	nextIndex = inc(nextIndex);
553	checkNext();
554	return x;
555	}
556	finally {
557	lock.unlock();
558	}
559	}
560
561	public void remove() {
562	lock.lock();
563	try {
564	int i = lastRet;
565	if (i == -1)
566	throw new IllegalStateException();
567	lastRet = -1;
568
569	int ti = takeIndex;
570	removeAt(i);
571	// back up cursor (reset to front if was first element)
572	nextIndex = (i == ti) ? takeIndex : i;
573	checkNext();
574	}
575	finally {
576	lock.unlock();
577	}
578	}
579	}
580
581	/**
582	* Save the state to a stream (that is, serialize it).
583	*
584	* @serialData The maximumSize is emitted (int), followed by all of
585	* its elements (each an <tt>E</tt>) in the proper order.
586	* @param s the stream
587	*/
588	private void writeObject(java.io.ObjectOutputStream s)
589	throws java.io.IOException {
590
591	// Write out element count, and any hidden stuff
592	s.defaultWriteObject();
593	// Write out maximumSize == items length
594	s.writeInt(items.length);
595
596	// Write out all elements in the proper order.
597	int i = takeIndex;
598	int k = 0;
599	while (k++ < count) {
600	s.writeObject(items[i]);
601	i = inc(i);
602	}
603	}
604
605	/**
606	* Reconstitute this queue instance from a stream (that is,
607	* deserialize it).
608	* @param s the stream
609	*/
610	private void readObject(java.io.ObjectInputStream s)
611	throws java.io.IOException, ClassNotFoundException {
612	// Read in size, and any hidden stuff
613	s.defaultReadObject();
614	int size = count;
615
616	// Read in array length and allocate array
617	int arrayLength = s.readInt();
618
619	// We use deserializedItems here because "items" is final
620	deserializedItems = (E[]) new Object[arrayLength];
621
622	// Read in all elements in the proper order into deserializedItems
623	for (int i = 0; i < size; i++)
624	deserializedItems[i] = (E)s.readObject();
625	}
626
627	/**
628	* Throw away the object created with readObject, and replace it
629	* with a usable ArrayBlockingQueue.
630	* @return the ArrayBlockingQueue
631	*/
632	private Object readResolve() throws java.io.ObjectStreamException {
633	E[] array = deserializedItems;
634	deserializedItems = null;
635	return new ArrayBlockingQueue(array.length, array, count, lock);
636	}
637	}