util/concurrent/ConcurrentLinkedQueue.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.*;
import java.util.concurrent.atomic.*;


/**
 * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.  
 * 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.
 * A <tt>ConcurrentLinkedQueue</tt> is an especially good choice when 
 * many threads will share access to a common queue.
 * This queue does not permit <tt>null</tt> elements.
 *
 * <p>This implementation employs an efficient &quot;wait-free&quot; 
 * algorithm based on one described in <a
 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
 * Algorithms</a> by Maged M. Michael and Michael L. Scott.)
 *
 * <p>Beware that, unlike in most collections, the <tt>size</tt> method
 * is <em>NOT</em> a constant-time operation. Because of the
 * asynchronous nature of these queues, determining the current number
 * of elements requires an O(n) traversal.
 * @since 1.5
 * @author Doug Lea
 *
 **/
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
        implements Queue<E>, java.io.Serializable {

    /*
     * This is a straight adaptation of Michael & Scott algorithm.
     * For explanation, read the paper.  The only (minor) algorithmic
     * difference is that this version supports lazy deletion of
     * internal nodes (method remove(Object)) -- remove CAS'es item
     * fields to null. The normal queue operations unlink but then
     * pass over nodes with null item fields. Similarly, iteration
     * methods ignore those with nulls.
     */

    // Atomics support

    private static final 
        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, AtomicLinkedNode> 
        tailUpdater = 
        AtomicReferenceFieldUpdater.newUpdater
        (ConcurrentLinkedQueue.class, AtomicLinkedNode.class, "tail");
    private static final 
        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, AtomicLinkedNode> 
        headUpdater = 
        AtomicReferenceFieldUpdater.newUpdater
        (ConcurrentLinkedQueue.class,  AtomicLinkedNode.class, "head");

    private boolean casTail(AtomicLinkedNode cmp, AtomicLinkedNode val) {
        return tailUpdater.compareAndSet(this, cmp, val);
    }

    private boolean casHead(AtomicLinkedNode cmp, AtomicLinkedNode val) {
        return headUpdater.compareAndSet(this, cmp, val);
    }


    /**
     * Pointer to header node, initialized to a dummy node.  The first
     * actual node is at head.getNext().
     */
    private transient volatile AtomicLinkedNode head = new AtomicLinkedNode(null, null);

    /** Pointer to last node on list **/
    private transient volatile AtomicLinkedNode tail = head;


    /**
     * Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
     */
    public ConcurrentLinkedQueue() {}

    /**
     * Creates a <tt>ConcurrentLinkedQueue</tt> 
     * initially containing the elements of the given collection,
     * added in traversal order of the collection's iterator.
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if <tt>c</tt> or any element within it
     * is <tt>null</tt>
     */
    public ConcurrentLinkedQueue(Collection<? extends E> c) {
        for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
            add(it.next());
    }

    // Have to override just to update the javadoc 

    /**
     * Adds the specified element to the tail of this queue.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     *
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean add(E o) {
        return super.add(o);
    }

    /**
     * Adds all of the elements in the specified collection to this queue.
     * The behavior of this operation is undefined if
     * the specified collection is modified while the operation is in
     * progress.  (This implies that the behavior of this call is undefined if
     * the specified collection is this queue, and this queue is nonempty.)
     * <p>
     * This implementation iterates over the specified collection, and adds
     * each object returned by the iterator to this queue's tail, in turn.
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean addAll(Collection<? extends E> c) {
        return super.addAll(c);
    }

    /**
     * @throws NullPointerException if the specified element is <tt>null</tt>
     */
    public boolean offer(E o) {
        if (o == null) throw new NullPointerException();
        AtomicLinkedNode n = new AtomicLinkedNode(o, null);
        for(;;) {
            AtomicLinkedNode t = tail;
            AtomicLinkedNode s = t.getNext();
            if (t == tail) {
                if (s == null) {
                    if (t.casNext(s, n)) {
                        casTail(t, n);
                        return true;
                    }
                }
                else {
                    casTail(t, s);
                }
            }
        }
    }

    public E poll() {
        for (;;) {
            AtomicLinkedNode h = head;
            AtomicLinkedNode t = tail;
            AtomicLinkedNode first = h.getNext();
            if (h == head) {
                if (h == t) {
                    if (first == null)
                        return null;
                    else
                        casTail(t, first);
                }
                else if (casHead(h, first)) {
                    E item = (E)first.getItem();
                    if (item != null) {
                        first.setItem(null);
                        return item;
                    }
                    // else skip over deleted item, continue loop,
                }
            }
        }
    }

    public E peek() { // same as poll except don't remove item
        for (;;) {
            AtomicLinkedNode h = head;
            AtomicLinkedNode t = tail;
            AtomicLinkedNode first = h.getNext();
            if (h == head) {
                if (h == t) {
                    if (first == null)
                        return null;
                    else
                        casTail(t, first);
                }
                else {
                    E item = (E)first.getItem();
                    if (item != null)
                        return item;
                    else // remove deleted node and continue
                        casHead(h, first);
                }
            }
        }
    }

