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 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 per the spec for {@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 per the spec for {@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;
    }

    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;
    }

    /**
     * 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:	1.35
Committed:	Tue May 17 17:57:41 2005 UTC (19 years ago) by jsr166
Branch:	MAIN
Changes since 1.34:	+1 -1 lines
Log Message:	doc fixes
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.24	* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5	dl	1.1	*/
6
7			package java.util.concurrent;
8			import java.util.*;
9			import java.util.concurrent.atomic.*;
10
11
12			/**
13	jsr166	1.29	* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
14	dholmes	1.6	* 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	dl	1.17	* 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	jsr166	1.29	* A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when
22	dl	1.19	* many threads will share access to a common collection.
23	dholmes	1.6	* This queue does not permit <tt>null</tt> elements.
24	dl	1.1	*
25	jsr166	1.29	* <p>This implementation employs an efficient "wait-free"
26	dholmes	1.6	* algorithm based on one described in <a
27	dl	1.1	* href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
28			* Fast, and Practical Non-Blocking and Blocking Concurrent Queue
29	dl	1.15	* Algorithms</a> by Maged M. Michael and Michael L. Scott.
30	dl	1.1	*
31	dholmes	1.7	* <p>Beware that, unlike in most collections, the <tt>size</tt> method
32	dl	1.1	* is <em>NOT</em> a constant-time operation. Because of the
33			* asynchronous nature of these queues, determining the current number
34	dl	1.15	* of elements requires a traversal of the elements.
35	dl	1.18	*
36	dl	1.27	* <p>This class and its iterator implement all of the
37			* <em>optional</em> methods of the {@link Collection} and {@link
38	jsr166	1.29	* Iterator} interfaces.
39	dl	1.18	*
40	dl	1.25	* <p>This class is a member of the
41			* <a href="{@docRoot}/../guide/collections/index.html">
42			* Java Collections Framework</a>.
43			*
44	dl	1.1	* @since 1.5
45			* @author Doug Lea
46	dl	1.21	* @param <E> the type of elements held in this collection
47	tim	1.2	*
48	dl	1.25	*/
49	dl	1.1	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
50			implements Queue<E>, java.io.Serializable {
51	dl	1.14	private static final long serialVersionUID = 196745693267521676L;
52	dl	1.1
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	dl	1.23	private static class Node<E> {
64	dl	1.22	private volatile E item;
65	dl	1.23	private volatile Node<E> next;
66	jsr166	1.29
67			private static final
68			AtomicReferenceFieldUpdater<Node, Node>
69	dl	1.13	nextUpdater =
70			AtomicReferenceFieldUpdater.newUpdater
71	dl	1.23	(Node.class, Node.class, "next");
72	jsr166	1.29	private static final
73			AtomicReferenceFieldUpdater<Node, Object>
74	dl	1.13	itemUpdater =
75			AtomicReferenceFieldUpdater.newUpdater
76	dl	1.23	(Node.class, Object.class, "item");
77	jsr166	1.29
78	dl	1.23	Node(E x) { item = x; }
79	jsr166	1.29
80	dl	1.23	Node(E x, Node<E> n) { item = x; next = n; }
81	jsr166	1.29
82	dl	1.22	E getItem() {
83	dl	1.13	return item;
84			}
85	jsr166	1.29
86	dl	1.22	boolean casItem(E cmp, E val) {
87	dl	1.13	return itemUpdater.compareAndSet(this, cmp, val);
88			}
89	jsr166	1.29
90	dl	1.22	void setItem(E val) {
91	dl	1.13	itemUpdater.set(this, val);
92			}
93	jsr166	1.29
94	dl	1.23	Node<E> getNext() {
95	dl	1.13	return next;
96			}
97	jsr166	1.29
98	dl	1.23	boolean casNext(Node<E> cmp, Node<E> val) {
99	dl	1.13	return nextUpdater.compareAndSet(this, cmp, val);
100			}
101	jsr166	1.29
102	dl	1.23	void setNext(Node<E> val) {
103	dl	1.13	nextUpdater.set(this, val);
104			}
105	jsr166	1.29
106	dl	1.13	}
107	dl	1.1
108	jsr166	1.29	private static final
109			AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
110			tailUpdater =
111	dl	1.5	AtomicReferenceFieldUpdater.newUpdater
112	dl	1.23	(ConcurrentLinkedQueue.class, Node.class, "tail");
113	jsr166	1.29	private static final
114			AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
115			headUpdater =
116	dl	1.5	AtomicReferenceFieldUpdater.newUpdater
117	dl	1.23	(ConcurrentLinkedQueue.class, Node.class, "head");
118	dl	1.1
119	dl	1.23	private boolean casTail(Node<E> cmp, Node<E> val) {
120	dl	1.1	return tailUpdater.compareAndSet(this, cmp, val);
121			}
122
123	dl	1.23	private boolean casHead(Node<E> cmp, Node<E> val) {
124	dl	1.1	return headUpdater.compareAndSet(this, cmp, val);
125			}
126
127
128	tim	1.2	/**
129	dl	1.1	* Pointer to header node, initialized to a dummy node. The first
130			* actual node is at head.getNext().
