util/concurrent/LinkedBlockingQueue.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.atomic.*;
import java.util.concurrent.locks.*;
import java.util.*;

/**
 * An optionally-bounded blocking queue based on linked nodes.  Linked
 * queues typically have higher throughput than array-based queues but
 * less predicatble performance in most concurrent applications.
 * 
 * <p> The optional capacity bound constructor argument serves as a
 * way to prevent unlmited queue expansion.  Linked nodes are
 * dynamically created upon each insertion unless this would bring the
 * queue above capacity.
 * @since 1.5
 * @author Doug Lea
 * 
 **/
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    /*
     * A variant of the "two lock queue" algorithm.  The putLock gates
     * entry to put (and offer), and has an associated condition for
     * waiting puts.  Similarly for the takeLock.  The "count" field
     * that they both rely on is maintained as an atomic to avoid
     * needing to get both locks in most cases. Also, to minimize need
     * for puts to get takeLock and vice-versa, cascading notifies are
     * used. When a put notices that it has enabled at least one take,
     * it signals taker. That taker in turn signals others if more
     * items have been entered since the signal. And symmetrically for
     * takes signalling puts. Operations such as remove(Object) and 
     * iterators acquire both locks.
    */

    /**
     * Linked list node class
     */
    static class Node<E> {
        /** The item, volatile to ensure barrier separating write and read */
        volatile E item;
        Node<E> next;
        Node(E x) { item = x; }
    }

    /** The capacity bound, or Integer.MAX_VALUE if none */
    private final int capacity;

    /** Current number of elements */
    private transient final AtomicInteger count = new AtomicInteger(0);

    /** Head of linked list */
    private transient Node<E> head;

    /** Tail of lined list */
    private transient Node<E> last;

    /** Lock held by take, poll, etc */
    private final ReentrantLock takeLock = new ReentrantLock();

    /** Wait queue for waiting takes */
    private final Condition notEmpty = takeLock.newCondition();

    /** Lock held by put, offer, etc */
    private final ReentrantLock putLock = new ReentrantLock();

    /** Wait queue for waiting puts */
    private final Condition notFull = putLock.newCondition();

    /**
     * Signal a waiting take. Called only from put/offer (which do not
     * otherwise ordinarily lock takeLock.)
     */
    private void signalNotEmpty() {
        takeLock.lock();
        try {
            notEmpty.signal();
        }
        finally {
            takeLock.unlock();
        }
    }

    /**
     * Signal a waiting put. Called only from take/poll.
     */
    private void signalNotFull() {
        putLock.lock();
        try {
            notFull.signal();
        }
        finally {
            putLock.unlock();
        }
    }

    /**
     * Create a node and link it and end of queue
     * @param x the item
     */
    private void insert(E x) {
        last = last.next = new Node<E>(x);
    }

    /**
     * Remove a node from head of queue,
     * @return the node
     */
    private E extract() {
        Node<E> first = head.next;
        head = first;
        E x = (E)first.item;
        first.item = null;
        return x;
    }

    /**
     * Lock to prevent both puts and takes. 
     */
    private void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

    /**
     * Unlock to allow both puts and takes. 
     */
    private void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }


    /**
     * Creates a LinkedBlockingQueue with no intrinsic capacity constraint.
     */
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }

    /**
     * Creates a LinkedBlockingQueue with the given capacity constraint.
     * @param capacity the maminum number of elements to hold without blocking.
     */
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new NullPointerException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }

    /**
     * Creates a LinkedBlockingQueue without an intrinsic capacity
     * constraint, initially holding the given elements, added in
     * traveral order of the collection's iterator.
     * @param initialElements the elements to initially contain
     */
    public LinkedBlockingQueue(Collection<E> initialElements) {
        this(Integer.MAX_VALUE);
        for (Iterator<E> it = initialElements.iterator(); it.hasNext();) 
            add(it.next());
    }

    public int size() {
        return count.get();
    }

    public int remainingCapacity() {
        return capacity - count.get();
    }

    public void put(E x) throws InterruptedException {
        if (x == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset
        // local var holding count  negative to indicate failure unless set.
        int c = -1; 
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from
             * capacity. Similarly for all other uses of count in
             * other wait guards.
             */
            try {
                while (count.get() == capacity) 
                    notFull.await();
            }
            catch (InterruptedException ie) {
                notFull.signal(); // propagate to a non-interrupted thread
                throw ie;
            }
            insert(x);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        }
        finally {
            putLock.unlock();
        }
        if (c == 0) 
            signalNotEmpty();
    }

