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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.concurrent.atomic.*; |
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import java.util.concurrent.locks.*; |
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import java.util.*; |
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|
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/** |
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* An optionally-bounded {@linkplain BlockingQueue blocking queue} based on |
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* linked nodes. |
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* This queue orders elements FIFO (first-in-first-out). |
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* The <em>head</em> of the queue is that element that has been on the |
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* queue the longest time. |
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* The <em>tail</em> of the queue is that element that has been on the |
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* queue the shortest time. New elements |
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* are inserted at the tail of the queue, and the queue retrieval |
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* operations obtain elements at the head of the queue. |
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* Linked queues typically have higher throughput than array-based queues but |
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* less predictable performance in most concurrent applications. |
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* |
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* <p> The optional capacity bound constructor argument serves as a |
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* way to prevent excessive queue expansion. The capacity, if unspecified, |
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* is equal to {@link Integer#MAX_VALUE}. Linked nodes are |
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* dynamically created upon each insertion unless this would bring the |
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* queue above capacity. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this collection |
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* |
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*/ |
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public class LinkedBlockingQueue<E> extends AbstractQueue<E> |
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implements BlockingQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -6903933977591709194L; |
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|
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/* |
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* A variant of the "two lock queue" algorithm. The putLock gates |
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* entry to put (and offer), and has an associated condition for |
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* waiting puts. Similarly for the takeLock. The "count" field |
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* that they both rely on is maintained as an atomic to avoid |
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* needing to get both locks in most cases. Also, to minimize need |
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* for puts to get takeLock and vice-versa, cascading notifies are |
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* used. When a put notices that it has enabled at least one take, |
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* it signals taker. That taker in turn signals others if more |
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* items have been entered since the signal. And symmetrically for |
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* takes signalling puts. Operations such as remove(Object) and |
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* iterators acquire both locks. |
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*/ |
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|
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/** |
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* Linked list node class |
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*/ |
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static class Node<E> { |
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/** The item, volatile to ensure barrier separating write and read */ |
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volatile E item; |
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Node<E> next; |
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Node(E x) { item = x; } |
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} |
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|
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/** The capacity bound, or Integer.MAX_VALUE if none */ |
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private final int capacity; |
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|
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/** Current number of elements */ |
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private final AtomicInteger count = new AtomicInteger(0); |
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|
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/** Head of linked list */ |
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private transient Node<E> head; |
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|
