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. New elements
 * are inserted at the tail of the queue, and the queue retrieval
 * operations obtain elements at the head of the queue.
 * A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when 
 * many threads will share access to a common collection.
 * 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 a traversal of the elements.
 *
 * <p>This class implements all of the <em>optional</em> methods
 * of the {@link Collection} and {@link Iterator} interfaces.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 *
 **/
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
        implements Queue<E>, java.io.Serializable {
    private static final long serialVersionUID = 196745693267521676L;

    /*
     * 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.
     */

    private static class Node<E> {
        private volatile E item;
        private volatile Node<E> next;
        
        private static final 
            AtomicReferenceFieldUpdater<Node, Node> 
            nextUpdater =
            AtomicReferenceFieldUpdater.newUpdater
            (Node.class, Node.class, "next");
        private static final 
            AtomicReferenceFieldUpdater<Node, Object> 
            itemUpdater =
            AtomicReferenceFieldUpdater.newUpdater
            (Node.class, Object.class, "item");
        
        Node(E x) { item = x; }
        
        Node(E x, Node<E> n) { item = x; next = n; }
        
        E getItem() {
            return item;
        }
        
        boolean casItem(E cmp, E val) {
            return itemUpdater.compareAndSet(this, cmp, val);
        }
        
        void setItem(E val) {
            itemUpdater.set(this, val);
        }
        
        Node<E> getNext() {
            return next;
        }
        
        boolean casNext(Node<E> cmp, Node<E> val) {
            return nextUpdater.compareAndSet(this, cmp, val);
        }
        
        void setNext(Node<E> val) {
            nextUpdater.set(this, val);
        }
        
    }

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

    private boolean casTail(Node<E> cmp, Node<E> val) {
        return tailUpdater.compareAndSet(this, cmp, val);
    }

    private boolean casHead(Node<E> cmp, Node<E> 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 Node<E> head = new Node<E>(null, null);

    /** Pointer to last node on list **/
    private transient volatile Node<E> 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.
     * @param o the element to add.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Collection.add</tt>).
     *
     * @throws NullPointerException if the specified element is <tt>null</tt>
     */
    public boolean add(E o) {
        return offer(o);
    }

    /**
     * Inserts the specified element to the tail of this queue.
     *
     * @param o the element to add.
     * @return <tt>true</tt> (as per the general contract of
     * <tt>Queue.offer</tt>).
     * @throws NullPointerException if the specified element is <tt>null</tt>
     */
    public boolean offer(E o) {
        if (o == null) throw new NullPointerException();
        Node<E> n = new Node<E>(o, null);
        for(;;) {
            Node<E> t = tail;
            Node<E> 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 (;;) {
            Node<E> h = head;
            Node<E> t = tail;
            Node<E> 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 = 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 (;;) {
            Node<E> h = head;
            Node<E> t = tail;
            Node<E> first = h.getNext();
            if (h == head) {
                if (h == t) {
                    if (first == null)
                        return null;
                    else
                        casTail(t, first);
                } else {
                    E item = 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.)
     */
    Node<E> first() {
        for (;;) {
            Node<E> h = head;
            Node<E> t = tail;
            Node<E> 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;
    }

    /**
     * Returns the number of elements in this queue.  If this queue
     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * <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.
     *
     * @return  the number of elements in this queue.
     */
    public int size() {
        int count = 0;
        for (Node<E> 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 (Node<E> p = first(); p != null; p = p.getNext()) {
            E item = p.getItem();
            if (item != null &&
                o.equals(item))
                return true;
        }
        return false;
    }

    public boolean remove(Object o) {
        if (o == null) return false;
        for (Node<E> p = first(); p != null; p = p.getNext()) {
            E 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 (Node<E> p = first(); p != null; p = p.getNext()) {
            E item = 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;
        Node<E> p;
        for (p = first(); p != null && k < a.length; p = p.getNext()) {
            E 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 (Node<E> q = first(); q != null; q = q.getNext()) {
            E item = 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 Node<E> 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 Node<E> 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 = nextItem;

            Node<E> p = (nextNode == null)? first() : nextNode.getNext();
            for (;;) {
                if (p == null) {
                    nextNode = null;
                    nextItem = null;
                    return x;
                }
                E item = 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() {
            Node<E> 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 (Node<E> 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();
        head = new Node<E>(null, null);
        tail = head;
        // Read in all elements and place in queue
        for (;;) {
            E item = (E)s.readObject();
            if (item == null)
                break;
            else
                offer(item);
        }
    }