    /**
     * Returns the first actual (non-header) node on list.  This is yet
     * another variant of poll/peek; here returning out the first
     * node, not element (so we cannot collapse with peek() without
     * introducing race.)
     */
    AtomicLinkedNode first() {
        for (;;) {
            AtomicLinkedNode h = head;
            AtomicLinkedNode t = tail;
            AtomicLinkedNode first = h.getNext();
            if (h == head) {
                if (h == t) {
                    if (first == null)
                        return null;
                    else
                        casTail(t, first);
                }
                else {
                    if (first.getItem() != null)
                        return first;
                    else // remove deleted node and continue
                        casHead(h, first);
                }
            }
        }
    }


    public boolean isEmpty() {
        return first() == null;
    }

    /**
     * @inheritDoc
     *
     * <p>Beware that, unlike in most collections, this method is
     * <em>NOT</em> a constant-time operation. Because of the
     * asynchronous nature of these queues, determining the current
     * number of elements requires an O(n) traversal.
     */
    public int size() {
        int count = 0;
        for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
            if (p.getItem() != null) {
                // Collections.size() spec says to max out
                if (++count == Integer.MAX_VALUE)
                    break;
            }
        }
        return count;
    }

    public boolean contains(Object o) {
        if (o == null) return false;
        for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
            Object item = p.getItem();
            if (item != null &&
                o.equals(item))
                return true;
        }
        return false;
    }

    /**
     * Removes a single instance of the specified element from this
     * queue, if it is present.  More formally,
     * removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
     * o.equals(e))</tt>, if the queue contains one or more such
     * elements.  Returns <tt>true</tt> if the queue contained the
     * specified element (or equivalently, if the queue changed as a
     * result of the call).
     *
     * <p>This implementation iterates over the queue looking for the
     * specified element.  If it finds the element, it removes the element
     * from the queue using the iterator's remove method.<p>
     *
     */
    public boolean remove(Object o) {
        if (o == null) return false;
        for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
            Object item = p.getItem();
            if (item != null &&
                o.equals(item) &&
                p.casItem(item, null))
                return true;
        }
        return false;
    }

    public Object[] toArray() {
        // Use ArrayList to deal with resizing.
        ArrayList<E> al = new ArrayList<E>();
        for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
            E item = (E) p.getItem();
            if (item != null)
                al.add(item);
        }
        return al.toArray();
    }

    public <T> T[] toArray(T[] a) {
        // try to use sent-in array
        int k = 0;
        AtomicLinkedNode p;
        for (p = first(); p != null && k < a.length; p = p.getNext()) {
            Object item = p.getItem();
            if (item != null)
                a[k++] = (T)item;
        }
        if (p == null) {
            if (k < a.length)
                a[k] = null;
            return a;
        }

        // If won't fit, use ArrayList version
        ArrayList<E> al = new ArrayList<E>();
        for (AtomicLinkedNode q = first(); q != null; q = q.getNext()) {
            E item = (E) q.getItem();
            if (item != null)
                al.add(item);
        }
        return (T[])al.toArray(a);
    }

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The returned iterator is a "weakly consistent" iterator that
     * will never throw {@link java.util.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 in proper sequence.
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        /**
         * Next node to return item for.
         */
        private AtomicLinkedNode nextNode;

        /**
         * 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;

        /**
         * Node of the last returned item, to support remove.
         */
        private AtomicLinkedNode lastRet;

        Itr() {
            advance();
        }

        /**
         * Move to next valid node.
         * Return item to return for next(), or null if no such.
         */
        private E advance() {
            lastRet = nextNode;
            E x = (E)nextItem;

            AtomicLinkedNode p = (nextNode == null)? first() : nextNode.getNext();
            for (;;) {
                if (p == null) {
                    nextNode = null;
                    nextItem = null;
                    return x;
                }
                E item = (E)p.getItem();
                if (item != null) {
                    nextNode = p;
                    nextItem = item;
                    return x;
                }
                else // skip over nulls
                    p = p.getNext();
            }
        }

        public boolean hasNext() {
            return nextNode != null;
        }

        public E next() {
            if (nextNode == null) throw new NoSuchElementException();
            return advance();
        }

        public void remove() {
            AtomicLinkedNode l = lastRet;
            if (l == null) throw new IllegalStateException();
            // rely on a future traversal to relink.
            l.setItem(null);
            lastRet = null;
        }
    }