131			*/
132	dl	1.23	private transient volatile Node<E> head = new Node<E>(null, null);
133	dl	1.1
134			/ Pointer to last node on list /
135	dl	1.23	private transient volatile Node<E> tail = head;
136	dl	1.1
137
138			/**
139	dholmes	1.6	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
140	dl	1.1	*/
141			public ConcurrentLinkedQueue() {}
142
143			/**
144	jsr166	1.29	* Creates a <tt>ConcurrentLinkedQueue</tt>
145	dholmes	1.7	* initially containing the elements of the given collection,
146	dholmes	1.6	* added in traversal order of the collection's iterator.
147			* @param c the collection of elements to initially contain
148	jsr166	1.34	* @throws NullPointerException if the specified collection or any
149			* of its elements are null
150	dl	1.1	*/
151	dholmes	1.6	public ConcurrentLinkedQueue(Collection<? extends E> c) {
152			for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
153	dl	1.1	add(it.next());
154			}
155
156	jsr166	1.29	// Have to override just to update the javadoc
157	dholmes	1.6
158			/**
159	jsr166	1.35	* Inserts the specified element at the tail of this queue.
160	dholmes	1.7	*
161	jsr166	1.33	* @return <tt>true</tt> (as per the spec for {@link Collection#add})
162	jsr166	1.32	* @throws NullPointerException if the specified element is null
163	dholmes	1.6	*/
164	jsr166	1.31	public boolean add(E e) {
165			return offer(e);
166	dholmes	1.6	}
167
168			/**
169	jsr166	1.32	* Inserts the specified element at the tail of this queue.
170	dl	1.17	*
171	jsr166	1.32	* @return <tt>true</tt> (as per the spec for {@link Queue#offer})
172			* @throws NullPointerException if the specified element is null
173	dholmes	1.6	*/
174	jsr166	1.31	public boolean offer(E e) {
175			if (e == null) throw new NullPointerException();
176			Node<E> n = new Node<E>(e, null);
177	dl	1.1	for(;;) {
178	dl	1.23	Node<E> t = tail;
179			Node<E> s = t.getNext();
180	dl	1.1	if (t == tail) {
181			if (s == null) {
182			if (t.casNext(s, n)) {
183	tim	1.2	casTail(t, n);
184	dl	1.1	return true;
185			}
186	tim	1.12	} else {
187	dl	1.1	casTail(t, s);
188			}
189			}
190			}
191			}
192
193			public E poll() {
194			for (;;) {
195	dl	1.23	Node<E> h = head;
196			Node<E> t = tail;
197			Node<E> first = h.getNext();
198	dl	1.1	if (h == head) {
199			if (h == t) {
200			if (first == null)
201			return null;
202			else
203			casTail(t, first);
204	tim	1.12	} else if (casHead(h, first)) {
205	dl	1.22	E item = first.getItem();
206	dl	1.1	if (item != null) {
207			first.setItem(null);
208			return item;
209			}
210	tim	1.2	// else skip over deleted item, continue loop,
211	dl	1.1	}
212			}
213			}
214			}
215
216			public E peek() { // same as poll except don't remove item
217			for (;;) {
218	dl	1.23	Node<E> h = head;
219			Node<E> t = tail;
220			Node<E> first = h.getNext();
221	dl	1.1	if (h == head) {
222			if (h == t) {
223			if (first == null)
224			return null;
225			else
226			casTail(t, first);
227	tim	1.12	} else {
228	dl	1.22	E item = first.getItem();
229	dl	1.1	if (item != null)
230			return item;
231			else // remove deleted node and continue
232			casHead(h, first);
233			}
234			}
235			}
236			}
237
238			/**
239	dholmes	1.6	* Returns the first actual (non-header) node on list. This is yet
240	dl	1.1	* another variant of poll/peek; here returning out the first
241			* node, not element (so we cannot collapse with peek() without
242			* introducing race.)
243			*/
244	dl	1.23	Node<E> first() {
245	dl	1.1	for (;;) {
246	dl	1.23	Node<E> h = head;
247			Node<E> t = tail;
248			Node<E> first = h.getNext();
249	dl	1.1	if (h == head) {
250			if (h == t) {
251			if (first == null)
252			return null;
253			else
254			casTail(t, first);
255	tim	1.12	} else {
256	dl	1.1	if (first.getItem() != null)
257			return first;
258			else // remove deleted node and continue
259			casHead(h, first);
260			}
261			}
262			}
263			}
264
265
266	dl	1.28	/**
267	jsr166	1.30	* Returns <tt>true</tt> if this queue contains no elements.
