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 implements 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);
                }
            }
        }
    }


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

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

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

    public boolean remove(Object o) {
        if (o == null) return false;
        for (Node<E> p = first(); p != null; p = p.getNext()) {
            E item = p.getItem();
            if (item != null &&
                o.equals(item) &&
                p.casItem(item, null))
                return true;
        }
        return false;
    }

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

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

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

    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The returned iterator is a "weakly consistent" iterator that
     * will never throw {@link java.util.ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     *
     * @return an iterator over the elements in this queue in proper sequence.
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

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

        /**
         * nextItem holds on to item fields because once we claim
         * that an element exists in hasNext(), we must return it in
         * the following next() call even if it was in the process of
         * being removed when hasNext() was called.
         **/
        private E nextItem;

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

        Itr() {
            advance();
        }

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

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

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

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

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

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

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

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

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

    /**
     * Reconstitute the Queue instance from a stream (that is,
     * deserialize it).
     * @param s the stream
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        // Read in capacity, and any hidden stuff
        s.defaultReadObject();
        head = new Node<E>(null, null);
        tail = head;
        // Read in all elements and place in queue
        for (;;) {
            E item = (E)s.readObject();
            if (item == null)
                break;
            else
                offer(item);
        }
    }

}
Revision:	1.25
Committed:	Tue Jan 27 11:36:31 2004 UTC (20 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.24:	+5 -1 lines
Log Message:	Add Collection framework membership doc
#	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	dholmes	1.7	* 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	dl	1.19	* A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when
22			* 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	dholmes	1.6	* <p>This implementation employs an efficient "wait-free"
26			* 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			* <p>This class implements all of the <em>optional</em> methods
37			* of the {@link Collection} and {@link Iterator} interfaces.
38			*
39	dl	1.25	* <p>This class is a member of the
40			* <a href="{@docRoot}/../guide/collections/index.html">
41			* Java Collections Framework</a>.
42			*
43	dl	1.1	* @since 1.5
44			* @author Doug Lea
45	dl	1.21	* @param <E> the type of elements held in this collection
46	tim	1.2	*
47	dl	1.25	*/
48	dl	1.1	public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
49			implements Queue<E>, java.io.Serializable {
50	dl	1.14	private static final long serialVersionUID = 196745693267521676L;
51	dl	1.1
52			/*
53			* This is a straight adaptation of Michael & Scott algorithm.
54			* For explanation, read the paper. The only (minor) algorithmic
55			* difference is that this version supports lazy deletion of
56			* internal nodes (method remove(Object)) -- remove CAS'es item
