--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/java/util/concurrent/LinkedBlockingQueue.java Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,836 @@
+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation. Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+/*
+ * This file is available under and governed by the GNU General Public
+ * License version 2 only, as published by the Free Software Foundation.
+ * However, the following notice accompanied the original version of this
+ * file:
+ *
+ * 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.concurrent.atomic.*;
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * An optionally-bounded {@linkplain BlockingQueue blocking 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.
+ * Linked queues typically have higher throughput than array-based queues but
+ * less predictable performance in most concurrent applications.
+ *
+ * <p> The optional capacity bound constructor argument serves as a
+ * way to prevent excessive queue expansion. The capacity, if unspecified,
+ * is equal to {@link Integer#MAX_VALUE}. Linked nodes are
+ * dynamically created upon each insertion unless this would bring the
+ * queue above capacity.
+ *
+ * <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}/../technotes/guides/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 LinkedBlockingQueue<E> extends AbstractQueue<E>
+ implements BlockingQueue<E>, java.io.Serializable {
+ private static final long serialVersionUID = -6903933977591709194L;
+
+ /*
+ * A variant of the "two lock queue" algorithm. The putLock gates
+ * entry to put (and offer), and has an associated condition for
+ * waiting puts. Similarly for the takeLock. The "count" field
+ * that they both rely on is maintained as an atomic to avoid
+ * needing to get both locks in most cases. Also, to minimize need
+ * for puts to get takeLock and vice-versa, cascading notifies are
+ * used. When a put notices that it has enabled at least one take,
+ * it signals taker. That taker in turn signals others if more
+ * items have been entered since the signal. And symmetrically for
+ * takes signalling puts. Operations such as remove(Object) and
+ * iterators acquire both locks.
+ */
+
+ /**
+ * Linked list node class
+ */
+ static class Node<E> {
+ /** The item, volatile to ensure barrier separating write and read */
+ volatile E item;
+ Node<E> next;
+ Node(E x) { item = x; }
+ }
+
+ /** The capacity bound, or Integer.MAX_VALUE if none */
+ private final int capacity;
+
+ /** Current number of elements */
+ private final AtomicInteger count = new AtomicInteger(0);
+
+ /** Head of linked list */
+ private transient Node<E> head;
+
+ /** Tail of linked list */
+ private transient Node<E> last;
+
+ /** Lock held by take, poll, etc */
+ private final ReentrantLock takeLock = new ReentrantLock();
+
+ /** Wait queue for waiting takes */
+ private final Condition notEmpty = takeLock.newCondition();
+
+ /** Lock held by put, offer, etc */
+ private final ReentrantLock putLock = new ReentrantLock();
+
+ /** Wait queue for waiting puts */
+ private final Condition notFull = putLock.newCondition();
+
+ /**
+ * Signals a waiting take. Called only from put/offer (which do not
+ * otherwise ordinarily lock takeLock.)
+ */
+ private void signalNotEmpty() {
+ final ReentrantLock takeLock = this.takeLock;
+ takeLock.lock();
+ try {
+ notEmpty.signal();
+ } finally {
+ takeLock.unlock();
+ }
+ }
+
+ /**
+ * Signals a waiting put. Called only from take/poll.
+ */
+ private void signalNotFull() {
+ final ReentrantLock putLock = this.putLock;
+ putLock.lock();
+ try {
+ notFull.signal();
+ } finally {
+ putLock.unlock();
+ }
+ }
+
+ /**
+ * Creates a node and links it at end of queue.
+ * @param x the item
+ */
+ private void insert(E x) {
+ last = last.next = new Node<E>(x);
+ }
+
+ /**
+ * Removes a node from head of queue,
+ * @return the node
+ */
+ private E extract() {
+ Node<E> first = head.next;
+ head = first;
+ E x = first.item;
+ first.item = null;
+ return x;
+ }
+
+ /**
+ * Lock to prevent both puts and takes.
+ */
+ private void fullyLock() {
+ putLock.lock();
+ takeLock.lock();
+ }
+
+ /**
+ * Unlock to allow both puts and takes.
+ */
+ private void fullyUnlock() {
+ takeLock.unlock();
+ putLock.unlock();
+ }
+
+
+ /**
+ * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
+ * {@link Integer#MAX_VALUE}.