    public boolean offer(E x, long timeout, TimeUnit unit) throws InterruptedException {
        if (x == null) throw new NullPointerException();
        putLock.lockInterruptibly();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        try {
            for (;;) {
                if (count.get() < capacity) {
                    insert(x);
                    c = count.getAndIncrement();
                    if (c + 1 < capacity)
                        notFull.signal();
                    break;
                }
                if (nanos <= 0)
                    return false;
                try {
                    nanos = notFull.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notFull.signal(); // propagate to a non-interrupted thread
                    throw ie;
                }
            }
        }
        finally {
            putLock.unlock();
        }
        if (c == 0) 
            signalNotEmpty();
        return true;
    }

    public boolean offer(E x) {
        if (x == null) throw new NullPointerException();
        if (count.get() == capacity)
            return false;
        putLock.tryLock();
        int c = -1; 
        try {
            if (count.get() < capacity) {
                insert(x);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        }
        finally {
            putLock.unlock();
        }
        if (c == 0) 
            signalNotEmpty();
        return c >= 0;
    }


    public E take() throws InterruptedException {
        E x;
        int c = -1;
        takeLock.lockInterruptibly();
        try {
            try {
                while (count.get() == 0) 
                    notEmpty.await();
            }
            catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to a non-interrupted thread
                throw ie;
            }

            x = extract();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        }
        finally {
            takeLock.unlock();
        }
        if (c == capacity) 
            signalNotFull();
        return x;
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E x = null;
        int c = -1;
        takeLock.lockInterruptibly();
        long nanos = unit.toNanos(timeout);
        try {
            for (;;) {
                if (count.get() > 0) {
                    x = extract();
                    c = count.getAndDecrement();
                    if (c > 1)
                        notEmpty.signal();
                    break;
                }
                if (nanos <= 0)
                    return null;
                try {
                    nanos = notEmpty.awaitNanos(nanos);
                }
                catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to a non-interrupted thread
                    throw ie;
                }
            }
        }
        finally {
            takeLock.unlock();
        }
        if (c == capacity) 
            signalNotFull();
        return x;
    }

    public E poll() {
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1; 
        takeLock.tryLock();
        try {
            if (count.get() > 0) {
                x = extract();
                c = count.getAndDecrement();
                if (c > 1)
                    notEmpty.signal();
            }
        }
        finally {
            takeLock.unlock();
        }
        if (c == capacity) 
            signalNotFull();
        return x;
    }


    public E peek() {
        if (count.get() == 0)
            return null;
        takeLock.tryLock();
        try {
            Node<E> first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
        }
        finally {
            takeLock.unlock();
        }
    }

    public boolean remove(Object x) {
        if (x == null) return false;
        boolean removed = false;
        fullyLock();
        try {
            Node<E> trail = head;
            Node<E> p = head.next;
            while (p != null) {
                if (x.equals(p.item)) {
                    removed = true;
                    break;
                }
                trail = p;
                p = p.next;
            }
            if (removed) {
                p.item = null;
                trail.next = p.next;
                if (count.getAndDecrement() == capacity)
                    notFull.signalAll();
            }
        }
        finally {
            fullyUnlock();
        }
        return removed;
    }

    public Object[] toArray() {
        fullyLock();
        try {
            int size = count.get();
            Object[] a = new Object[size];                
            int k = 0;
            for (Node<E> p = head.next; p != null; p = p.next) 
                a[k++] = p.item;
            return a;
        }
        finally {
            fullyUnlock();
        }
    }

    public <T> T[] toArray(T[] a) {
        fullyLock();
        try {
            int size = count.get();
            if (a.length < size)
                a = (T[])java.lang.reflect.Array.newInstance
                    (a.getClass().getComponentType(), size);
            
            int k = 0;
            for (Node p = head.next; p != null; p = p.next) 
                a[k++] = (T)p.item;
            return a;
        }
        finally {
            fullyUnlock();
        }
    }

    public String toString() {
        fullyLock();
        try {
            return super.toString();
        }
        finally {
            fullyUnlock();
        }
    }

    public Iterator<E> iterator() {
      return new Itr();
    }

    private class Itr implements Iterator<E> {
        /* 
         * Basic weak-consistent iterator.  At all times hold the next
         * item to hand out so that if hasNext() reports true, we will
         * still have it to return even if lost race with a take etc.
         */
        Node<E> current;
        Node<E> lastRet;
        E currentElement;
        
