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>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentLinkedQueue}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code ConcurrentLinkedQueue} in another thread.
 *
 * <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 the specified collection or any
     *         of its elements are null
     */
    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

    /**
     * Inserts the specified element at the tail of this queue.
     *
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

    /**
     * Inserts the specified element at the tail of this queue.
     *
     * @return <tt>true</tt> (as specified by {@link Queue#offer})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        Node<E> n = new Node<E>(e, 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;
    }

    /**
     * Returns <tt>true</tt> if this queue contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this queue contains
     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
     *
     * @param o object to be checked for containment in this queue
     * @return <tt>true</tt> if this queue contains the specified element
     */
    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;
    }

    /**
     * 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.equals(e)</tt>, if this queue contains one or more such
     * elements.
     * Returns <tt>true</tt> if this queue contained the specified element
     * (or equivalently, if this queue changed as a result of the call).
     *
     * @param o element to be removed from this queue, if present
     * @return <tt>true</tt> if this queue changed as a result of the call
     */
    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;
    }

    /**
     * Returns an array containing all of the elements in this queue, in
     * proper sequence.
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this queue.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this queue
     */
    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();
    }

    /**
     * Returns an array containing all of the elements in this queue, in
     * proper sequence; the runtime type of the returned array is that of
     * the specified array.  If the queue fits in the specified array, it
     * is returned therein.  Otherwise, a new array is allocated with the
     * runtime type of the specified array and the size of this queue.
     *
     * <p>If this queue fits in the specified array with room to spare
     * (i.e., the array has more elements than this queue), the element in
     * the array immediately following the end of the queue is set to
     * <tt>null</tt>.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
     * The following code can be used to dump the queue into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of the queue are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose
     * @return an array containing all of the elements in this queue
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this queue
     * @throws NullPointerException if the specified array is null
     */
    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:	1.43
Committed:	Wed Sep 14 21:45:12 2005 UTC (18 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.42:	+7 -6 lines
Log Message:	happens-before
#	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>Memory consistency effects: As with other concurrent
41	* collections, actions in a thread prior to placing an object into a
42	* {@code ConcurrentLinkedQueue}
43	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
44	* actions subsequent to the access or removal of that element from
45	* the {@code ConcurrentLinkedQueue} in another thread.
46	*
47	* <p>This class is a member of the
48	* <a href="{@docRoot}/../guide/collections/index.html">
49	* Java Collections Framework</a>.
50	*
51	* @since 1.5
52	* @author Doug Lea
53	* @param <E> the type of elements held in this collection
54	*
55	*/
56	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
57	implements Queue<E>, java.io.Serializable {
58	private static final long serialVersionUID = 196745693267521676L;
59
60	/*
61	* This is a straight adaptation of Michael & Scott algorithm.
62	* For explanation, read the paper. The only (minor) algorithmic
63	* difference is that this version supports lazy deletion of
64	* internal nodes (method remove(Object)) -- remove CAS'es item
65	* fields to null. The normal queue operations unlink but then
66	* pass over nodes with null item fields. Similarly, iteration
67	* methods ignore those with nulls.
68	*/
69
70	private static class Node<E> {
71	private volatile E item;
72	private volatile Node<E> next;
73
74	private static final
75	AtomicReferenceFieldUpdater<Node, Node>
76	nextUpdater =
77	AtomicReferenceFieldUpdater.newUpdater
78	(Node.class, Node.class, "next");
79	private static final
80	AtomicReferenceFieldUpdater<Node, Object>
81	itemUpdater =
82	AtomicReferenceFieldUpdater.newUpdater
83	(Node.class, Object.class, "item");
84
85	Node(E x) { item = x; }
86
87	Node(E x, Node<E> n) { item = x; next = n; }
88
89	E getItem() {
90	return item;
91	}
92
93	boolean casItem(E cmp, E val) {
94	return itemUpdater.compareAndSet(this, cmp, val);
95	}
96
97	void setItem(E val) {
98	itemUpdater.set(this, val);
99	}
100
101	Node<E> getNext() {
102	return next;
103	}
104
105	boolean casNext(Node<E> cmp, Node<E> val) {
106	return nextUpdater.compareAndSet(this, cmp, val);
107	}
108
109	void setNext(Node<E> val) {
110	nextUpdater.set(this, val);
111	}
112
113	}
114
115	private static final
116	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
117	tailUpdater =
118	AtomicReferenceFieldUpdater.newUpdater
119	(ConcurrentLinkedQueue.class, Node.class, "tail");
120	private static final
121	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
122	headUpdater =
123	AtomicReferenceFieldUpdater.newUpdater
124	(ConcurrentLinkedQueue.class, Node.class, "head");
125
126	private boolean casTail(Node<E> cmp, Node<E> val) {
127	return tailUpdater.compareAndSet(this, cmp, val);
128	}
129
130	private boolean casHead(Node<E> cmp, Node<E> val) {
131	return headUpdater.compareAndSet(this, cmp, val);
132	}
133
134
135	/**
136	* Pointer to header node, initialized to a dummy node. The first
137	* actual node is at head.getNext().