57			* fields to null. The normal queue operations unlink but then
58			* pass over nodes with null item fields. Similarly, iteration
59			* methods ignore those with nulls.
60			*/
61
62	dl	1.23	private static class Node<E> {
63	dl	1.22	private volatile E item;
64	dl	1.23	private volatile Node<E> next;
65	dl	1.13
66			private static final
67	dl	1.23	AtomicReferenceFieldUpdater<Node, Node>
68	dl	1.13	nextUpdater =
69			AtomicReferenceFieldUpdater.newUpdater
70	dl	1.23	(Node.class, Node.class, "next");
71	dl	1.13	private static final
72	dl	1.23	AtomicReferenceFieldUpdater<Node, Object>
73	dl	1.13	itemUpdater =
74			AtomicReferenceFieldUpdater.newUpdater
75	dl	1.23	(Node.class, Object.class, "item");
76	dl	1.13
77	dl	1.23	Node(E x) { item = x; }
78	dl	1.13
79	dl	1.23	Node(E x, Node<E> n) { item = x; next = n; }
80	dl	1.13
81	dl	1.22	E getItem() {
82	dl	1.13	return item;
83			}
84
85	dl	1.22	boolean casItem(E cmp, E val) {
86	dl	1.13	return itemUpdater.compareAndSet(this, cmp, val);
87			}
88
89	dl	1.22	void setItem(E val) {
90	dl	1.13	itemUpdater.set(this, val);
91			}
92
93	dl	1.23	Node<E> getNext() {
94	dl	1.13	return next;
95			}
96
97	dl	1.23	boolean casNext(Node<E> cmp, Node<E> val) {
98	dl	1.13	return nextUpdater.compareAndSet(this, cmp, val);
99			}
100
101	dl	1.23	void setNext(Node<E> val) {
102	dl	1.13	nextUpdater.set(this, val);
103			}
104
105			}
106	dl	1.1
107	dl	1.5	private static final
108	dl	1.23	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
109	dl	1.5	tailUpdater =
110			AtomicReferenceFieldUpdater.newUpdater
111	dl	1.23	(ConcurrentLinkedQueue.class, Node.class, "tail");
112	dl	1.5	private static final
113	dl	1.23	AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
114	dl	1.5	headUpdater =
115			AtomicReferenceFieldUpdater.newUpdater
116	dl	1.23	(ConcurrentLinkedQueue.class, Node.class, "head");
117	dl	1.1
118	dl	1.23	private boolean casTail(Node<E> cmp, Node<E> val) {
119	dl	1.1	return tailUpdater.compareAndSet(this, cmp, val);
120			}
121
122	dl	1.23	private boolean casHead(Node<E> cmp, Node<E> val) {
123	dl	1.1	return headUpdater.compareAndSet(this, cmp, val);
124			}
125
126
127	tim	1.2	/**
128	dl	1.1	* Pointer to header node, initialized to a dummy node. The first
129			* actual node is at head.getNext().
130			*/
131	dl	1.23	private transient volatile Node<E> head = new Node<E>(null, null);
132	dl	1.1
133			/ Pointer to last node on list /
134	dl	1.23	private transient volatile Node<E> tail = head;
135	dl	1.1
136
137			/**
138	dholmes	1.6	* Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
139	dl	1.1	*/
140			public ConcurrentLinkedQueue() {}
141
142			/**
143	dholmes	1.6	* Creates a <tt>ConcurrentLinkedQueue</tt>
144	dholmes	1.7	* initially containing the elements of the given collection,
145	dholmes	1.6	* added in traversal order of the collection's iterator.
146			* @param c the collection of elements to initially contain
147			* @throws NullPointerException if <tt>c</tt> or any element within it
148			* is <tt>null</tt>
149	dl	1.1	*/
150	dholmes	1.6	public ConcurrentLinkedQueue(Collection<? extends E> c) {
151			for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
152	dl	1.1	add(it.next());
153			}
154
155	dholmes	1.7	// Have to override just to update the javadoc
156	dholmes	1.6
157			/**
158	dholmes	1.7	* Adds the specified element to the tail of this queue.
159	dl	1.17	* @param o the element to add.
160	dholmes	1.7	* @return <tt>true</tt> (as per the general contract of
161			* <tt>Collection.add</tt>).
162			*
163	dl	1.17	* @throws NullPointerException if the specified element is <tt>null</tt>
164	dholmes	1.6	*/
165			public boolean add(E o) {
166	dl	1.17	return offer(o);
167	dholmes	1.6	}
168
169			/**
170	dl	1.17	* Inserts the specified element to the tail of this queue.
171			*
172			* @param o the element to add.
173			* @return <tt>true</tt> (as per the general contract of
174			* <tt>Queue.offer</tt>).
175	dholmes	1.6	* @throws NullPointerException if the specified element is <tt>null</tt>
176			*/
177			public boolean offer(E o) {
178			if (o == null) throw new NullPointerException();
179	dl	1.23	Node<E> n = new Node<E>(o, null);