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

        // Write out any hidden stuff
        s.defaultWriteObject();

        // Write out all elements in the proper order.
        for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
            Object item = p.getItem();
            if (item != null)
                s.writeObject(item);
        }

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

    /**
     * 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:	1.10
Committed:	Wed Aug 6 18:42:49 2003 UTC (20 years, 10 months ago) by tim
Branch:	MAIN
Changes since 1.9:	+1 -1 lines
Log Message:	Fixes to minor errors found by DocCheck, includes stray Locks reference removal
#	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.*;
9	import java.util.concurrent.atomic.*;
10
11
12	/**
13	* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
14	* This queue orders elements FIFO (first-in-first-out).
15	* The <em>head</em> of the queue is that element that has been on the
16	* queue the longest time.
17	* The <em>tail</em> of the queue is that element that has been on the
18	* queue the shortest time.
19	* A <tt>ConcurrentLinkedQueue</tt> is an especially good choice when
20	* many threads will share access to a common queue.
21	* This queue does not permit <tt>null</tt> elements.
22	*
23	* <p>This implementation employs an efficient "wait-free"
24	* algorithm based on one described in <a
25	* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
26	* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
27	* Algorithms</a> by Maged M. Michael and Michael L. Scott.)
28	*
29	* <p>Beware that, unlike in most collections, the <tt>size</tt> method
30	* is <em>NOT</em> a constant-time operation. Because of the
31	* asynchronous nature of these queues, determining the current number
32	* of elements requires an O(n) traversal.
33	* @since 1.5
34	* @author Doug Lea
35	*
36	**/
37	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
38	implements Queue<E>, java.io.Serializable {
39
40	/*
41	* This is a straight adaptation of Michael & Scott algorithm.
42	* For explanation, read the paper. The only (minor) algorithmic
43	* difference is that this version supports lazy deletion of
44	* internal nodes (method remove(Object)) -- remove CAS'es item
45	* fields to null. The normal queue operations unlink but then
46	* pass over nodes with null item fields. Similarly, iteration
47	* methods ignore those with nulls.
48	*/
49
50	// Atomics support
51
52	private static final
53	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, AtomicLinkedNode>
54	tailUpdater =
55	AtomicReferenceFieldUpdater.newUpdater
56	(ConcurrentLinkedQueue.class, AtomicLinkedNode.class, "tail");
57	private static final
58	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, AtomicLinkedNode>
59	headUpdater =
60	AtomicReferenceFieldUpdater.newUpdater
61	(ConcurrentLinkedQueue.class, AtomicLinkedNode.class, "head");
62
63	private boolean casTail(AtomicLinkedNode cmp, AtomicLinkedNode val) {
64	return tailUpdater.compareAndSet(this, cmp, val);
65	}
66
67	private boolean casHead(AtomicLinkedNode cmp, AtomicLinkedNode val) {
68	return headUpdater.compareAndSet(this, cmp, val);
69	}
70
71
72	/**
73	* Pointer to header node, initialized to a dummy node. The first
74	* actual node is at head.getNext().
75	*/
76	private transient volatile AtomicLinkedNode head = new AtomicLinkedNode(null, null);
77
78	/ Pointer to last node on list /
79	private transient volatile AtomicLinkedNode tail = head;
80
81
82	/**
83	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
84	*/
85	public ConcurrentLinkedQueue() {}
86
87	/**
88	* Creates a <tt>ConcurrentLinkedQueue</tt>
89	* initially containing the elements of the given collection,
90	* added in traversal order of the collection's iterator.
91	* @param c the collection of elements to initially contain
92	* @throws NullPointerException if <tt>c</tt> or any element within it
93	* is <tt>null</tt>
94	*/
95	public ConcurrentLinkedQueue(Collection<? extends E> c) {
96	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
97	add(it.next());
98	}
99
100	// Have to override just to update the javadoc
101
102	/**
103	* Adds the specified element to the tail of this queue.
104	* @return <tt>true</tt> (as per the general contract of
105	* <tt>Collection.add</tt>).
106	*
107	* @throws NullPointerException {@inheritDoc}