        Itr() {
            fullyLock();
            try {
                current = head.next;
                if (current != null)
                    currentElement = current.item;
            }
            finally {
                fullyUnlock();
            }
        }
        
        public boolean hasNext() {
            return current != null;
        }

        public E next() {
            fullyLock();
            try {
                if (current == null)
                    throw new NoSuchElementException();
                E x = currentElement;
                lastRet = current;
                current = current.next;
                if (current != null)
                    currentElement = current.item;
                return x;
            }
            finally {
                fullyUnlock();
            }
            
        }

        public void remove() {
            if (lastRet == null)
                throw new IllegalStateException();
            fullyLock();
            try {
                Node<E> node = lastRet;
                lastRet = null;
                Node<E> trail = head;
                Node<E> p = head.next;
                while (p != null && p != node) {
                    trail = p;
                    p = p.next;
                }
                if (p == node) {
                    p.item = null;
                    trail.next = p.next;
                    int c = count.getAndDecrement();
                    if (c == capacity)
                        notFull.signalAll();
                }
            }
            finally {
                fullyUnlock();
            }
        }
    }

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

        fullyLock(); 
        try {
            // Write out any hidden stuff, plus capacity
            s.defaultWriteObject();

            // Write out all elements in the proper order.
            for (Node<E> p = head.next; p != null; p = p.next) 
                s.writeObject(p.item);

            // Use trailing null as sentinel
            s.writeObject(null);
        }
        finally {
            fullyUnlock();
        }
    }

    /**
     * Reconstitute the 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 capacity, and any hidden stuff
        s.defaultReadObject();

        // Read in all elements and place in queue
        for (;;) {
            E item = (E)s.readObject();
            if (item == null)
                break;
            add(item);
        }
    }
}