138	*/
139	private transient volatile Node<E> head = new Node<E>(null, null);
140
141	/ Pointer to last node on list /
142	private transient volatile Node<E> tail = head;
143
144
145	/**
146	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
147	*/
148	public ConcurrentLinkedQueue() {}
149
150	/**
151	* Creates a <tt>ConcurrentLinkedQueue</tt>
152	* initially containing the elements of the given collection,
153	* added in traversal order of the collection's iterator.
154	* @param c the collection of elements to initially contain
155	* @throws NullPointerException if the specified collection or any
156	* of its elements are null
157	*/
158	public ConcurrentLinkedQueue(Collection<? extends E> c) {
159	for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
160	add(it.next());
161	}
162
163	// Have to override just to update the javadoc
164
165	/**
166	* Inserts the specified element at the tail of this queue.
167	*
168	* @return <tt>true</tt> (as specified by {@link Collection#add})
169	* @throws NullPointerException if the specified element is null
170	*/
171	public boolean add(E e) {
172	return offer(e);
173	}
174
175	/**
176	* Inserts the specified element at the tail of this queue.
177	*
178	* @return <tt>true</tt> (as specified by {@link Queue#offer})
179	* @throws NullPointerException if the specified element is null
180	*/
181	public boolean offer(E e) {
182	if (e == null) throw new NullPointerException();
183	Node<E> n = new Node<E>(e, null);
184	for (;;) {
185	Node<E> t = tail;
186	Node<E> s = t.getNext();
187	if (t == tail) {
188	if (s == null) {
189	if (t.casNext(s, n)) {
190	casTail(t, n);
191	return true;
192	}
193	} else {
194	casTail(t, s);
195	}
196	}
197	}
198	}
199
200	public E poll() {
201	for (;;) {
202	Node<E> h = head;
203	Node<E> t = tail;
204	Node<E> first = h.getNext();
205	if (h == head) {
206	if (h == t) {
207	if (first == null)
208	return null;
209	else
210	casTail(t, first);
211	} else if (casHead(h, first)) {
212	E item = first.getItem();
213	if (item != null) {
214	first.setItem(null);
215	return item;
216	}
217	// else skip over deleted item, continue loop,
218	}
219	}
220	}
221	}
222
223	public E peek() { // same as poll except don't remove item
224	for (;;) {
225	Node<E> h = head;
226	Node<E> t = tail;
227	Node<E> first = h.getNext();
228	if (h == head) {
229	if (h == t) {
230	if (first == null)
231	return null;
232	else
233	casTail(t, first);
234	} else {
235	E item = first.getItem();
236	if (item != null)
237	return item;
238	else // remove deleted node and continue
239	casHead(h, first);
240	}
241	}
242	}
243	}
244
245	/**
246	* Returns the first actual (non-header) node on list. This is yet
247	* another variant of poll/peek; here returning out the first
248	* node, not element (so we cannot collapse with peek() without
249	* introducing race.)
250	*/
251	Node<E> first() {
252	for (;;) {
253	Node<E> h = head;
254	Node<E> t = tail;
255	Node<E> first = h.getNext();
256	if (h == head) {
257	if (h == t) {
258	if (first == null)
259	return null;
260	else
261	casTail(t, first);
262	} else {
263	if (first.getItem() != null)
264	return first;
265	else // remove deleted node and continue
266	casHead(h, first);
267	}
268	}
269	}
270	}
271
272
273	/**
274	* Returns <tt>true</tt> if this queue contains no elements.