108	*/
109	public boolean add(E o) {
110	return super.add(o);
111	}
112
113	/**
114	* Adds all of the elements in the specified collection to this queue.
115	* The behavior of this operation is undefined if
116	* the specified collection is modified while the operation is in
117	* progress. (This implies that the behavior of this call is undefined if
118	* the specified collection is this queue, and this queue is nonempty.)
119	* <p>
120	* This implementation iterates over the specified collection, and adds
121	* each object returned by the iterator to this queue's tail, in turn.
122	* @throws NullPointerException {@inheritDoc}
123	*/
124	public boolean addAll(Collection<? extends E> c) {
125	return super.addAll(c);
126	}
127
128	/**
129	* @throws NullPointerException if the specified element is <tt>null</tt>
130	*/
131	public boolean offer(E o) {
132	if (o == null) throw new NullPointerException();
133	AtomicLinkedNode n = new AtomicLinkedNode(o, null);
134	for(;;) {
135	AtomicLinkedNode t = tail;
136	AtomicLinkedNode s = t.getNext();
137	if (t == tail) {
138	if (s == null) {
139	if (t.casNext(s, n)) {
140	casTail(t, n);
141	return true;
142	}
143	}
144	else {
145	casTail(t, s);
146	}
147	}
148	}
149	}
150
151	public E poll() {
152	for (;;) {
153	AtomicLinkedNode h = head;
154	AtomicLinkedNode t = tail;
155	AtomicLinkedNode first = h.getNext();
156	if (h == head) {
157	if (h == t) {
158	if (first == null)
159	return null;
160	else
161	casTail(t, first);
162	}
163	else if (casHead(h, first)) {
164	E item = (E)first.getItem();
165	if (item != null) {
166	first.setItem(null);
167	return item;
168	}
169	// else skip over deleted item, continue loop,
170	}
171	}
172	}
173	}
174
175	public E peek() { // same as poll except don't remove item
176	for (;;) {
177	AtomicLinkedNode h = head;
178	AtomicLinkedNode t = tail;
179	AtomicLinkedNode first = h.getNext();
180	if (h == head) {
181	if (h == t) {
182	if (first == null)
183	return null;
184	else
185	casTail(t, first);
186	}
187	else {
188	E item = (E)first.getItem();
189	if (item != null)
190	return item;
191	else // remove deleted node and continue
192	casHead(h, first);
193	}
194	}
195	}
196	}
197
198	/**
199	* Returns the first actual (non-header) node on list. This is yet
200	* another variant of poll/peek; here returning out the first
201	* node, not element (so we cannot collapse with peek() without
202	* introducing race.)
203	*/
204	AtomicLinkedNode first() {
205	for (;;) {
206	AtomicLinkedNode h = head;
207	AtomicLinkedNode t = tail;
208	AtomicLinkedNode first = h.getNext();
209	if (h == head) {
210	if (h == t) {
211	if (first == null)
212	return null;
213	else
214	casTail(t, first);
215	}
216	else {
217	if (first.getItem() != null)
218	return first;
219	else // remove deleted node and continue
220	casHead(h, first);
221	}
222	}
223	}
224	}
225
226
227	public boolean isEmpty() {
228	return first() == null;
229	}
230
231	/**
232	* @inheritDoc
233	*
234	* <p>Beware that, unlike in most collections, this method is
235	* <em>NOT</em> a constant-time operation. Because of the
236	* asynchronous nature of these queues, determining the current
237	* number of elements requires an O(n) traversal.
238	*/
239	public int size() {
240	int count = 0;
241	for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
242	if (p.getItem() != null) {
243	// Collections.size() spec says to max out
244	if (++count == Integer.MAX_VALUE)
245	break;
246	}
247	}
248	return count;
249	}
250
251	public boolean contains(Object o) {
252	if (o == null) return false;
253	for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
254	Object item = p.getItem();
255	if (item != null &&
256	o.equals(item))
257	return true;
258	}
259	return false;
260	}
261
262	/**
263	* Removes a single instance of the specified element from this
264	* queue, if it is present. More formally,
265	* removes an element <tt>e</tt> such that <tt>(o==null ? e==null :
266	* o.equals(e))</tt>, if the queue contains one or more such
267	* elements. Returns <tt>true</tt> if the queue contained the
268	* specified element (or equivalently, if the queue changed as a
269	* result of the call).
270	*
271	* <p>This implementation iterates over the queue looking for the
272	* specified element. If it finds the element, it removes the element
273	* from the queue using the iterator's remove method.<p>
274	*
275	*/