180	dl	1.1	for(;;) {
181	dl	1.23	Node<E> t = tail;
182			Node<E> s = t.getNext();
183	dl	1.1	if (t == tail) {
184			if (s == null) {
185			if (t.casNext(s, n)) {
186	tim	1.2	casTail(t, n);
187	dl	1.1	return true;
188			}
189	tim	1.12	} else {
190	dl	1.1	casTail(t, s);
191			}
192			}
193			}
194			}
195
196			public E poll() {
197			for (;;) {
198	dl	1.23	Node<E> h = head;
199			Node<E> t = tail;
200			Node<E> first = h.getNext();
201	dl	1.1	if (h == head) {
202			if (h == t) {
203			if (first == null)
204			return null;
205			else
206			casTail(t, first);
207	tim	1.12	} else if (casHead(h, first)) {
208	dl	1.22	E item = first.getItem();
209	dl	1.1	if (item != null) {
210			first.setItem(null);
211			return item;
212			}
213	tim	1.2	// else skip over deleted item, continue loop,
214	dl	1.1	}
215			}
216			}
217			}
218
219			public E peek() { // same as poll except don't remove item
220			for (;;) {
221	dl	1.23	Node<E> h = head;
222			Node<E> t = tail;
223			Node<E> first = h.getNext();
224	dl	1.1	if (h == head) {
225			if (h == t) {
226			if (first == null)
227			return null;
228			else
229			casTail(t, first);
230	tim	1.12	} else {
231	dl	1.22	E item = first.getItem();
232	dl	1.1	if (item != null)
233			return item;
234			else // remove deleted node and continue
235			casHead(h, first);
236			}
237			}
238			}
239			}
240
241			/**
242	dholmes	1.6	* Returns the first actual (non-header) node on list. This is yet
243	dl	1.1	* another variant of poll/peek; here returning out the first
244			* node, not element (so we cannot collapse with peek() without
245			* introducing race.)
246			*/
247	dl	1.23	Node<E> first() {
248	dl	1.1	for (;;) {
249	dl	1.23	Node<E> h = head;
250			Node<E> t = tail;
251			Node<E> first = h.getNext();
252	dl	1.1	if (h == head) {
253			if (h == t) {
254			if (first == null)
255			return null;
256			else
257			casTail(t, first);
258	tim	1.12	} else {
259	dl	1.1	if (first.getItem() != null)
260			return first;
261			else // remove deleted node and continue
262			casHead(h, first);
263			}
264			}
265			}
266			}
267
268
269			public boolean isEmpty() {
270			return first() == null;
271			}
272
273			/**
274	dl	1.17	* Returns the number of elements in this queue. If this queue
275			* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
276			* <tt>Integer.MAX_VALUE</tt>.
277	tim	1.2	*
278	dl	1.17	* <p>Beware that, unlike in most collections, this method is
279	dl	1.1	* <em>NOT</em> a constant-time operation. Because of the
280			* asynchronous nature of these queues, determining the current
281			* number of elements requires an O(n) traversal.
282	dl	1.17	*
283			* @return the number of elements in this queue.
284	tim	1.2	*/
285	dl	1.1	public int size() {
286			int count = 0;
287	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
288	dl	1.8	if (p.getItem() != null) {
289			// Collections.size() spec says to max out
290			if (++count == Integer.MAX_VALUE)
291			break;
292			}
293	dl	1.1	}
294			return count;
295			}
296
297	dholmes	1.6	public boolean contains(Object o) {
298			if (o == null) return false;
299	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
300	dl	1.22	E item = p.getItem();
301	tim	1.2	if (item != null &&
302	dholmes	1.6	o.equals(item))
303	dl	1.1	return true;
304			}
305			return false;
306			}
307
308	dholmes	1.6	public boolean remove(Object o) {
309			if (o == null) return false;
310	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
311	dl	1.22	E item = p.getItem();
312	tim	1.2	if (item != null &&
313	dholmes	1.6	o.equals(item) &&
314	dl	1.1	p.casItem(item, null))
315			return true;
316			}
317			return false;
318			}
319	tim	1.2
320	dl	1.1	public Object[] toArray() {
321			// Use ArrayList to deal with resizing.
322	tim	1.3	ArrayList<E> al = new ArrayList<E>();
323	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
324	dl	1.22	E item = p.getItem();
325	dl	1.1	if (item != null)
326			al.add(item);
327			}
328			return al.toArray();
329			}
330
331			public <T> T[] toArray(T[] a) {
332			// try to use sent-in array
333			int k = 0;
334	dl	1.23	Node<E> p;
335	dl	1.1	for (p = first(); p != null && k < a.length; p = p.getNext()) {
336	dl	1.22	E item = p.getItem();
