util/concurrent/ConcurrentLinkedQueue.java

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

package java.util.concurrent;
import java.util.*;
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 and its iterator implement all of the
 * <em>optional</em> methods of the {@link Collection} and {@link
 * Iterator} interfaces.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../guide/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this collection
 *
 */
public class 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);
                }
            }
        }
    }


    /**
     * Returns <tt>true</tt> if this queue contains no elements.
     *
     * @return <tt>true</tt> if this queue contains no elements.
     */
    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 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();
        }

        /**
         * Moves to next valid node and returns 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:	1.30
Committed:	Fri Apr 29 02:00:39 2005 UTC (19 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.29:	+2 -2 lines
Log Message:	doc fixes
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/licenses/publicdomain
5	*/
6
7	package java.util.concurrent;
8	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 and its iterator implement all of the
37	* <em>optional</em> methods of the {@link Collection} and {@link
38	* Iterator} interfaces.
39	*
40	* <p>This class is a member of the
41	* <a href="{@docRoot}/../guide/collections/index.html">
42	* Java Collections Framework</a>.
43	*
44	* @since 1.5
45	* @author Doug Lea
46	* @param <E> the type of elements held in this collection
47	*
48	*/
49	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
50	implements Queue<E>, java.io.Serializable {
51	private static final long serialVersionUID = 196745693267521676L;
52
53	/*
54	* This is a straight adaptation of Michael & Scott algorithm.
55	* For explanation, read the paper. The only (minor) algorithmic
56	* difference is that this version supports lazy deletion of
57	* internal nodes (method remove(Object)) -- remove CAS'es item
58	* fields to null. The normal queue operations unlink but then
59	* pass over nodes with null item fields. Similarly, iteration
60	* methods ignore those with nulls.
61	*/
62
63	private static class Node<E> {
64	private volatile E item;
65	private volatile Node<E> next;
66
67	private static final
68	AtomicReferenceFieldUpdater<Node, Node>
69	nextUpdater =
70	AtomicReferenceFieldUpdater.newUpdater
71	(Node.class, Node.class, "next");
72	private static final
73	AtomicReferenceFieldUpdater<Node, Object>
74	itemUpdater =
75	AtomicReferenceFieldUpdater.newUpdater
76	(Node.class, Object.class, "item");
77
78	Node(E x) { item = x; }
79
80	Node(E x, Node<E> n) { item = x; next = n; }
81
82	E getItem() {
83	return item;
84	}
85
86	boolean casItem(E cmp, E val) {
87	return itemUpdater.compareAndSet(this, cmp, val);
88	}
89
90	void setItem(E val) {
91	itemUpdater.set(this, val);
92	}
93
94	Node<E> getNext() {
95	return next;
96	}
97
98	boolean casNext(Node<E> cmp, Node<E> val) {
99	return nextUpdater.compareAndSet(this, cmp, val);
100	}
101
102	void setNext(Node<E> val) {
103	nextUpdater.set(this, val);
104	}
105
106	}
107
108	private static final
109	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
110	tailUpdater =
111	AtomicReferenceFieldUpdater.newUpdater
112	(ConcurrentLinkedQueue.class, Node.class, "tail");
113	private static final
114	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
115	headUpdater =
116	AtomicReferenceFieldUpdater.newUpdater
117	(ConcurrentLinkedQueue.class, Node.class, "head");
118
119	private boolean casTail(Node<E> cmp, Node<E> val) {
120	return tailUpdater.compareAndSet(this, cmp, val);
121	}
122
123	private boolean casHead(Node<E> cmp, Node<E> val) {
124	return headUpdater.compareAndSet(this, cmp, val);
125	}
126
127
128	/**
129	* Pointer to header node, initialized to a dummy node. The first
130	* actual node is at head.getNext().
131	*/
132	private transient volatile Node<E> head = new Node<E>(null, null);
133
134	/ Pointer to last node on list /
135	private transient volatile Node<E> tail = head;
136
137
138	/**
139	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
140	*/
141	public ConcurrentLinkedQueue() {}
142
143	/**
144	* Creates a <tt>ConcurrentLinkedQueue</tt>
145	* initially containing the elements of the given collection,
146	* added in traversal order of the collection's iterator.
147	* @param c the collection of elements to initially contain
148	* @throws NullPointerException if <tt>c</tt> or any element within it
149	* is <tt>null</tt>.