Revision:	1.7
Committed:	Tue Jul 8 00:46:34 2003 UTC (20 years, 11 months ago) by dl
Branch:	MAIN
CVS Tags:	JSR166_PRELIMINARY_TEST_RELEASE_2
Changes since 1.6:	+1 -0 lines
Log Message:	Locks in subpackage; fairness params added
#	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.atomic.*;
9	import java.util.concurrent.locks.*;
10	import java.util.*;
11
12	/**
13	* An optionally-bounded blocking queue based on linked nodes. Linked
14	* queues typically have higher throughput than array-based queues but
15	* less predicatble performance in most concurrent applications.
16	*
17	* <p> The optional capacity bound constructor argument serves as a
18	* way to prevent unlmited queue expansion. Linked nodes are
19	* dynamically created upon each insertion unless this would bring the
20	* queue above capacity.
21	* @since 1.5
22	* @author Doug Lea
23	*
24	**/
25	public class LinkedBlockingQueue<E> extends AbstractQueue<E>
26	implements BlockingQueue<E>, java.io.Serializable {
27
28	/*
29	* A variant of the "two lock queue" algorithm. The putLock gates
30	* entry to put (and offer), and has an associated condition for
31	* waiting puts. Similarly for the takeLock. The "count" field
32	* that they both rely on is maintained as an atomic to avoid
33	* needing to get both locks in most cases. Also, to minimize need
34	* for puts to get takeLock and vice-versa, cascading notifies are
35	* used. When a put notices that it has enabled at least one take,
36	* it signals taker. That taker in turn signals others if more
37	* items have been entered since the signal. And symmetrically for
38	* takes signalling puts. Operations such as remove(Object) and
39	* iterators acquire both locks.
40	*/
41
42	/**
43	* Linked list node class
44	*/
45	static class Node<E> {
46	/** The item, volatile to ensure barrier separating write and read */
47	volatile E item;
48	Node<E> next;
49	Node(E x) { item = x; }
50	}
51
52	/** The capacity bound, or Integer.MAX_VALUE if none */
53	private final int capacity;
54
55	/** Current number of elements */
56	private transient final AtomicInteger count = new AtomicInteger(0);
57
58	/** Head of linked list */
59	private transient Node<E> head;
60
61	/** Tail of lined list */
62	private transient Node<E> last;
63
64	/** Lock held by take, poll, etc */
65	private final ReentrantLock takeLock = new ReentrantLock();
66
67	/** Wait queue for waiting takes */
68	private final Condition notEmpty = takeLock.newCondition();
69
70	/** Lock held by put, offer, etc */
71	private final ReentrantLock putLock = new ReentrantLock();
72
73	/** Wait queue for waiting puts */
74	private final Condition notFull = putLock.newCondition();
75
76	/**
77	* Signal a waiting take. Called only from put/offer (which do not
78	* otherwise ordinarily lock takeLock.)
79	*/
80	private void signalNotEmpty() {
81	takeLock.lock();
82	try {
83	notEmpty.signal();
84	}
85	finally {
86	takeLock.unlock();
87	}
88	}
89
90	/**
91	* Signal a waiting put. Called only from take/poll.
92	*/
93	private void signalNotFull() {
94	putLock.lock();
95	try {
96	notFull.signal();
97	}
98	finally {
99	putLock.unlock();
100	}
101	}
102
103	/**
104	* Create a node and link it and end of queue
105	* @param x the item
106	*/
107	private void insert(E x) {
108	last = last.next = new Node<E>(x);
109	}
110
111	/**
112	* Remove a node from head of queue,
113	* @return the node
114	*/
115	private E extract() {
116	Node<E> first = head.next;
117	head = first;
118	E x = (E)first.item;
119	first.item = null;
120	return x;
121	}
122
123	/**
124	* Lock to prevent both puts and takes.
125	*/
126	private void fullyLock() {
127	putLock.lock();
128	takeLock.lock();
129	}
130
131	/**
132	* Unlock to allow both puts and takes.
133	*/
134	private void fullyUnlock() {
135	takeLock.unlock();
136	putLock.unlock();
137	}
138
139
140	/**
141	* Creates a LinkedBlockingQueue with no intrinsic capacity constraint.
142	*/
143	public LinkedBlockingQueue() {
144	this(Integer.MAX_VALUE);
145	}
146
147	/**
148	* Creates a LinkedBlockingQueue with the given capacity constraint.
149	* @param capacity the maminum number of elements to hold without blocking.
150	*/
151	public LinkedBlockingQueue(int capacity) {
152	if (capacity <= 0) throw new NullPointerException();
153	this.capacity = capacity;
154	last = head = new Node<E>(null);