+ */
+ public LinkedBlockingQueue() {
+ this(Integer.MAX_VALUE);
+ }
+
+ /**
+ * Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
+ *
+ * @param capacity the capacity of this queue
+ * @throws IllegalArgumentException if <tt>capacity</tt> is not greater
+ * than zero
+ */
+ public LinkedBlockingQueue(int capacity) {
+ if (capacity <= 0) throw new IllegalArgumentException();
+ this.capacity = capacity;
+ last = head = new Node<E>(null);
+ }
+
+ /**
+ * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
+ * {@link Integer#MAX_VALUE}, 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 LinkedBlockingQueue(Collection<? extends E> c) {
+ this(Integer.MAX_VALUE);
+ for (E e : c)
+ add(e);
+ }
+
+
+ // this doc comment is overridden to remove the reference to collections
+ // greater in size than Integer.MAX_VALUE
+ /**
+ * Returns the number of elements in this queue.
+ *
+ * @return the number of elements in this queue
+ */
+ public int size() {
+ return count.get();
+ }
+
+ // this doc comment is a modified copy of the inherited doc comment,
+ // without the reference to unlimited queues.
+ /**
+ * Returns the number of additional elements that this queue can ideally
+ * (in the absence of memory or resource constraints) accept without
+ * blocking. This is always equal to the initial capacity of this queue
+ * less the current <tt>size</tt> of this queue.
+ *
+ * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+ * an element will succeed by inspecting <tt>remainingCapacity</tt>
+ * because it may be the case that another thread is about to
+ * insert or remove an element.
+ */
+ public int remainingCapacity() {
+ return capacity - count.get();
+ }
+
+ /**
+ * Inserts the specified element at the tail of this queue, waiting if
+ * necessary for space to become available.
+ *
+ * @throws InterruptedException {@inheritDoc}
+ * @throws NullPointerException {@inheritDoc}
+ */
+ public void put(E e) throws InterruptedException {
+ if (e == null) throw new NullPointerException();
+ // Note: convention in all put/take/etc is to preset
+ // local var holding count negative to indicate failure unless set.
+ int c = -1;
+ final ReentrantLock putLock = this.putLock;
+ final AtomicInteger count = this.count;
+ putLock.lockInterruptibly();
+ try {
+ /*
+ * Note that count is used in wait guard even though it is
+ * not protected by lock. This works because count can
+ * only decrease at this point (all other puts are shut
+ * out by lock), and we (or some other waiting put) are
+ * signalled if it ever changes from
+ * capacity. Similarly for all other uses of count in
+ * other wait guards.
+ */
+ try {
+ while (count.get() == capacity)
+ notFull.await();
+ } catch (InterruptedException ie) {
+ notFull.signal(); // propagate to a non-interrupted thread
+ throw ie;
+ }
+ insert(e);
+ c = count.getAndIncrement();
+ if (c + 1 < capacity)
+ notFull.signal();
+ } finally {
+ putLock.unlock();
+ }
+ if (c == 0)
+ signalNotEmpty();
+ }
+
+ /**
+ * Inserts the specified element at the tail of this queue, waiting if
+ * necessary up to the specified wait time for space to become available.
+ *
+ * @return <tt>true</tt> if successful, or <tt>false</tt> if
+ * the specified waiting time elapses before space is available.
+ * @throws InterruptedException {@inheritDoc}
+ * @throws NullPointerException {@inheritDoc}
+ */
+ public boolean offer(E e, long timeout, TimeUnit unit)
+ throws InterruptedException {
+
+ if (e == null) throw new NullPointerException();
+ long nanos = unit.toNanos(timeout);
+ int c = -1;
+ final ReentrantLock putLock = this.putLock;
+ final AtomicInteger count = this.count;
+ putLock.lockInterruptibly();
+ try {
+ for (;;) {
+ if (count.get() < capacity) {
+ insert(e);
+ c = count.getAndIncrement();
+ if (c + 1 < capacity)
+ notFull.signal();
+ break;
+ }
+ if (nanos <= 0)
+ return false;
+ try {
+ nanos = notFull.awaitNanos(nanos);
+ } catch (InterruptedException ie) {
+ notFull.signal(); // propagate to a non-interrupted thread
+ throw ie;
+ }
+ }
+ } finally {
+ putLock.unlock();
+ }
+ if (c == 0)
+ signalNotEmpty();
+ return true;
+ }
+
+ /**
+ * Inserts the specified element at the tail of this queue if it is
+ * possible to do so immediately without exceeding the queue's capacity,
+ * returning <tt>true</tt> upon success and <tt>false</tt> if this queue
+ * is full.
+ * When using a capacity-restricted queue, this method is generally
+ * preferable to method {@link BlockingQueue#add add}, which can fail to
+ * insert an element only by throwing an exception.