150	*/
151	public ConcurrentLinkedQueue(Collection<? extends E> c) {
152	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
153	add(it.next());
154	}
155
156	// Have to override just to update the javadoc
157
158	/**
159	* Adds the specified element to the tail of this queue.
160	* @param o the element to add.
161	* @return <tt>true</tt> (as per the general contract of
162	* <tt>Collection.add</tt>).
163	*
164	* @throws NullPointerException if the specified element is <tt>null</tt>.
165	*/
166	public boolean add(E o) {
167	return offer(o);
168	}
169
170	/**
171	* Inserts the specified element to the tail of this queue.
172	*
173	* @param o the element to add.
174	* @return <tt>true</tt> (as per the general contract of
175	* <tt>Queue.offer</tt>).
176	* @throws NullPointerException if the specified element is <tt>null</tt>.
177	*/
178	public boolean offer(E o) {
179	if (o == null) throw new NullPointerException();
180	Node<E> n = new Node<E>(o, null);
181	for(;;) {
182	Node<E> t = tail;
183	Node<E> s = t.getNext();
184	if (t == tail) {
185	if (s == null) {
186	if (t.casNext(s, n)) {
187	casTail(t, n);
188	return true;
189	}
190	} else {
191	casTail(t, s);
192	}
193	}
194	}
195	}
196
197	public E poll() {
198	for (;;) {
199	Node<E> h = head;
200	Node<E> t = tail;
201	Node<E> first = h.getNext();
202	if (h == head) {
203	if (h == t) {
204	if (first == null)
205	return null;
206	else
207	casTail(t, first);
208	} else if (casHead(h, first)) {
209	E item = first.getItem();
210	if (item != null) {
211	first.setItem(null);
212	return item;
213	}
214	// else skip over deleted item, continue loop,
215	}
216	}
217	}
218	}
219
220	public E peek() { // same as poll except don't remove item
221	for (;;) {
222	Node<E> h = head;
223	Node<E> t = tail;
224	Node<E> first = h.getNext();
225	if (h == head) {
226	if (h == t) {
227	if (first == null)
228	return null;
229	else
230	casTail(t, first);
231	} else {
232	E item = first.getItem();
233	if (item != null)
234	return item;
235	else // remove deleted node and continue
236	casHead(h, first);
237	}
238	}
239	}
240	}
241
242	/**
243	* Returns the first actual (non-header) node on list. This is yet
244	* another variant of poll/peek; here returning out the first
245	* node, not element (so we cannot collapse with peek() without
246	* introducing race.)
247	*/
248	Node<E> first() {
249	for (;;) {
250	Node<E> h = head;
251	Node<E> t = tail;
252	Node<E> first = h.getNext();
253	if (h == head) {
254	if (h == t) {
255	if (first == null)
256	return null;
257	else
258	casTail(t, first);
259	} else {
260	if (first.getItem() != null)
261	return first;
262	else // remove deleted node and continue
263	casHead(h, first);
264	}
265	}
266	}
267	}
268
269
270	/**
271	* Returns <tt>true</tt> if this queue contains no elements.
272	*
273	* @return <tt>true</tt> if this queue contains no elements.
274	*/
275	public boolean isEmpty() {
276	return first() == null;
277	}
278
279	/**
280	* Returns the number of elements in this queue. If this queue
281	* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
282	* <tt>Integer.MAX_VALUE</tt>.
283	*
284	* <p>Beware that, unlike in most collections, this method is
285	* <em>NOT</em> a constant-time operation. Because of the
286	* asynchronous nature of these queues, determining the current
287	* number of elements requires an O(n) traversal.
288	*
289	* @return the number of elements in this queue.
290	*/
291	public int size() {
292	int count = 0;
293	for (Node<E> p = first(); p != null; p = p.getNext()) {
294	if (p.getItem() != null) {
295	// Collections.size() spec says to max out
296	if (++count == Integer.MAX_VALUE)
297	break;
298	}
299	}
300	return count;
301	}
302
303	public boolean contains(Object o) {
304	if (o == null) return false;
305	for (Node<E> p = first(); p != null; p = p.getNext()) {
306	E item = p.getItem();
307	if (item != null &&
308	o.equals(item))
309	return true;
310	}
311	return false;
312	}
313
314	public boolean remove(Object o) {
315	if (o == null) return false;
316	for (Node<E> p = first(); p != null; p = p.getNext()) {
317	E item = p.getItem();
318	if (item != null &&
319	o.equals(item) &&
320	p.casItem(item, null))
321	return true;
322	}
323	return false;
324	}
325
326	public Object[] toArray() {
327	// Use ArrayList to deal with resizing.