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/** Tail of linked list */ |
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private transient Node<E> last; |
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|
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/** Lock held by take, poll, etc */ |
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private final ReentrantLock takeLock = new ReentrantLock(); |
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|
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/** Wait queue for waiting takes */ |
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private final Condition notEmpty = takeLock.newCondition(); |
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|
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/** Lock held by put, offer, etc */ |
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private final ReentrantLock putLock = new ReentrantLock(); |
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|
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/** Wait queue for waiting puts */ |
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private final Condition notFull = putLock.newCondition(); |
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|
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/** |
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* Signals a waiting take. Called only from put/offer (which do not |
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* otherwise ordinarily lock takeLock.) |
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*/ |
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private void signalNotEmpty() { |
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final ReentrantLock takeLock = this.takeLock; |
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takeLock.lock(); |
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try { |
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notEmpty.signal(); |
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} finally { |
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takeLock.unlock(); |
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} |
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} |
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|
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/** |
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* Signals a waiting put. Called only from take/poll. |
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*/ |
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private void signalNotFull() { |
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final ReentrantLock putLock = this.putLock; |
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putLock.lock(); |
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try { |
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notFull.signal(); |
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} finally { |
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putLock.unlock(); |
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} |
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} |
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|
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/** |
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* Creates a node and links it at end of queue. |
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* @param x the item |
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*/ |
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private void insert(E x) { |
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last = last.next = new Node<E>(x); |
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} |
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|
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/** |
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* Removes a node from head of queue, |
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* @return the node |
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*/ |
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private E extract() { |
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Node<E> first = head.next; |
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head = first; |
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E x = first.item; |
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first.item = null; |
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return x; |
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} |
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|
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/** |
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* Lock to prevent both puts and takes. |
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*/ |
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private void fullyLock() { |
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putLock.lock(); |
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takeLock.lock(); |
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} |
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|
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/** |
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* Unlock to allow both puts and takes. |
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*/ |
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private void fullyUnlock() { |
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takeLock.unlock(); |
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putLock.unlock(); |