275	*
276	* @return <tt>true</tt> if this queue contains no elements
277	*/
278	public boolean isEmpty() {
279	return first() == null;
280	}
281
282	/**
283	* Returns the number of elements in this queue. If this queue
284	* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
285	* <tt>Integer.MAX_VALUE</tt>.
286	*
287	* <p>Beware that, unlike in most collections, this method is
288	* <em>NOT</em> a constant-time operation. Because of the
289	* asynchronous nature of these queues, determining the current
290	* number of elements requires an O(n) traversal.
291	*
292	* @return the number of elements in this queue
293	*/
294	public int size() {
295	int count = 0;
296	for (Node<E> p = first(); p != null; p = p.getNext()) {
297	if (p.getItem() != null) {
298	// Collections.size() spec says to max out
299	if (++count == Integer.MAX_VALUE)
300	break;
301	}
302	}
303	return count;
304	}
305
306	/**
307	* Returns <tt>true</tt> if this queue contains the specified element.
308	* More formally, returns <tt>true</tt> if and only if this queue contains
309	* at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
310	*
311	* @param o object to be checked for containment in this queue
312	* @return <tt>true</tt> if this queue contains the specified element
313	*/
314	public boolean contains(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	return true;
321	}
322	return false;
323	}
324
325	/**
326	* Removes a single instance of the specified element from this queue,
327	* if it is present. More formally, removes an element <tt>e</tt> such
328	* that <tt>o.equals(e)</tt>, if this queue contains one or more such
329	* elements.
330	* Returns <tt>true</tt> if this queue contained the specified element
331	* (or equivalently, if this queue changed as a result of the call).
332	*
333	* @param o element to be removed from this queue, if present
334	* @return <tt>true</tt> if this queue changed as a result of the call
335	*/
336	public boolean remove(Object o) {
337	if (o == null) return false;
338	for (Node<E> p = first(); p != null; p = p.getNext()) {
339	E item = p.getItem();
340	if (item != null &&
341	o.equals(item) &&
342	p.casItem(item, null))
343	return true;
344	}
345	return false;
346	}
347
348	/**
349	* Returns an array containing all of the elements in this queue, in
350	* proper sequence.
351	*
352	* <p>The returned array will be "safe" in that no references to it are
353	* maintained by this queue. (In other words, this method must allocate
354	* a new array). The caller is thus free to modify the returned array.
355	*
356	* <p>This method acts as bridge between array-based and collection-based
357	* APIs.
358	*
359	* @return an array containing all of the elements in this queue
360	*/
361	public Object[] toArray() {
362	// Use ArrayList to deal with resizing.
363	ArrayList<E> al = new ArrayList<E>();
364	for (Node<E> p = first(); p != null; p = p.getNext()) {
365	E item = p.getItem();
366	if (item != null)
367	al.add(item);
368	}
369	return al.toArray();
370	}
371
372	/**
373	* Returns an array containing all of the elements in this queue, in
374	* proper sequence; the runtime type of the returned array is that of
375	* the specified array. If the queue fits in the specified array, it
376	* is returned therein. Otherwise, a new array is allocated with the
377	* runtime type of the specified array and the size of this queue.
378	*
379	* <p>If this queue fits in the specified array with room to spare
380	* (i.e., the array has more elements than this queue), the element in
381	* the array immediately following the end of the queue is set to
382	* <tt>null</tt>.
383	*
384	* <p>Like the {@link #toArray()} method, this method acts as bridge between
385	* array-based and collection-based APIs. Further, this method allows
386	* precise control over the runtime type of the output array, and may,
387	* under certain circumstances, be used to save allocation costs.
388	*
389	* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
390	* The following code can be used to dump the queue into a newly
391	* allocated array of <tt>String</tt>:
392	*
393	* <pre>
394	* String[] y = x.toArray(new String[0]);</pre>
395	*
396	* Note that <tt>toArray(new Object[0])</tt> is identical in function to
397	* <tt>toArray()</tt>.
