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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.locks.*; |
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import java.util.*; |
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|
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/** |
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* A {@linkplain BlockingQueue blocking queue} in which each insert |
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* operation must wait for a corresponding remove operation by another |
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* thread, and vice versa. A synchronous queue does not have any |
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* internal capacity, not even a capacity of one. You cannot |
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* <tt>peek</tt> at a synchronous queue because an element is only |
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* present when you try to remove it; you cannot insert an element |
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* (using any method) unless another thread is trying to remove it; |
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* you cannot iterate as there is nothing to iterate. The |
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* <em>head</em> of the queue is the element that the first queued |
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* inserting thread is trying to add to the queue; if there is no such |
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* queued thread then no element is available for removal and |
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* <tt>poll()</tt> will return <tt>null</tt>. For purposes of other |
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* <tt>Collection</tt> methods (for example <tt>contains</tt>), a |
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* <tt>SynchronousQueue</tt> acts as an empty collection. This queue |
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* does not permit <tt>null</tt> elements. |
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* |
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* <p>Synchronous queues are similar to rendezvous channels used in |
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* CSP and Ada. They are well suited for handoff designs, in which an |
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* object running in one thread must sync up with an object running |
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* in another thread in order to hand it some information, event, or |
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* task. |
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* |
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* <p> This class supports an optional fairness policy for ordering |
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* waiting producer and consumer threads. By default, this ordering |
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* is not guaranteed. However, a queue constructed with fairness set |
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* to <tt>true</tt> grants threads access in FIFO order. Fairness |
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* generally decreases throughput but reduces variability and avoids |
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* starvation. |
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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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public class SynchronousQueue<E> extends AbstractQueue<E> |
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implements BlockingQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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|
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/* |
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This implementation divides actions into two cases for puts: |
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|
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* An arriving producer that does not already have a waiting consumer |
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creates a node holding item, and then waits for a consumer to take it. |
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* An arriving producer that does already have a waiting consumer fills |
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the slot node created by the consumer, and notifies it to continue. |
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|
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And symmetrically, two for takes: |
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|
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* An arriving consumer that does not already have a waiting producer |
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creates an empty slot node, and then waits for a producer to fill it. |
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* An arriving consumer that does already have a waiting producer takes |
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item from the node created by the producer, and notifies it to continue. |
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|
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When a put or take waiting for the actions of its counterpart |
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aborts due to interruption or timeout, it marks the node |
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it created as "CANCELLED", which causes its counterpart to retry |
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the entire put or take sequence. |
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|
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This requires keeping two simple queues, waitingProducers and |
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waitingConsumers. Each of these can be FIFO (preserves fairness) |
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or LIFO (improves throughput). |
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*/ |
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|
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/** Lock protecting both wait queues */ |
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private final ReentrantLock qlock; |
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/** Queue holding waiting puts */ |
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private final WaitQueue waitingProducers; |
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/** Queue holding waiting takes */ |
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private final WaitQueue waitingConsumers; |
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|
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/** |
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* Creates a <tt>SynchronousQueue</tt> with nonfair access policy. |
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*/ |
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public SynchronousQueue() { |
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this(false); |
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} |
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|
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/** |
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* Creates a <tt>SynchronousQueue</tt> with specified fairness policy. |
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* @param fair if true, threads contend in FIFO order for access; |
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* otherwise the order is unspecified. |
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*/ |
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public SynchronousQueue(boolean fair) { |
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if (fair) { |
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qlock = new ReentrantLock(true); |
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waitingProducers = new FifoWaitQueue(); |
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waitingConsumers = new FifoWaitQueue(); |
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} |
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else { |
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qlock = new ReentrantLock(); |
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waitingProducers = new LifoWaitQueue(); |
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waitingConsumers = new LifoWaitQueue(); |
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} |
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} |
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|
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/** |
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* Queue to hold waiting puts/takes; specialized to Fifo/Lifo below. |
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* These queues have all transient fields, but are serializable |
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* in order to recover fairness settings when deserialized. |
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*/ |
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static abstract class WaitQueue implements java.io.Serializable { |
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/** Creates, adds, and returns node for x. */ |
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abstract Node enq(Object x); |
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/** Removes and returns node, or null if empty. */ |
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abstract Node deq(); |
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/** Removes a cancelled node to avoid garbage retention. */ |
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abstract void unlink(Node node); |
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/** Returns true if a cancelled node might be on queue. */ |
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abstract boolean shouldUnlink(Node node); |
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} |
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|
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/** |
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* FIFO queue to hold waiting puts/takes. |
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*/ |
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static final class FifoWaitQueue extends WaitQueue implements java.io.Serializable { |