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} |
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|
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|
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/** |
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* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of |
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* {@link Integer#MAX_VALUE}. |
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*/ |
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public LinkedBlockingQueue() { |
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this(Integer.MAX_VALUE); |
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} |
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|
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/** |
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* Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity. |
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* |
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* @param capacity the capacity of this queue. |
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* @throws IllegalArgumentException if <tt>capacity</tt> is not greater |
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* than zero. |
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*/ |
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public LinkedBlockingQueue(int capacity) { |
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if (capacity <= 0) throw new IllegalArgumentException(); |
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this.capacity = capacity; |
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last = head = new Node<E>(null); |
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} |
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|
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/** |
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* Creates a <tt>LinkedBlockingQueue</tt> with a capacity of |
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* {@link Integer#MAX_VALUE}, initially containing the elements of the |
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* given collection, |
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* added in traversal order of the collection's iterator. |
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* @param c the collection of elements to initially contain |
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* @throws NullPointerException if <tt>c</tt> or any element within it |
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* is <tt>null</tt>. |
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*/ |
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public LinkedBlockingQueue(Collection<? extends E> c) { |
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this(Integer.MAX_VALUE); |
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for (E e : c) |
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add(e); |
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} |
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|
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|
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// this doc comment is overridden to remove the reference to collections |
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// greater in size than Integer.MAX_VALUE |
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/** |
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* Returns the number of elements in this queue. |
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* |
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* @return the number of elements in this queue. |
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*/ |
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public int size() { |
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return count.get(); |
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} |
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|
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// this doc comment is a modified copy of the inherited doc comment, |
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// without the reference to unlimited queues. |
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/** |
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* Returns the number of elements that this queue can ideally (in |
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* the absence of memory or resource constraints) accept without |
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* blocking. This is always equal to the initial capacity of this queue |
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* less the current <tt>size</tt> of this queue. |
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* <p>Note that you <em>cannot</em> always tell if |
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* an attempt to <tt>add</tt> an element will succeed by |
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* inspecting <tt>remainingCapacity</tt> because it may be the |
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* case that a waiting consumer is ready to <tt>take</tt> an |
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* element out of an otherwise full queue. |
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*/ |
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public int remainingCapacity() { |
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return capacity - count.get(); |
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} |