268	dl	1.28	*
269	jsr166	1.33	* @return <tt>true</tt> if this queue contains no elements
270	dl	1.28	*/
271	dl	1.1	public boolean isEmpty() {
272			return first() == null;
273			}
274
275			/**
276	dl	1.17	* Returns the number of elements in this queue. If this queue
277			* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
278			* <tt>Integer.MAX_VALUE</tt>.
279	tim	1.2	*
280	dl	1.17	* <p>Beware that, unlike in most collections, this method is
281	dl	1.1	* <em>NOT</em> a constant-time operation. Because of the
282			* asynchronous nature of these queues, determining the current
283			* number of elements requires an O(n) traversal.
284	dl	1.17	*
285	jsr166	1.33	* @return the number of elements in this queue
286	tim	1.2	*/
287	dl	1.1	public int size() {
288			int count = 0;
289	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
290	dl	1.8	if (p.getItem() != null) {
291			// Collections.size() spec says to max out
292			if (++count == Integer.MAX_VALUE)
293			break;
294			}
295	dl	1.1	}
296			return count;
297			}
298
299	dholmes	1.6	public boolean contains(Object o) {
300			if (o == null) return false;
301	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
302	dl	1.22	E item = p.getItem();
303	tim	1.2	if (item != null &&
304	dholmes	1.6	o.equals(item))
305	dl	1.1	return true;
306			}
307			return false;
308			}
309
310	dholmes	1.6	public boolean remove(Object o) {
311			if (o == null) return false;
312	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
313	dl	1.22	E item = p.getItem();
314	tim	1.2	if (item != null &&
315	dholmes	1.6	o.equals(item) &&
316	dl	1.1	p.casItem(item, null))
317			return true;
318			}
319			return false;
320			}
321	tim	1.2
322	jsr166	1.33	/**
323			* Returns an array containing all of the elements in this queue, in
324			* proper sequence.
325			*
326			* <p>The returned array will be "safe" in that no references to it are
327			* maintained by this queue. (In other words, this method must allocate
328			* a new array). The caller is thus free to modify the returned array.
329			*
330			* <p>This method acts as bridge between array-based and collection-based
331			* APIs.
332			*
333			* @return an array containing all of the elements in this queue
334			*/
335	dl	1.1	public Object[] toArray() {
336			// Use ArrayList to deal with resizing.
337	tim	1.3	ArrayList<E> al = new ArrayList<E>();
338	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
339	dl	1.22	E item = p.getItem();
340	dl	1.1	if (item != null)
341			al.add(item);
342			}
343			return al.toArray();
344			}
345
346	jsr166	1.33	/**
347			* Returns an array containing all of the elements in this queue, in
348			* proper sequence; the runtime type of the returned array is that of
349			* the specified array. If the queue fits in the specified array, it
350			* is returned therein. Otherwise, a new array is allocated with the
351			* runtime type of the specified array and the size of this queue.
352			*
353			* <p>If this queue fits in the specified array with room to spare
354			* (i.e., the array has more elements than this queue), the element in
355			* the array immediately following the end of the queue is set to
356			* <tt>null</tt>.
357			*
358			* <p>Like the {@link #toArray()} method, this method acts as bridge between
359			* array-based and collection-based APIs. Further, this method allows
360			* precise control over the runtime type of the output array, and may,
361			* under certain circumstances, be used to save allocation costs.
362			*
363			* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
364			* The following code can be used to dump the queue into a newly
365			* allocated array of <tt>String</tt>:
366			*
367			* <pre>
368			* String[] y = x.toArray(new String[0]);</pre>
369			*
370			* Note that <tt>toArray(new Object[0])</tt> is identical in function to
371			* <tt>toArray()</tt>.
372			*
373			* @param a the array into which the elements of the queue are to
374			* be stored, if it is big enough; otherwise, a new array of the
375			* same runtime type is allocated for this purpose
376			* @return an array containing all of the elements in this queue
377			* @throws ArrayStoreException if the runtime type of the specified array
378			* is not a supertype of the runtime type of every element in
379			* this queue
380			* @throws NullPointerException if the specified array is null
381			*/
382	dl	1.1	public <T> T[] toArray(T[] a) {
383			// try to use sent-in array
384			int k = 0;
385	dl	1.23	Node<E> p;
386	dl	1.1	for (p = first(); p != null && k < a.length; p = p.getNext()) {
387	dl	1.22	E item = p.getItem();
388	dl	1.1	if (item != null)
389			a[k++] = (T)item;
390			}
391			if (p == null) {
392			if (k < a.length)
393			a[k] = null;
394			return a;
395			}
396
397			// If won't fit, use ArrayList version
398	tim	1.3	ArrayList<E> al = new ArrayList<E>();
399	dl	1.23	for (Node<E> q = first(); q != null; q = q.getNext()) {
400	dl	1.22	E item = q.getItem();
401	dl	1.1	if (item != null)
402			al.add(item);
403			}
404			return (T[])al.toArray(a);
405			}
406
407	dholmes	1.7	/**
408			* Returns an iterator over the elements in this queue in proper sequence.