}
Revision:	1.23
Committed:	Tue Dec 23 19:38:09 2003 UTC (20 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.22:	+44 -44 lines
Log Message:	cache finals across volatiles; avoid readResolve; doc improvments; timed invokeAll interleaves
#	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. New elements
19	* are inserted at the tail of the queue, and the queue retrieval
20	* operations obtain elements at the head of the queue.
21	* A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when
22	* many threads will share access to a common collection.
23	* This queue does not permit <tt>null</tt> elements.
24	*
25	* <p>This implementation employs an efficient "wait-free"
26	* algorithm based on one described in <a
27	* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
28	* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
29	* Algorithms</a> by Maged M. Michael and Michael L. Scott.
30	*
31	* <p>Beware that, unlike in most collections, the <tt>size</tt> method
32	* is <em>NOT</em> a constant-time operation. Because of the
33	* asynchronous nature of these queues, determining the current number
34	* of elements requires a traversal of the elements.
35	*
36	* <p>This class implements all of the <em>optional</em> methods
37	* of the {@link Collection} and {@link Iterator} interfaces.
38	*
39	* @since 1.5
40	* @author Doug Lea
41	* @param <E> the type of elements held in this collection
42	*
43	**/
44	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
45	implements Queue<E>, java.io.Serializable {
46	private static final long serialVersionUID = 196745693267521676L;
47
48	/*
49	* This is a straight adaptation of Michael & Scott algorithm.
50	* For explanation, read the paper. The only (minor) algorithmic
51	* difference is that this version supports lazy deletion of
52	* internal nodes (method remove(Object)) -- remove CAS'es item
53	* fields to null. The normal queue operations unlink but then
54	* pass over nodes with null item fields. Similarly, iteration
55	* methods ignore those with nulls.
56	*/
57
58	private static class Node<E> {
59	private volatile E item;
60	private volatile Node<E> next;
61
62	private static final
63	AtomicReferenceFieldUpdater<Node, Node>
64	nextUpdater =
65	AtomicReferenceFieldUpdater.newUpdater
66	(Node.class, Node.class, "next");
67	private static final
68	AtomicReferenceFieldUpdater<Node, Object>
69	itemUpdater =
70	AtomicReferenceFieldUpdater.newUpdater
71	(Node.class, Object.class, "item");
72
73	Node(E x) { item = x; }
74
75	Node(E x, Node<E> n) { item = x; next = n; }
76
77	E getItem() {
78	return item;
79	}
80
81	boolean casItem(E cmp, E val) {
82	return itemUpdater.compareAndSet(this, cmp, val);
83	}
84
85	void setItem(E val) {
86	itemUpdater.set(this, val);
87	}
88
89	Node<E> getNext() {
90	return next;
91	}
92
93	boolean casNext(Node<E> cmp, Node<E> val) {
94	return nextUpdater.compareAndSet(this, cmp, val);
95	}
96
97	void setNext(Node<E> val) {
98	nextUpdater.set(this, val);
99	}
100
101	}
102
103	private static final
104	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
105	tailUpdater =
106	AtomicReferenceFieldUpdater.newUpdater
107	(ConcurrentLinkedQueue.class, Node.class, "tail");
108	private static final
109	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
110	headUpdater =
111	AtomicReferenceFieldUpdater.newUpdater
112	(ConcurrentLinkedQueue.class, Node.class, "head");
113
114	private boolean casTail(Node<E> cmp, Node<E> val) {
115	return tailUpdater.compareAndSet(this, cmp, val);
116	}
117
118	private boolean casHead(Node<E> cmp, Node<E> val) {
119	return headUpdater.compareAndSet(this, cmp, val);
120	}
121
122
123	/**
124	* Pointer to header node, initialized to a dummy node. The first
125	* actual node is at head.getNext().
126	*/
127	private transient volatile Node<E> head = new Node<E>(null, null);
128
129	/ Pointer to last node on list /
130	private transient volatile Node<E> tail = head;
131
132
133	/**
134	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
135	*/
136	public ConcurrentLinkedQueue() {}
137
138	/**
139	* Creates a <tt>ConcurrentLinkedQueue</tt>
140	* initially containing the elements of the given collection,