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|
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/** |
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* Adds the specified element to the tail of this queue, waiting if |
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* necessary for space to become available. |
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* @param o the element to add |
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* @throws InterruptedException if interrupted while waiting. |
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* @throws NullPointerException if the specified element is <tt>null</tt>. |
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*/ |
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public void put(E o) throws InterruptedException { |
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if (o == null) throw new NullPointerException(); |
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// Note: convention in all put/take/etc is to preset |
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// local var holding count negative to indicate failure unless set. |
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int c = -1; |
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final ReentrantLock putLock = this.putLock; |
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final AtomicInteger count = this.count; |
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putLock.lockInterruptibly(); |
240 |
try { |
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/* |
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* Note that count is used in wait guard even though it is |
243 |
* not protected by lock. This works because count can |
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* only decrease at this point (all other puts are shut |
245 |
* out by lock), and we (or some other waiting put) are |
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* signalled if it ever changes from |
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* capacity. Similarly for all other uses of count in |
248 |
* other wait guards. |
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*/ |
250 |
try { |
251 |
while (count.get() == capacity) |
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notFull.await(); |
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} catch (InterruptedException ie) { |
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notFull.signal(); // propagate to a non-interrupted thread |
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throw ie; |
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} |
257 |
insert(o); |
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c = count.getAndIncrement(); |
259 |
if (c + 1 < capacity) |
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notFull.signal(); |
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} finally { |
262 |
putLock.unlock(); |
263 |
} |
264 |
if (c == 0) |
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signalNotEmpty(); |
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} |
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|
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/** |
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* Inserts the specified element at the tail of this queue, waiting if |
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* necessary up to the specified wait time for space to become available. |
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* @param o the element to add |
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* @param timeout how long to wait before giving up, in units of |
273 |
* <tt>unit</tt> |
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* @param unit a <tt>TimeUnit</tt> determining how to interpret the |
275 |
* <tt>timeout</tt> parameter |
276 |
* @return <tt>true</tt> if successful, or <tt>false</tt> if |
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* the specified waiting time elapses before space is available. |
278 |
* @throws InterruptedException if interrupted while waiting. |
279 |
* @throws NullPointerException if the specified element is <tt>null</tt>. |
280 |
*/ |
281 |
public boolean offer(E o, long timeout, TimeUnit unit) |
282 |
throws InterruptedException { |
283 |
|
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if (o == null) throw new NullPointerException(); |
285 |
long nanos = unit.toNanos(timeout); |
286 |
int c = -1; |
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final ReentrantLock putLock = this.putLock; |
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final AtomicInteger count = this.count; |
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putLock.lockInterruptibly(); |
290 |
try { |
291 |
for (;;) { |
292 |
if (count.get() < capacity) { |
293 |
insert(o); |
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c = count.getAndIncrement(); |
295 |
if (c + 1 < capacity) |
296 |
notFull.signal(); |
297 |
break; |
298 |
} |
299 |
if (nanos <= 0) |
300 |
return false; |
301 |
try { |
302 |
nanos = notFull.awaitNanos(nanos); |
303 |
} catch (InterruptedException ie) { |
304 |
notFull.signal(); // propagate to a non-interrupted thread |
305 |
throw ie; |
306 |
} |
307 |
} |
308 |
} finally { |
309 |
putLock.unlock(); |
310 |
} |
311 |
if (c == 0) |
312 |
signalNotEmpty(); |
313 |
return true; |
314 |
} |
315 |
|
316 |
/** |
317 |
* Inserts the specified element at the tail of this queue if possible, |
318 |
* returning immediately if this queue is full. |
319 |
* |
320 |
* @param o the element to add. |
321 |