155	}
156
157	/**
158	* Creates a LinkedBlockingQueue without an intrinsic capacity
159	* constraint, initially holding the given elements, added in
160	* traveral order of the collection's iterator.
161	* @param initialElements the elements to initially contain
162	*/
163	public LinkedBlockingQueue(Collection<E> initialElements) {
164	this(Integer.MAX_VALUE);
165	for (Iterator<E> it = initialElements.iterator(); it.hasNext();)
166	add(it.next());
167	}
168
169	public int size() {
170	return count.get();
171	}
172
173	public int remainingCapacity() {
174	return capacity - count.get();
175	}
176
177	public void put(E x) throws InterruptedException {
178	if (x == null) throw new NullPointerException();
179	// Note: convention in all put/take/etc is to preset
180	// local var holding count negative to indicate failure unless set.
181	int c = -1;
182	putLock.lockInterruptibly();
183	try {
184	/*
185	* Note that count is used in wait guard even though it is
186	* not protected by lock. This works because count can
187	* only decrease at this point (all other puts are shut
188	* out by lock), and we (or some other waiting put) are
189	* signalled if it ever changes from
190	* capacity. Similarly for all other uses of count in
191	* other wait guards.
192	*/
193	try {
194	while (count.get() == capacity)
195	notFull.await();
196	}
197	catch (InterruptedException ie) {
198	notFull.signal(); // propagate to a non-interrupted thread
199	throw ie;
200	}
201	insert(x);
202	c = count.getAndIncrement();
203	if (c + 1 < capacity)
204	notFull.signal();
205	}
206	finally {
207	putLock.unlock();
208	}
209	if (c == 0)
210	signalNotEmpty();
211	}
212
213	public boolean offer(E x, long timeout, TimeUnit unit) throws InterruptedException {
214	if (x == null) throw new NullPointerException();
215	putLock.lockInterruptibly();
216	long nanos = unit.toNanos(timeout);
217	int c = -1;
218	try {
219	for (;;) {
220	if (count.get() < capacity) {
221	insert(x);
222	c = count.getAndIncrement();
223	if (c + 1 < capacity)
224	notFull.signal();
225	break;
226	}
227	if (nanos <= 0)
228	return false;
229	try {
230	nanos = notFull.awaitNanos(nanos);
231	}
232	catch (InterruptedException ie) {
233	notFull.signal(); // propagate to a non-interrupted thread
234	throw ie;
235	}
236	}
237	}
238	finally {
239	putLock.unlock();
240	}
241	if (c == 0)
242	signalNotEmpty();
243	return true;
244	}
245
246	public boolean offer(E x) {
247	if (x == null) throw new NullPointerException();
248	if (count.get() == capacity)
249	return false;
250	putLock.tryLock();
251	int c = -1;
252	try {
253	if (count.get() < capacity) {
254	insert(x);
255	c = count.getAndIncrement();
256	if (c + 1 < capacity)
257	notFull.signal();
258	}
259	}
260	finally {
261	putLock.unlock();
262	}
263	if (c == 0)
264	signalNotEmpty();
265	return c >= 0;
266	}
267
268
269	public E take() throws InterruptedException {
270	E x;
271	int c = -1;
272	takeLock.lockInterruptibly();
273	try {
274	try {
275	while (count.get() == 0)
276	notEmpty.await();
277	}
278	catch (InterruptedException ie) {
279	notEmpty.signal(); // propagate to a non-interrupted thread
280	throw ie;
281	}
282
283	x = extract();
284	c = count.getAndDecrement();
285	if (c > 1)
286	notEmpty.signal();
287	}
288	finally {
289	takeLock.unlock();
290	}
291	if (c == capacity)
292	signalNotFull();
293	return x;
294	}
295
296	public E poll(long timeout, TimeUnit unit) throws InterruptedException {
297	E x = null;
298	int c = -1;
299	takeLock.lockInterruptibly();
300	long nanos = unit.toNanos(timeout);
301	try {
302	for (;;) {
303	if (count.get() > 0) {
304	x = extract();
305	c = count.getAndDecrement();
306	if (c > 1)
307	notEmpty.signal();
308	break;
309	}
310	if (nanos <= 0)
311	return null;
312	try {
313	nanos = notEmpty.awaitNanos(nanos);
314	}
315	catch (InterruptedException ie) {
316	notEmpty.signal(); // propagate to a non-interrupted thread
317	throw ie;
318	}
319	}
320	}
321	finally {
322	takeLock.unlock();
323	}
324	if (c == capacity)
325	signalNotFull();
326	return x;
327	}
328
329	public E poll() {
330	if (count.get() == 0)
331	return null;
332	E x = null;
333	int c = -1;
334	takeLock.tryLock();
335	try {
336	if (count.get() > 0) {
337	x = extract();
338	c = count.getAndDecrement();
339	if (c > 1)
340	notEmpty.signal();
341	}
342	}
343	finally {
344	takeLock.unlock();