141	* added in traversal order of the collection's iterator.
142	* @param c the collection of elements to initially contain
143	* @throws NullPointerException if <tt>c</tt> or any element within it
144	* is <tt>null</tt>
145	*/
146	public ConcurrentLinkedQueue(Collection<? extends E> c) {
147	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
148	add(it.next());
149	}
150
151	// Have to override just to update the javadoc
152
153	/**
154	* Adds the specified element to the tail of this queue.
155	* @param o the element to add.
156	* @return <tt>true</tt> (as per the general contract of
157	* <tt>Collection.add</tt>).
158	*
159	* @throws NullPointerException if the specified element is <tt>null</tt>
160	*/
161	public boolean add(E o) {
162	return offer(o);
163	}
164
165	/**
166	* Inserts the specified element to the tail of this queue.
167	*
168	* @param o the element to add.
169	* @return <tt>true</tt> (as per the general contract of
170	* <tt>Queue.offer</tt>).
171	* @throws NullPointerException if the specified element is <tt>null</tt>
172	*/
173	public boolean offer(E o) {
174	if (o == null) throw new NullPointerException();
175	Node<E> n = new Node<E>(o, null);
176	for(;;) {
177	Node<E> t = tail;
178	Node<E> s = t.getNext();
179	if (t == tail) {
180	if (s == null) {
181	if (t.casNext(s, n)) {
182	casTail(t, n);
183	return true;
184	}
185	} else {
186	casTail(t, s);
187	}
188	}
189	}
190	}
191
192	public E poll() {
193	for (;;) {
194	Node<E> h = head;
195	Node<E> t = tail;
196	Node<E> first = h.getNext();
197	if (h == head) {
198	if (h == t) {
199	if (first == null)
200	return null;
201	else
202	casTail(t, first);
203	} else if (casHead(h, first)) {
204	E item = first.getItem();
205	if (item != null) {
206	first.setItem(null);
207	return item;
208	}
209	// else skip over deleted item, continue loop,
210	}
211	}
212	}
213	}
214
215	public E peek() { // same as poll except don't remove item
216	for (;;) {
217	Node<E> h = head;
218	Node<E> t = tail;
219	Node<E> first = h.getNext();
220	if (h == head) {
221	if (h == t) {
222	if (first == null)
223	return null;
224	else
225	casTail(t, first);
226	} else {
227	E item = first.getItem();
228	if (item != null)
229	return item;
230	else // remove deleted node and continue
231	casHead(h, first);
232	}
233	}
234	}
235	}
236
237	/**
238	* Returns the first actual (non-header) node on list. This is yet
239	* another variant of poll/peek; here returning out the first
240	* node, not element (so we cannot collapse with peek() without
241	* introducing race.)
242	*/
243	Node<E> first() {
244	for (;;) {
245	Node<E> h = head;
246	Node<E> t = tail;
247	Node<E> first = h.getNext();
248	if (h == head) {
249	if (h == t) {
250	if (first == null)
251	return null;
252	else
253	casTail(t, first);
254	} else {
255	if (first.getItem() != null)
256	return first;
257	else // remove deleted node and continue
258	casHead(h, first);
259	}
260	}
261	}
262	}
263
264
265	public boolean isEmpty() {
266	return first() == null;
267	}
268
269	/**
270	* Returns the number of elements in this queue. If this queue
271	* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
272	* <tt>Integer.MAX_VALUE</tt>.
273	*
274	* <p>Beware that, unlike in most collections, this method is
275	* <em>NOT</em> a constant-time operation. Because of the
276	* asynchronous nature of these queues, determining the current
277	* number of elements requires an O(n) traversal.
278	*
279	* @return the number of elements in this queue.
280	*/
281	public int size() {
282	int count = 0;
283	for (Node<E> p = first(); p != null; p = p.getNext()) {
284	if (p.getItem() != null) {
285	// Collections.size() spec says to max out
286	if (++count == Integer.MAX_VALUE)
287	break;
288	}
289	}
290	return count;
291	}
292
293	public boolean contains(Object o) {
294	if (o == null) return false;
295	for (Node<E> p = first(); p != null; p = p.getNext()) {
296	E item = p.getItem();
297	if (item != null &&
298	o.equals(item))
299	return true;
300	}
301	return false;
302	}
303
304	public boolean remove(Object o) {
305	if (o == null) return false;
306	for (Node<E> p = first(); p != null; p = p.getNext()) {
307	E item = p.getItem();
308	if (item != null &&