* @return <tt>true</tt> if it was possible to add the element to |
322 |
* this queue, else <tt>false</tt> |
323 |
* @throws NullPointerException if the specified element is <tt>null</tt>. |
324 |
*/ |
325 |
public boolean offer(E o) { |
326 |
if (o == null) throw new NullPointerException(); |
327 |
final AtomicInteger count = this.count; |
328 |
if (count.get() == capacity) |
329 |
return false; |
330 |
int c = -1; |
331 |
final ReentrantLock putLock = this.putLock; |
332 |
putLock.lock(); |
333 |
try { |
334 |
if (count.get() < capacity) { |
335 |
insert(o); |
336 |
c = count.getAndIncrement(); |
337 |
if (c + 1 < capacity) |
338 |
notFull.signal(); |
339 |
} |
340 |
} finally { |
341 |
putLock.unlock(); |
342 |
} |
343 |
if (c == 0) |
344 |
signalNotEmpty(); |
345 |
return c >= 0; |
346 |
} |
347 |
|
348 |
|
349 |
public E take() throws InterruptedException { |
350 |
E x; |
351 |
int c = -1; |
352 |
final AtomicInteger count = this.count; |
353 |
final ReentrantLock takeLock = this.takeLock; |
354 |
takeLock.lockInterruptibly(); |
355 |
try { |
356 |
try { |
357 |
while (count.get() == 0) |
358 |
notEmpty.await(); |
359 |
} catch (InterruptedException ie) { |
360 |
notEmpty.signal(); // propagate to a non-interrupted thread |
361 |
throw ie; |
362 |
} |
363 |
|
364 |
x = extract(); |
365 |
c = count.getAndDecrement(); |
366 |
if (c > 1) |
367 |
notEmpty.signal(); |
368 |
} finally { |
369 |
takeLock.unlock(); |
370 |
} |
371 |
if (c == capacity) |
372 |
signalNotFull(); |
373 |
return x; |
374 |
} |
375 |
|
376 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
377 |
E x = null; |
378 |
int c = -1; |
379 |
long nanos = unit.toNanos(timeout); |
380 |
final AtomicInteger count = this.count; |
381 |
final ReentrantLock takeLock = this.takeLock; |
382 |
takeLock.lockInterruptibly(); |
383 |
try { |
384 |
for (;;) { |
385 |
if (count.get() > 0) { |
386 |
x = extract(); |
387 |
c = count.getAndDecrement(); |
388 |
if (c > 1) |
389 |
notEmpty.signal(); |
390 |
break; |
391 |
} |
392 |
if (nanos <= 0) |
393 |
return null; |
394 |
try { |
395 |
nanos = notEmpty.awaitNanos(nanos); |
396 |
} catch (InterruptedException ie) { |
397 |
notEmpty.signal(); // propagate to a non-interrupted thread |
398 |
throw ie; |
399 |
} |
400 |
} |
401 |
} finally { |
402 |
takeLock.unlock(); |
403 |
} |
404 |
if (c == capacity) |
405 |
signalNotFull(); |
406 |
return x; |
407 |
} |
408 |
|
409 |
public E poll() { |
410 |
final AtomicInteger count = this.count; |
411 |
if (count.get() == 0) |
412 |
return null; |
413 |
E x = null; |
414 |
int c = -1; |
415 |
final ReentrantLock takeLock = this.takeLock; |
416 |
takeLock.lock(); |
417 |
try { |
418 |
if (count.get() > 0) { |
419 |
x = extract(); |
420 |
c = count.getAndDecrement(); |
421 |
if (c > 1) |
422 |
notEmpty.signal(); |
423 |
} |
424 |
} finally { |
425 |
takeLock.unlock(); |
426 |
} |
427 |
if (c == capacity) |
428 |
signalNotFull(); |
429 |
return x; |
430 |
} |
431 |
|
432 |
|
433 |
public E peek() { |
434 |
if (count.get() == 0) |
435 |
return null; |
436 |
final ReentrantLock takeLock = this.takeLock; |
437 |
takeLock.lock(); |
438 |
try { |
439 |
Node<E> first = head.next; |
440 |
if (first == null) |
441 |
return null; |
442 |
else |
443 |
return first.item; |
444 |
} finally { |
445 |
takeLock.unlock(); |
446 |
} |
447 |
} |
448 |
|
449 |
/** |
450 |
* Removes a single instance of the specified element from this |
451 |
* queue, if it is present. |
452 |
*/ |
453 |
public boolean remove(Object o) { |
454 |
if (o == null) return false; |
455 |
boolean removed = false; |
456 |
fullyLock(); |
457 |
try { |
458 |
Node<E> trail = head; |
459 |
Node<E> p = head.next; |
460 |
while (p != null) { |
461 |
if (o.equals(p.item)) { |
462 |
removed = true; |
463 |
break; |
464 |
} |
465 |
trail = p; |
466 |
p = p.next; |
467 |
} |
468 |
if (removed) { |
469 |
p.item = null; |
470 |
trail.next = p.next; |
471 |
if (last == p) |
472 |
last = trail; |
473 |
if (count.getAndDecrement() == capacity) |
474 |
notFull.signalAll(); |
475 |
} |
476 |
} finally { |
477 |
fullyUnlock(); |
478 |
} |
479 |
return removed; |
480 |
} |
481 |
|
482 |
public Object[] toArray() { |
483 |
fullyLock(); |
484 |
try { |
485 |
int size = count.get(); |
486 |
Object[] a = new Object[size]; |
487 |
int k = 0; |
488 |
for (Node<E> p = head.next; p != null; p = p.next) |
489 |
a[k++] = p.item; |
490 |
return a; |
491 |
} finally { |
492 |
fullyUnlock(); |
493 |
} |
494 |
} |
495 |
|
496 |
public <T> T[] toArray(T[] a) { |
497 |
fullyLock(); |
498 |
try { |
499 |
int size = count.get(); |
500 |
if (a.length < size) |
501 |
a = (T[])java.lang.reflect.Array.newInstance |
502 |
(a.getClass().getComponentType(), size); |
503 |
|
504 |
int k = 0; |
505 |
for (Node p = head.next; p != null; p = p.next) |
506 |
a[k++] = (T)p.item; |
507 |
return a; |
508 |
} finally { |
509 |
fullyUnlock(); |
510 |
} |
511 |
} |
512 |
|
513 |
public String toString() { |
514 |
fullyLock(); |
515 |
try { |
516 |
return super.toString(); |
517 |
} finally { |
518 |
fullyUnlock(); |
519 |
} |
520 |
} |
521 |
|
522 |
/** |
523 |
* Atomically removes all of the elements from this queue. |
524 |
* The queue will be empty after this call returns. |
525 |
*/ |
526 |
public void clear() { |
527 |
fullyLock(); |
528 |
try { |
529 |
head.next = null; |
530 |
assert head.item == null; |
531 |
last = head; |
532 |
if (count.getAndSet(0) == capacity) |
533 |
notFull.signalAll(); |
534 |
} finally { |
535 |
fullyUnlock(); |
536 |
} |
537 |
} |
538 |
|
539 |
public int drainTo(Collection<? super E> c) { |
540 |
if (c == null) |
541 |
throw new NullPointerException(); |
542 |
if (c == this) |
543 |