398	*
399	* @param a the array into which the elements of the queue are to
400	* be stored, if it is big enough; otherwise, a new array of the
401	* same runtime type is allocated for this purpose
402	* @return an array containing all of the elements in this queue
403	* @throws ArrayStoreException if the runtime type of the specified array
404	* is not a supertype of the runtime type of every element in
405	* this queue
406	* @throws NullPointerException if the specified array is null
407	*/
408	public <T> T[] toArray(T[] a) {
409	// try to use sent-in array
410	int k = 0;
411	Node<E> p;
412	for (p = first(); p != null && k < a.length; p = p.getNext()) {
413	E item = p.getItem();
414	if (item != null)
415	a[k++] = (T)item;
416	}
417	if (p == null) {
418	if (k < a.length)
419	a[k] = null;
420	return a;
421	}
422
423	// If won't fit, use ArrayList version
424	ArrayList<E> al = new ArrayList<E>();
425	for (Node<E> q = first(); q != null; q = q.getNext()) {
426	E item = q.getItem();
427	if (item != null)
428	al.add(item);
429	}
430	return (T[])al.toArray(a);
431	}
432
433	/**
434	* Returns an iterator over the elements in this queue in proper sequence.
435	* The returned iterator is a "weakly consistent" iterator that
436	* will never throw {@link ConcurrentModificationException},
437	* and guarantees to traverse elements as they existed upon
438	* construction of the iterator, and may (but is not guaranteed to)
439	* reflect any modifications subsequent to construction.
440	*
441	* @return an iterator over the elements in this queue in proper sequence
442	*/
443	public Iterator<E> iterator() {
444	return new Itr();
445	}
446
447	private class Itr implements Iterator<E> {
448	/**
449	* Next node to return item for.
450	*/
451	private Node<E> nextNode;
452
453	/**
454	* nextItem holds on to item fields because once we claim
455	* that an element exists in hasNext(), we must return it in
456	* the following next() call even if it was in the process of
457	* being removed when hasNext() was called.
458	*/
459	private E nextItem;
460
461	/**
462	* Node of the last returned item, to support remove.
463	*/
464	private Node<E> lastRet;
465
466	Itr() {
467	advance();
468	}
469
470	/**
471	* Moves to next valid node and returns item to return for
472	* next(), or null if no such.
473	*/
474	private E advance() {
475	lastRet = nextNode;
476	E x = nextItem;
477
478	Node<E> p = (nextNode == null)? first() : nextNode.getNext();
479	for (;;) {
480	if (p == null) {
481	nextNode = null;
482	nextItem = null;
483	return x;
484	}
485	E item = p.getItem();
486	if (item != null) {
487	nextNode = p;
488	nextItem = item;
489	return x;
490	} else // skip over nulls
491	p = p.getNext();
492	}
493	}
494
495	public boolean hasNext() {
496	return nextNode != null;
497	}
498
499	public E next() {
500	if (nextNode == null) throw new NoSuchElementException();
501	return advance();
502	}
503
504	public void remove() {
505	Node<E> l = lastRet;
506	if (l == null) throw new IllegalStateException();
507	// rely on a future traversal to relink.
508	l.setItem(null);
509	lastRet = null;
510	}
511	}
512
513	/**
514	* Save the state to a stream (that is, serialize it).
515	*
516	* @serialData All of the elements (each an <tt>E</tt>) in
517	* the proper order, followed by a null
518	* @param s the stream
519	*/
520	private void writeObject(java.io.ObjectOutputStream s)
521	throws java.io.IOException {
522
523	// Write out any hidden stuff
524	s.defaultWriteObject();
525
526	// Write out all elements in the proper order.
527	for (Node<E> p = first(); p != null; p = p.getNext()) {
528	Object item = p.getItem();
529	if (item != null)
530	s.writeObject(item);
531	}
532
533	// Use trailing null as sentinel
534	s.writeObject(null);
535	}
536
537	/**
538	* Reconstitute the Queue instance from a stream (that is,
539	* deserialize it).
540	* @param s the stream
541	*/
542	private void readObject(java.io.ObjectInputStream s)
543	throws java.io.IOException, ClassNotFoundException {
544	// Read in capacity, and any hidden stuff
545	s.defaultReadObject();
546	head = new Node<E>(null, null);
547	tail = head;
548	// Read in all elements and place in queue
549	for (;;) {
550	E item = (E)s.readObject();
551	if (item == null)
552	break;
553	else
554	offer(item);
555	}
556	}
557
558	}