328	ArrayList<E> al = new ArrayList<E>();
329	for (Node<E> p = first(); p != null; p = p.getNext()) {
330	E item = p.getItem();
331	if (item != null)
332	al.add(item);
333	}
334	return al.toArray();
335	}
336
337	public <T> T[] toArray(T[] a) {
338	// try to use sent-in array
339	int k = 0;
340	Node<E> p;
341	for (p = first(); p != null && k < a.length; p = p.getNext()) {
342	E item = p.getItem();
343	if (item != null)
344	a[k++] = (T)item;
345	}
346	if (p == null) {
347	if (k < a.length)
348	a[k] = null;
349	return a;
350	}
351
352	// If won't fit, use ArrayList version
353	ArrayList<E> al = new ArrayList<E>();
354	for (Node<E> q = first(); q != null; q = q.getNext()) {
355	E item = q.getItem();
356	if (item != null)
357	al.add(item);
358	}
359	return (T[])al.toArray(a);
360	}
361
362	/**
363	* Returns an iterator over the elements in this queue in proper sequence.
364	* The returned iterator is a "weakly consistent" iterator that
365	* will never throw {@link ConcurrentModificationException},
366	* and guarantees to traverse elements as they existed upon
367	* construction of the iterator, and may (but is not guaranteed to)
368	* reflect any modifications subsequent to construction.
369	*
370	* @return an iterator over the elements in this queue in proper sequence.
371	*/
372	public Iterator<E> iterator() {
373	return new Itr();
374	}
375
376	private class Itr implements Iterator<E> {
377	/**
378	* Next node to return item for.
379	*/
380	private Node<E> nextNode;
381
382	/**
383	* nextItem holds on to item fields because once we claim
384	* that an element exists in hasNext(), we must return it in
385	* the following next() call even if it was in the process of
386	* being removed when hasNext() was called.
387	*/
388	private E nextItem;
389
390	/**
391	* Node of the last returned item, to support remove.
392	*/
393	private Node<E> lastRet;
394
395	Itr() {
396	advance();
397	}
398
399	/**
400	* Moves to next valid node and returns item to return for
401	* next(), or null if no such.
402	*/
403	private E advance() {
404	lastRet = nextNode;
405	E x = nextItem;
406
407	Node<E> p = (nextNode == null)? first() : nextNode.getNext();
408	for (;;) {
409	if (p == null) {
410	nextNode = null;
411	nextItem = null;
412	return x;
413	}
414	E item = p.getItem();
415	if (item != null) {
416	nextNode = p;
417	nextItem = item;
418	return x;
419	} else // skip over nulls
420	p = p.getNext();
421	}
422	}
423
424	public boolean hasNext() {
425	return nextNode != null;
426	}
427
428	public E next() {
429	if (nextNode == null) throw new NoSuchElementException();
430	return advance();
431	}
432
433	public void remove() {
434	Node<E> l = lastRet;
435	if (l == null) throw new IllegalStateException();
436	// rely on a future traversal to relink.
437	l.setItem(null);
438	lastRet = null;
439	}
440	}
441
442	/**
443	* Save the state to a stream (that is, serialize it).
444	*
445	* @serialData All of the elements (each an <tt>E</tt>) in
446	* the proper order, followed by a null
447	* @param s the stream
448	*/
449	private void writeObject(java.io.ObjectOutputStream s)
450	throws java.io.IOException {
451
452	// Write out any hidden stuff
453	s.defaultWriteObject();
454
455	// Write out all elements in the proper order.
456	for (Node<E> p = first(); p != null; p = p.getNext()) {
457	Object item = p.getItem();
458	if (item != null)
459	s.writeObject(item);
460	}
461
462	// Use trailing null as sentinel
463	s.writeObject(null);
464	}
465
466	/**
467	* Reconstitute the Queue instance from a stream (that is,
468	* deserialize it).
469	* @param s the stream
470	*/
471	private void readObject(java.io.ObjectInputStream s)
472	throws java.io.IOException, ClassNotFoundException {
473	// Read in capacity, and any hidden stuff
474	s.defaultReadObject();
475	head = new Node<E>(null, null);
476	tail = head;
477	// Read in all elements and place in queue
478	for (;;) {
479	E item = (E)s.readObject();
480	if (item == null)
481	break;
482	else
483	offer(item);
484	}
485	}
486
487	}