+ *
+ * @throws NullPointerException if the specified element is null
+ */
+ public boolean offer(E e) {
+ if (e == null) throw new NullPointerException();
+ final AtomicInteger count = this.count;
+ if (count.get() == capacity)
+ return false;
+ int c = -1;
+ final ReentrantLock putLock = this.putLock;
+ putLock.lock();
+ try {
+ if (count.get() < capacity) {
+ insert(e);
+ c = count.getAndIncrement();
+ if (c + 1 < capacity)
+ notFull.signal();
+ }
+ } finally {
+ putLock.unlock();
+ }
+ if (c == 0)
+ signalNotEmpty();
+ return c >= 0;
+ }
+
+
+ public E take() throws InterruptedException {
+ E x;
+ int c = -1;
+ final AtomicInteger count = this.count;
+ final ReentrantLock takeLock = this.takeLock;
+ takeLock.lockInterruptibly();
+ try {
+ try {
+ while (count.get() == 0)
+ notEmpty.await();
+ } catch (InterruptedException ie) {
+ notEmpty.signal(); // propagate to a non-interrupted thread
+ throw ie;
+ }
+
+ x = extract();
+ c = count.getAndDecrement();
+ if (c > 1)
+ notEmpty.signal();
+ } finally {
+ takeLock.unlock();
+ }
+ if (c == capacity)
+ signalNotFull();
+ return x;
+ }
+
+ public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+ E x = null;
+ int c = -1;
+ long nanos = unit.toNanos(timeout);
+ final AtomicInteger count = this.count;
+ final ReentrantLock takeLock = this.takeLock;
+ takeLock.lockInterruptibly();
+ try {
+ for (;;) {
+ if (count.get() > 0) {
+ x = extract();
+ c = count.getAndDecrement();
+ if (c > 1)
+ notEmpty.signal();
+ break;
+ }
+ if (nanos <= 0)
+ return null;
+ try {
+ nanos = notEmpty.awaitNanos(nanos);
+ } catch (InterruptedException ie) {
+ notEmpty.signal(); // propagate to a non-interrupted thread
+ throw ie;
+ }
+ }
+ } finally {
+ takeLock.unlock();
+ }
+ if (c == capacity)
+ signalNotFull();
+ return x;
+ }
+
+ public E poll() {
+ final AtomicInteger count = this.count;
+ if (count.get() == 0)
+ return null;
+ E x = null;
+ int c = -1;
+ final ReentrantLock takeLock = this.takeLock;
+ takeLock.lock();
+ try {
+ if (count.get() > 0) {
+ x = extract();
+ c = count.getAndDecrement();
+ if (c > 1)
+ notEmpty.signal();
+ }
+ } finally {
+ takeLock.unlock();
+ }
+ if (c == capacity)
+ signalNotFull();
+ return x;
+ }
+
+
+ public E peek() {
+ if (count.get() == 0)
+ return null;
+ final ReentrantLock takeLock = this.takeLock;
+ takeLock.lock();
+ try {
+ Node<E> first = head.next;
+ if (first == null)
+ return null;
+ else
+ return first.item;
+ } finally {
+ takeLock.unlock();
+ }
+ }
+
+ /**
+ * 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;
+ boolean removed = false;
+ fullyLock();
+ try {
+ Node<E> trail = head;
+ Node<E> p = head.next;
+ while (p != null) {
+ if (o.equals(p.item)) {
+ removed = true;
+ break;
+ }
+ trail = p;
+ p = p.next;
+ }
+ if (removed) {
+ p.item = null;
+ trail.next = p.next;
+ if (last == p)
+ last = trail;
+ if (count.getAndDecrement() == capacity)
+ notFull.signalAll();
+ }
+ } finally {
+ fullyUnlock();
+ }
+ return removed;
+ }
+
+ /**
+ * 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() {
+ fullyLock();
+ try {
+ int size = count.get();
+ Object[] a = new Object[size];
+ int k = 0;
+ for (Node<E> p = head.next; p != null; p = p.next)
+ a[k++] = p.item;
+ return a;
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ /**
+ * 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) {
+ fullyLock();
+ try {
+ int size = count.get();
+ if (a.length < size)
+ a = (T[])java.lang.reflect.Array.newInstance
+ (a.getClass().getComponentType(), size);
+
+ int k = 0;
+ for (Node p = head.next; p != null; p = p.next)
+ a[k++] = (T)p.item;
+ if (a.length > k)
+ a[k] = null;
+ return a;
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ public String toString() {
+ fullyLock();
+ try {
+ return super.toString();
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ /**
+ * Atomically removes all of the elements from this queue.
+ * The queue will be empty after this call returns.