276	public boolean remove(Object o) {
277	if (o == null) return false;
278	for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
279	Object item = p.getItem();
280	if (item != null &&
281	o.equals(item) &&
282	p.casItem(item, null))
283	return true;
284	}
285	return false;
286	}
287
288	public Object[] toArray() {
289	// Use ArrayList to deal with resizing.
290	ArrayList<E> al = new ArrayList<E>();
291	for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
292	E item = (E) p.getItem();
293	if (item != null)
294	al.add(item);
295	}
296	return al.toArray();
297	}
298
299	public <T> T[] toArray(T[] a) {
300	// try to use sent-in array
301	int k = 0;
302	AtomicLinkedNode p;
303	for (p = first(); p != null && k < a.length; p = p.getNext()) {
304	Object item = p.getItem();
305	if (item != null)
306	a[k++] = (T)item;
307	}
308	if (p == null) {
309	if (k < a.length)
310	a[k] = null;
311	return a;
312	}
313
314	// If won't fit, use ArrayList version
315	ArrayList<E> al = new ArrayList<E>();
316	for (AtomicLinkedNode q = first(); q != null; q = q.getNext()) {
317	E item = (E) q.getItem();
318	if (item != null)
319	al.add(item);
320	}
321	return (T[])al.toArray(a);
322	}
323
324	/**
325	* Returns an iterator over the elements in this queue in proper sequence.
326	* The returned iterator is a "weakly consistent" iterator that
327	* will never throw {@link java.util.ConcurrentModificationException},
328	* and guarantees to traverse elements as they existed upon
329	* construction of the iterator, and may (but is not guaranteed to)
330	* reflect any modifications subsequent to construction.
331	*
332	* @return an iterator over the elements in this queue in proper sequence.
333	*/
334	public Iterator<E> iterator() {
335	return new Itr();
336	}
337
338	private class Itr implements Iterator<E> {
339	/**
340	* Next node to return item for.
341	*/
342	private AtomicLinkedNode nextNode;
343
344	/**
345	* nextItem holds on to item fields because once we claim
346	* that an element exists in hasNext(), we must return it in
347	* the following next() call even if it was in the process of
348	* being removed when hasNext() was called.
349	**/
350	private E nextItem;
351
352	/**
353	* Node of the last returned item, to support remove.
354	*/
355	private AtomicLinkedNode lastRet;
356
357	Itr() {
358	advance();
359	}
360
361	/**
362	* Move to next valid node.
363	* Return item to return for next(), or null if no such.
364	*/
365	private E advance() {
366	lastRet = nextNode;
367	E x = (E)nextItem;
368
369	AtomicLinkedNode p = (nextNode == null)? first() : nextNode.getNext();
370	for (;;) {
371	if (p == null) {
372	nextNode = null;
373	nextItem = null;
374	return x;
375	}
376	E item = (E)p.getItem();
377	if (item != null) {
378	nextNode = p;
379	nextItem = item;
380	return x;
381	}
382	else // skip over nulls
383	p = p.getNext();
384	}
385	}
386
387	public boolean hasNext() {
388	return nextNode != null;
389	}
390
391	public E next() {
392	if (nextNode == null) throw new NoSuchElementException();
393	return advance();
394	}
395
396	public void remove() {
397	AtomicLinkedNode l = lastRet;
398	if (l == null) throw new IllegalStateException();
399	// rely on a future traversal to relink.
400	l.setItem(null);
401	lastRet = null;
402	}
403	}
404
405	/**
406	* Save the state to a stream (that is, serialize it).
407	*
408	* @serialData All of the elements (each an <tt>E</tt>) in
409	* the proper order, followed by a null
410	* @param s the stream
411	*/
412	private void writeObject(java.io.ObjectOutputStream s)
413	throws java.io.IOException {
414
415	// Write out any hidden stuff
416	s.defaultWriteObject();
417
418	// Write out all elements in the proper order.
419	for (AtomicLinkedNode p = first(); p != null; p = p.getNext()) {
420	Object item = p.getItem();
421	if (item != null)
422	s.writeObject(item);
423	}
424
425	// Use trailing null as sentinel
426	s.writeObject(null);
427	}
428
429	/**
430	* Reconstitute the Queue instance from a stream (that is,
431	* deserialize it).
432	* @param s the stream
433	*/
434	private void readObject(java.io.ObjectInputStream s)
435	throws java.io.IOException, ClassNotFoundException {
436	// Read in capacity, and any hidden stuff
437	s.defaultReadObject();
438
439	// Read in all elements and place in queue
440	for (;;) {
441	E item = (E)s.readObject();
442	if (item == null)
443	break;
444	add(item);
445	}
446	}
447
448	}