337	dl	1.1	if (item != null)
338			a[k++] = (T)item;
339			}
340			if (p == null) {
341			if (k < a.length)
342			a[k] = null;
343			return a;
344			}
345
346			// If won't fit, use ArrayList version
347	tim	1.3	ArrayList<E> al = new ArrayList<E>();
348	dl	1.23	for (Node<E> q = first(); q != null; q = q.getNext()) {
349	dl	1.22	E item = q.getItem();
350	dl	1.1	if (item != null)
351			al.add(item);
352			}
353			return (T[])al.toArray(a);
354			}
355
356	dholmes	1.7	/**
357			* Returns an iterator over the elements in this queue in proper sequence.
358	dl	1.9	* The returned iterator is a "weakly consistent" iterator that
359			* will never throw {@link java.util.ConcurrentModificationException},
360			* and guarantees to traverse elements as they existed upon
361			* construction of the iterator, and may (but is not guaranteed to)
362			* reflect any modifications subsequent to construction.
363	dholmes	1.7	*
364			* @return an iterator over the elements in this queue in proper sequence.
365			*/
366	dl	1.1	public Iterator<E> iterator() {
367			return new Itr();
368			}
369
370			private class Itr implements Iterator<E> {
371			/**
372			* Next node to return item for.
373			*/
374	dl	1.23	private Node<E> nextNode;
375	dl	1.1
376	tim	1.2	/**
377	dl	1.1	* nextItem holds on to item fields because once we claim
378			* that an element exists in hasNext(), we must return it in
379			* the following next() call even if it was in the process of
380			* being removed when hasNext() was called.
381			**/
382			private E nextItem;
383
384			/**
385			* Node of the last returned item, to support remove.
386			*/
387	dl	1.23	private Node<E> lastRet;
388	dl	1.1
389	tim	1.2	Itr() {
390	dl	1.1	advance();
391			}
392	tim	1.2
393	dl	1.1	/**
394	tim	1.2	* Move to next valid node.
395	dl	1.1	* Return item to return for next(), or null if no such.
396			*/
397	tim	1.2	private E advance() {
398	dl	1.1	lastRet = nextNode;
399	dl	1.22	E x = nextItem;
400	dl	1.1
401	dl	1.23	Node<E> p = (nextNode == null)? first() : nextNode.getNext();
402	dl	1.1	for (;;) {
403			if (p == null) {
404			nextNode = null;
405			nextItem = null;
406			return x;
407			}
408	dl	1.22	E item = p.getItem();
409	dl	1.1	if (item != null) {
410			nextNode = p;
411			nextItem = item;
412			return x;
413	tim	1.12	} else // skip over nulls
414	dl	1.1	p = p.getNext();
415			}
416			}
417	tim	1.2
418	dl	1.1	public boolean hasNext() {
419			return nextNode != null;
420			}
421	tim	1.2
422	dl	1.1	public E next() {
423			if (nextNode == null) throw new NoSuchElementException();
424			return advance();
425			}
426	tim	1.2
427	dl	1.1	public void remove() {
428	dl	1.23	Node<E> l = lastRet;
429	dl	1.1	if (l == null) throw new IllegalStateException();
430			// rely on a future traversal to relink.
431			l.setItem(null);
432			lastRet = null;
433			}
434			}
435
436			/**
437			* Save the state to a stream (that is, serialize it).
438			*
439			* @serialData All of the elements (each an <tt>E</tt>) in
440			* the proper order, followed by a null
441			* @param s the stream
442			*/
443			private void writeObject(java.io.ObjectOutputStream s)
444			throws java.io.IOException {
445
446			// Write out any hidden stuff
447			s.defaultWriteObject();
448	tim	1.2
449	dl	1.1	// Write out all elements in the proper order.
450	dl	1.23	for (Node<E> p = first(); p != null; p = p.getNext()) {
451	dl	1.1	Object item = p.getItem();
452			if (item != null)
453			s.writeObject(item);
454			}
455
456			// Use trailing null as sentinel
457			s.writeObject(null);
458			}
459
460			/**
461			* Reconstitute the Queue instance from a stream (that is,
462			* deserialize it).
463			* @param s the stream
464			*/
465			private void readObject(java.io.ObjectInputStream s)
466			throws java.io.IOException, ClassNotFoundException {
467	tim	1.2	// Read in capacity, and any hidden stuff
468			s.defaultReadObject();
469	dl	1.23	head = new Node<E>(null, null);
470	dl	1.16	tail = head;
471	tim	1.2	// Read in all elements and place in queue
472	dl	1.1	for (;;) {
473			E item = (E)s.readObject();
474			if (item == null)
475			break;
476	dl	1.16	else
477			offer(item);
478	dl	1.1	}
479			}
480
481			}