345	}
346	if (c == capacity)
347	signalNotFull();
348	return x;
349	}
350
351
352	public E peek() {
353	if (count.get() == 0)
354	return null;
355	takeLock.tryLock();
356	try {
357	Node<E> first = head.next;
358	if (first == null)
359	return null;
360	else
361	return first.item;
362	}
363	finally {
364	takeLock.unlock();
365	}
366	}
367
368	public boolean remove(Object x) {
369	if (x == null) return false;
370	boolean removed = false;
371	fullyLock();
372	try {
373	Node<E> trail = head;
374	Node<E> p = head.next;
375	while (p != null) {
376	if (x.equals(p.item)) {
377	removed = true;
378	break;
379	}
380	trail = p;
381	p = p.next;
382	}
383	if (removed) {
384	p.item = null;
385	trail.next = p.next;
386	if (count.getAndDecrement() == capacity)
387	notFull.signalAll();
388	}
389	}
390	finally {
391	fullyUnlock();
392	}
393	return removed;
394	}
395
396	public Object[] toArray() {
397	fullyLock();
398	try {
399	int size = count.get();
400	Object[] a = new Object[size];
401	int k = 0;
402	for (Node<E> p = head.next; p != null; p = p.next)
403	a[k++] = p.item;
404	return a;
405	}
406	finally {
407	fullyUnlock();
408	}
409	}
410
411	public <T> T[] toArray(T[] a) {
412	fullyLock();
413	try {
414	int size = count.get();
415	if (a.length < size)
416	a = (T[])java.lang.reflect.Array.newInstance
417	(a.getClass().getComponentType(), size);
418
419	int k = 0;
420	for (Node p = head.next; p != null; p = p.next)
421	a[k++] = (T)p.item;
422	return a;
423	}
424	finally {
425	fullyUnlock();
426	}
427	}
428
429	public String toString() {
430	fullyLock();
431	try {
432	return super.toString();
433	}
434	finally {
435	fullyUnlock();
436	}
437	}
438
439	public Iterator<E> iterator() {
440	return new Itr();
441	}
442
443	private class Itr implements Iterator<E> {
444	/*
445	* Basic weak-consistent iterator. At all times hold the next
446	* item to hand out so that if hasNext() reports true, we will
447	* still have it to return even if lost race with a take etc.
448	*/
449	Node<E> current;
450	Node<E> lastRet;
451	E currentElement;
452
453	Itr() {
454	fullyLock();
455	try {
456	current = head.next;
457	if (current != null)
458	currentElement = current.item;
459	}
460	finally {
461	fullyUnlock();
462	}
463	}
464
465	public boolean hasNext() {
466	return current != null;
467	}
468
469	public E next() {
470	fullyLock();
471	try {
472	if (current == null)
473	throw new NoSuchElementException();
474	E x = currentElement;
475	lastRet = current;
476	current = current.next;
477	if (current != null)
478	currentElement = current.item;
479	return x;
480	}
481	finally {
482	fullyUnlock();
483	}
484
485	}
486
487	public void remove() {
488	if (lastRet == null)
489	throw new IllegalStateException();
490	fullyLock();
491	try {
492	Node<E> node = lastRet;
493	lastRet = null;
494	Node<E> trail = head;
495	Node<E> p = head.next;
496	while (p != null && p != node) {
497	trail = p;
498	p = p.next;
499	}
500	if (p == node) {
501	p.item = null;
502	trail.next = p.next;
503	int c = count.getAndDecrement();
504	if (c == capacity)
505	notFull.signalAll();
506	}
507	}
508	finally {
509	fullyUnlock();
510	}
511	}
512	}
513
514	/**
515	* Save the state to a stream (that is, serialize it).
516	*
517	* @serialData The capacity is emitted (int), followed by all of
518	* its elements (each an <tt>Object</tt>) in the proper order,
519	* followed by a null
520	* @param s the stream
521	*/
522	private void writeObject(java.io.ObjectOutputStream s)
523	throws java.io.IOException {
524
525	fullyLock();
526	try {
527	// Write out any hidden stuff, plus capacity
528	s.defaultWriteObject();
529
530	// Write out all elements in the proper order.
531	for (Node<E> p = head.next; p != null; p = p.next)
532	s.writeObject(p.item);
533
534	// Use trailing null as sentinel
535	s.writeObject(null);
536	}
537	finally {
538	fullyUnlock();
539	}
540	}
541
542	/**
543	* Reconstitute the Queue instance from a stream (that is,
544	* deserialize it).
545	* @param s the stream
546	*/
547	private void readObject(java.io.ObjectInputStream s)
548	throws java.io.IOException, ClassNotFoundException {
549	// Read in capacity, and any hidden stuff
550	s.defaultReadObject();
551
552	// Read in all elements and place in queue
553	for (;;) {
554	E item = (E)s.readObject();
555	if (item == null)
556	break;
557	add(item);
558	}
559	}
560	}
561