+ */
+ public void clear() {
+ fullyLock();
+ try {
+ head.next = null;
+ assert head.item == null;
+ last = head;
+ if (count.getAndSet(0) == capacity)
+ notFull.signalAll();
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ /**
+ * @throws UnsupportedOperationException {@inheritDoc}
+ * @throws ClassCastException {@inheritDoc}
+ * @throws NullPointerException {@inheritDoc}
+ * @throws IllegalArgumentException {@inheritDoc}
+ */
+ public int drainTo(Collection<? super E> c) {
+ if (c == null)
+ throw new NullPointerException();
+ if (c == this)
+ throw new IllegalArgumentException();
+ Node<E> first;
+ fullyLock();
+ try {
+ first = head.next;
+ head.next = null;
+ assert head.item == null;
+ last = head;
+ if (count.getAndSet(0) == capacity)
+ notFull.signalAll();
+ } finally {
+ fullyUnlock();
+ }
+ // Transfer the elements outside of locks
+ int n = 0;
+ for (Node<E> p = first; p != null; p = p.next) {
+ c.add(p.item);
+ p.item = null;
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * @throws UnsupportedOperationException {@inheritDoc}
+ * @throws ClassCastException {@inheritDoc}
+ * @throws NullPointerException {@inheritDoc}
+ * @throws IllegalArgumentException {@inheritDoc}
+ */
+ public int drainTo(Collection<? super E> c, int maxElements) {
+ if (c == null)
+ throw new NullPointerException();
+ if (c == this)
+ throw new IllegalArgumentException();
+ fullyLock();
+ try {
+ int n = 0;
+ Node<E> p = head.next;
+ while (p != null && n < maxElements) {
+ c.add(p.item);
+ p.item = null;
+ p = p.next;
+ ++n;
+ }
+ if (n != 0) {
+ head.next = p;
+ assert head.item == null;
+ if (p == null)
+ last = head;
+ if (count.getAndAdd(-n) == capacity)
+ notFull.signalAll();
+ }
+ return n;
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ /**
+ * Returns an iterator over the elements in this queue in proper sequence.
+ * The returned <tt>Iterator</tt> 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> {
+ /*
+ * Basic weak-consistent iterator. At all times hold the next
+ * item to hand out so that if hasNext() reports true, we will
+ * still have it to return even if lost race with a take etc.
+ */
+ private Node<E> current;
+ private Node<E> lastRet;
+ private E currentElement;
+
+ Itr() {
+ final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+ final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+ putLock.lock();
+ takeLock.lock();
+ try {
+ current = head.next;
+ if (current != null)
+ currentElement = current.item;
+ } finally {
+ takeLock.unlock();
+ putLock.unlock();
+ }
+ }
+
+ public boolean hasNext() {
+ return current != null;
+ }
+
+ public E next() {
+ final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+ final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+ putLock.lock();
+ takeLock.lock();
+ try {
+ if (current == null)
+ throw new NoSuchElementException();
+ E x = currentElement;
+ lastRet = current;
+ current = current.next;
+ if (current != null)
+ currentElement = current.item;
+ return x;
+ } finally {
+ takeLock.unlock();
+ putLock.unlock();
+ }
+ }
+
+ public void remove() {
+ if (lastRet == null)
+ throw new IllegalStateException();
+ final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+ final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+ putLock.lock();
+ takeLock.lock();
+ try {
+ Node<E> node = lastRet;
+ lastRet = null;
+ Node<E> trail = head;
+ Node<E> p = head.next;
+ while (p != null && p != node) {
+ trail = p;
+ p = p.next;
+ }
+ if (p == node) {
+ p.item = null;
+ trail.next = p.next;
+ if (last == p)
+ last = trail;
+ int c = count.getAndDecrement();
+ if (c == capacity)
+ notFull.signalAll();
+ }
+ } finally {
+ takeLock.unlock();
+ putLock.unlock();
+ }
+ }
+ }
+
+ /**
+ * Save the state to a stream (that is, serialize it).
+ *
+ * @serialData The capacity is emitted (int), followed by all of
+ * its elements (each an <tt>Object</tt>) in the proper order,
+ * followed by a null
+ * @param s the stream
+ */
+ private void writeObject(java.io.ObjectOutputStream s)
+ throws java.io.IOException {
+
+ fullyLock();
+ try {
+ // Write out any hidden stuff, plus capacity
+ s.defaultWriteObject();
+
+ // Write out all elements in the proper order.
+ for (Node<E> p = head.next; p != null; p = p.next)
+ s.writeObject(p.item);
+
+ // Use trailing null as sentinel
+ s.writeObject(null);
+ } finally {
+ fullyUnlock();
+ }
+ }
+
+ /**
+ * Reconstitute this 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();
+
+ count.set(0);
+ last = head = new Node<E>(null);
+
+ // Read in all elements and place in queue
+ for (;;) {
+ E item = (E)s.readObject();
+ if (item == null)
+ break;
+ add(item);
+ }
+ }
+}