309	o.equals(item) &&
310	p.casItem(item, null))
311	return true;
312	}
313	return false;
314	}
315
316	public Object[] toArray() {
317	// Use ArrayList to deal with resizing.
318	ArrayList<E> al = new ArrayList<E>();
319	for (Node<E> p = first(); p != null; p = p.getNext()) {
320	E item = p.getItem();
321	if (item != null)
322	al.add(item);
323	}
324	return al.toArray();
325	}
326
327	public <T> T[] toArray(T[] a) {
328	// try to use sent-in array
329	int k = 0;
330	Node<E> p;
331	for (p = first(); p != null && k < a.length; p = p.getNext()) {
332	E item = p.getItem();
333	if (item != null)
334	a[k++] = (T)item;
335	}
336	if (p == null) {
337	if (k < a.length)
338	a[k] = null;
339	return a;
340	}
341
342	// If won't fit, use ArrayList version
343	ArrayList<E> al = new ArrayList<E>();
344	for (Node<E> q = first(); q != null; q = q.getNext()) {
345	E item = q.getItem();
346	if (item != null)
347	al.add(item);
348	}
349	return (T[])al.toArray(a);
350	}
351
352	/**
353	* Returns an iterator over the elements in this queue in proper sequence.
354	* The returned iterator is a "weakly consistent" iterator that
355	* will never throw {@link java.util.ConcurrentModificationException},
356	* and guarantees to traverse elements as they existed upon
357	* construction of the iterator, and may (but is not guaranteed to)
358	* reflect any modifications subsequent to construction.
359	*
360	* @return an iterator over the elements in this queue in proper sequence.
361	*/
362	public Iterator<E> iterator() {
363	return new Itr();
364	}
365
366	private class Itr implements Iterator<E> {
367	/**
368	* Next node to return item for.
369	*/
370	private Node<E> nextNode;
371
372	/**
373	* nextItem holds on to item fields because once we claim
374	* that an element exists in hasNext(), we must return it in
375	* the following next() call even if it was in the process of
376	* being removed when hasNext() was called.
377	**/
378	private E nextItem;
379
380	/**
381	* Node of the last returned item, to support remove.
382	*/
383	private Node<E> lastRet;
384
385	Itr() {
386	advance();
387	}
388
389	/**
390	* Move to next valid node.
391	* Return item to return for next(), or null if no such.
392	*/
393	private E advance() {
394	lastRet = nextNode;
395	E x = nextItem;
396
397	Node<E> p = (nextNode == null)? first() : nextNode.getNext();
398	for (;;) {
399	if (p == null) {
400	nextNode = null;
401	nextItem = null;
402	return x;
403	}
404	E item = p.getItem();
405	if (item != null) {
406	nextNode = p;
407	nextItem = item;
408	return x;
409	} else // skip over nulls
410	p = p.getNext();
411	}
412	}
413
414	public boolean hasNext() {
415	return nextNode != null;
416	}
417
418	public E next() {
419	if (nextNode == null) throw new NoSuchElementException();
420	return advance();
421	}
422
423	public void remove() {
424	Node<E> l = lastRet;
425	if (l == null) throw new IllegalStateException();
426	// rely on a future traversal to relink.
427	l.setItem(null);
428	lastRet = null;
429	}
430	}
431
432	/**
433	* Save the state to a stream (that is, serialize it).
434	*
435	* @serialData All of the elements (each an <tt>E</tt>) in
436	* the proper order, followed by a null
437	* @param s the stream
438	*/
439	private void writeObject(java.io.ObjectOutputStream s)
440	throws java.io.IOException {
441
442	// Write out any hidden stuff
443	s.defaultWriteObject();
444
445	// Write out all elements in the proper order.
446	for (Node<E> p = first(); p != null; p = p.getNext()) {
447	Object item = p.getItem();
448	if (item != null)
449	s.writeObject(item);
450	}
451
452	// Use trailing null as sentinel
453	s.writeObject(null);
454	}
455
456	/**
457	* Reconstitute the Queue instance from a stream (that is,
458	* deserialize it).
459	* @param s the stream
460	*/
461	private void readObject(java.io.ObjectInputStream s)
462	throws java.io.IOException, ClassNotFoundException {
463	// Read in capacity, and any hidden stuff
464	s.defaultReadObject();
465	head = new Node<E>(null, null);
466	tail = head;
467	// Read in all elements and place in queue
468	for (;;) {
469	E item = (E)s.readObject();
470	if (item == null)
471	break;
472	else
473	offer(item);
474	}
475	}
476
477	}