throw new IllegalArgumentException(); |
544 |
Node first; |
545 |
fullyLock(); |
546 |
try { |
547 |
first = head.next; |
548 |
head.next = null; |
549 |
assert head.item == null; |
550 |
last = head; |
551 |
if (count.getAndSet(0) == capacity) |
552 |
notFull.signalAll(); |
553 |
} finally { |
554 |
fullyUnlock(); |
555 |
} |
556 |
// Transfer the elements outside of locks |
557 |
int n = 0; |
558 |
for (Node<E> p = first; p != null; p = p.next) { |
559 |
c.add(p.item); |
560 |
p.item = null; |
561 |
++n; |
562 |
} |
563 |
return n; |
564 |
} |
565 |
|
566 |
public int drainTo(Collection<? super E> c, int maxElements) { |
567 |
if (c == null) |
568 |
throw new NullPointerException(); |
569 |
if (c == this) |
570 |
throw new IllegalArgumentException(); |
571 |
fullyLock(); |
572 |
try { |
573 |
int n = 0; |
574 |
Node<E> p = head.next; |
575 |
while (p != null && n < maxElements) { |
576 |
c.add(p.item); |
577 |
p.item = null; |
578 |
p = p.next; |
579 |
++n; |
580 |
} |
581 |
if (n != 0) { |
582 |
head.next = p; |
583 |
assert head.item == null; |
584 |
if (p == null) |
585 |
last = head; |
586 |
if (count.getAndAdd(-n) == capacity) |
587 |
notFull.signalAll(); |
588 |
} |
589 |
return n; |
590 |
} finally { |
591 |
fullyUnlock(); |
592 |
} |
593 |
} |
594 |
|
595 |
/** |
596 |
* Returns an iterator over the elements in this queue in proper sequence. |
597 |
* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that |
598 |
* will never throw {@link ConcurrentModificationException}, |
599 |
* and guarantees to traverse elements as they existed upon |
600 |
* construction of the iterator, and may (but is not guaranteed to) |
601 |
* reflect any modifications subsequent to construction. |
602 |
* |
603 |
* @return an iterator over the elements in this queue in proper sequence. |
604 |
*/ |
605 |
public Iterator<E> iterator() { |
606 |
return new Itr(); |
607 |
} |
608 |
|
609 |
private class Itr implements Iterator<E> { |
610 |
/* |
611 |
* Basic weak-consistent iterator. At all times hold the next |
612 |
* item to hand out so that if hasNext() reports true, we will |
613 |
* still have it to return even if lost race with a take etc. |
614 |
*/ |
615 |
private Node<E> current; |
616 |
private Node<E> lastRet; |
617 |
private E currentElement; |
618 |
|
619 |
Itr() { |
620 |
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock; |
621 |
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock; |
622 |
putLock.lock(); |
623 |
takeLock.lock(); |
624 |
try { |
625 |
current = head.next; |
626 |
if (current != null) |
627 |
currentElement = current.item; |
628 |
} finally { |
629 |
takeLock.unlock(); |
630 |
putLock.unlock(); |
631 |
} |
632 |
} |
633 |
|
634 |
public boolean hasNext() { |
635 |
return current != null; |
636 |
} |
637 |
|
638 |
public E next() { |
639 |
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock; |
640 |
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock; |
641 |
putLock.lock(); |
642 |
takeLock.lock(); |
643 |
try { |
644 |
if (current == null) |
645 |
throw new NoSuchElementException(); |
646 |
E x = currentElement; |
647 |
lastRet = current; |
648 |
current = current.next; |
649 |
if (current != null) |
650 |
currentElement = current.item; |
651 |
return x; |
652 |
} finally { |
653 |
takeLock.unlock(); |
654 |
putLock.unlock(); |
655 |
} |
656 |
} |
657 |
|
658 |
public void remove() { |
659 |
if (lastRet == null) |
660 |
throw new IllegalStateException(); |
661 |
final ReentrantLock putLock = LinkedBlockingQueue.this.putLock; |
662 |
final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock; |
663 |
putLock.lock(); |
664 |
takeLock.lock(); |
665 |
try { |
666 |
Node<E> node = lastRet; |
667 |
lastRet = null; |
668 |
Node<E> trail = head; |
669 |
Node<E> p = head.next; |
670 |
while (p != null && p != node) { |
671 |
trail = p; |
672 |
p = p.next; |
673 |
} |
674 |
if (p == node) { |
675 |
p.item = null; |
676 |
trail.next = p.next; |
677 |
if (last == p) |
678 |
last = trail; |
679 |
int c = count.getAndDecrement(); |
680 |
if (c == capacity) |
681 |
notFull.signalAll(); |
682 |
} |
683 |
} finally { |
684 |
takeLock.unlock(); |
685 |
putLock.unlock(); |
686 |
} |
687 |
} |
688 |
} |
689 |
|
690 |
/** |
691 |
* Save the state to a stream (that is, serialize it). |
692 |
* |
693 |
* @serialData The capacity is emitted (int), followed by all of |
694 |
* its elements (each an <tt>Object</tt>) in the proper order, |
695 |
* followed by a null |
696 |
* @param s the stream |
697 |
*/ |
698 |
private void writeObject(java.io.ObjectOutputStream s) |
699 |
throws java.io.IOException { |
700 |
|
701 |
fullyLock(); |
702 |
try { |
703 |
// Write out any hidden stuff, plus capacity |
704 |
s.defaultWriteObject(); |
705 |
|
706 |
// Write out all elements in the proper order. |
707 |
for (Node<E> p = head.next; p != null; p = p.next) |
708 |
s.writeObject(p.item); |
709 |
|
710 |
// Use trailing null as sentinel |
711 |
s.writeObject(null); |
712 |
} finally { |
713 |
fullyUnlock(); |
714 |
} |
715 |
} |
716 |
|
717 |
/** |
718 |
* Reconstitute this queue instance from a stream (that is, |
719 |
* deserialize it). |
720 |
* @param s the stream |
721 |
*/ |
722 |
private void readObject(java.io.ObjectInputStream s) |
723 |
throws java.io.IOException, ClassNotFoundException { |
724 |
// Read in capacity, and any hidden stuff |
725 |
s.defaultReadObject(); |
726 |
|
727 |
count.set(0); |
728 |
last = head = new Node<E>(null); |
729 |
|
730 |
// Read in all elements and place in queue |
731 |
for (;;) { |
732 |
E item = (E)s.readObject(); |
733 |
if (item == null) |
734 |
break; |
735 |
add(item); |
736 |
} |
737 |
} |
738 |
} |