409	dl	1.9	* The returned iterator is a "weakly consistent" iterator that
410	jsr166	1.29	* will never throw {@link ConcurrentModificationException},
411	dl	1.9	* and guarantees to traverse elements as they existed upon
412			* construction of the iterator, and may (but is not guaranteed to)
413			* reflect any modifications subsequent to construction.
414	dholmes	1.7	*
415	jsr166	1.33	* @return an iterator over the elements in this queue in proper sequence
416	dholmes	1.7	*/
417	dl	1.1	public Iterator<E> iterator() {
418			return new Itr();
419			}
420
421			private class Itr implements Iterator<E> {
422			/**
423			* Next node to return item for.
424			*/
425	dl	1.23	private Node<E> nextNode;
426	dl	1.1
427	tim	1.2	/**
428	dl	1.1	* nextItem holds on to item fields because once we claim
429			* that an element exists in hasNext(), we must return it in
430			* the following next() call even if it was in the process of
431			* being removed when hasNext() was called.
432	jsr166	1.29	*/
433	dl	1.1	private E nextItem;
434
435			/**
436			* Node of the last returned item, to support remove.
437			*/
438	dl	1.23	private Node<E> lastRet;
439	dl	1.1
440	tim	1.2	Itr() {
441	dl	1.1	advance();
442			}
443	tim	1.2
444	dl	1.1	/**
445	dl	1.26	* Moves to next valid node and returns item to return for
446			* next(), or null if no such.
447	dl	1.1	*/
448	tim	1.2	private E advance() {
449	dl	1.1	lastRet = nextNode;
450	dl	1.22	E x = nextItem;
451	dl	1.1
452	dl	1.23	Node<E> p = (nextNode == null)? first() : nextNode.getNext();
453	dl	1.1	for (;;) {
454			if (p == null) {
455			nextNode = null;
456			nextItem = null;
457			return x;
458			}
459	dl	1.22	E item = p.getItem();
460	dl	1.1	if (item != null) {
461			nextNode = p;
462			nextItem = item;
463			return x;
464	tim	1.12	} else // skip over nulls
465	dl	1.1	p = p.getNext();
466			}
467			}
468	tim	1.2
469	dl	1.1	public boolean hasNext() {
470			return nextNode != null;
471			}
472	tim	1.2
473	dl	1.1	public E next() {
474			if (nextNode == null) throw new NoSuchElementException();
475			return advance();
476			}
477	tim	1.2
478	dl	1.1	public void remove() {
479	dl	1.23	Node<E> l = lastRet;
480	dl	1.1	if (l == null) throw new IllegalStateException();
481			// rely on a future traversal to relink.
482			l.setItem(null);
483			lastRet = null;
484			}
485			}
486
487			/**
488			* Save the state to a stream (that is, serialize it).
489			*
490			* @serialData All of the elements (each an <tt>E</tt>) in
491			* the proper order, followed by a null
492			* @param s the stream
493			*/
494			private void writeObject(java.io.ObjectOutputStream s)
495			throws java.io.IOException {
496
497			// Write out any hidden stuff
498			s.defaultWriteObject();
499	tim	1.2
500	dl	1.1	// Write out all elements in the proper order.
501	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
502	dl	1.1	Object item = p.getItem();
503			if (item != null)
504			s.writeObject(item);
505			}
506
507			// Use trailing null as sentinel
508			s.writeObject(null);
509			}
510
511			/**
512			* Reconstitute the Queue instance from a stream (that is,
513			* deserialize it).
514			* @param s the stream
515			*/
516			private void readObject(java.io.ObjectInputStream s)
517			throws java.io.IOException, ClassNotFoundException {
518	tim	1.2	// Read in capacity, and any hidden stuff
519			s.defaultReadObject();
520	dl	1.23	head = new Node<E>(null, null);
521	dl	1.16	tail = head;
522	tim	1.2	// Read in all elements and place in queue
523	dl	1.1	for (;;) {
524			E item = (E)s.readObject();
525			if (item == null)
526			break;
527	dl	1.16	else
528			offer(item);
529	dl	1.1	}
530			}
531
532			}