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private static final long serialVersionUID = -3623113410248163686L; |
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private transient Node head; |
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private transient Node last; |
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|
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Node enq(Object x) { |
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Node p = new Node(x); |
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if (last == null) |
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last = head = p; |
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else |
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last = last.next = p; |
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return p; |
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} |
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|
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Node deq() { |
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Node p = head; |
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if (p != null) { |
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if ((head = p.next) == null) |
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last = null; |
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p.next = null; |
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} |
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return p; |
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} |
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|
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boolean shouldUnlink(Node node) { |
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return (node == last || node.next != null); |
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} |
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|
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void unlink(Node node) { |
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Node p = head; |
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Node trail = null; |
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while (p != null) { |
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if (p == node) { |
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Node next = p.next; |
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if (trail == null) |
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head = next; |
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else |
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trail.next = next; |
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if (last == node) |
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last = trail; |
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break; |
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} |
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trail = p; |
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p = p.next; |
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} |
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} |
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} |
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|
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/** |
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* LIFO queue to hold waiting puts/takes. |
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*/ |
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static final class LifoWaitQueue extends WaitQueue implements java.io.Serializable { |
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private static final long serialVersionUID = -3633113410248163686L; |
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private transient Node head; |
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|
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Node enq(Object x) { |
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return head = new Node(x, head); |
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} |
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|
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Node deq() { |
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Node p = head; |
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if (p != null) { |
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head = p.next; |
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p.next = null; |
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} |
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return p; |
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} |
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|
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boolean shouldUnlink(Node node) { |
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// Return false if already dequeued or is bottom node (in which |
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// case we might retain at most one garbage node) |
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return (node == head || node.next != null); |
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} |
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|
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void unlink(Node node) { |
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Node p = head; |
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Node trail = null; |
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while (p != null) { |
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if (p == node) { |
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Node next = p.next; |
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if (trail == null) |
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head = next; |
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else |
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trail.next = next; |
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break; |
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} |
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trail = p; |
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p = p.next; |
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} |
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} |
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} |
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|
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/** |
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* Unlinks the given node from consumer queue. Called by cancelled |
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* (timeout, interrupt) waiters to avoid garbage retention in the |
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* absence of producers. |
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*/ |
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private void unlinkCancelledConsumer(Node node) { |
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// Use a form of double-check to avoid unnecessary locking and |
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// traversal. The first check outside lock might |
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// conservatively report true. |
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if (waitingConsumers.shouldUnlink(node)) { |
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qlock.lock(); |
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try { |
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if (waitingConsumers.shouldUnlink(node)) |
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waitingConsumers.unlink(node); |
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} finally { |
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qlock.unlock(); |
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} |
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} |
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} |
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|
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/** |
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* Unlinks the given node from producer queue. Symmetric |
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* to unlinkCancelledConsumer. |
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*/ |
249 |
private void unlinkCancelledProducer(Node node) { |
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if (waitingProducers.shouldUnlink(node)) { |
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qlock.lock(); |
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try { |
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if (waitingProducers.shouldUnlink(node)) |
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waitingProducers.unlink(node); |
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} finally { |
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qlock.unlock(); |
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} |
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} |
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} |
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|
261 |
/** |
262 |
* Nodes each maintain an item and handle waits and signals for |
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* getting and setting it. The class extends |
264 |
* AbstractQueuedSynchronizer to manage blocking, using AQS state |
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* 0 for waiting, 1 for ack, -1 for cancelled. |
266 |
*/ |
267 |
static final class Node extends AbstractQueuedSynchronizer { |
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private static final long serialVersionUID = -2631493897867746127L; |
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|
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/** Synchronization state value representing that node acked */ |
271 |
private static final int ACK = 1; |
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/** Synchronization state value representing that node cancelled */ |
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private static final int CANCEL = -1; |
274 |
|
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/** The item being transferred */ |
276 |
Object item; |
277 |
/** Next node in wait queue */ |
278 |
Node next; |
279 |
|
280 |
/** Creates a node with initial item */ |
281 |
Node(Object x) { item = x; } |
282 |
|
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/** Creates a node with initial item and next */ |
284 |
Node(Object x, Node n) { item = x; next = n; } |
285 |
|
286 |
/** |
287 |
* Implements AQS base acquire to succeed if not in WAITING state |
288 |
*/ |
289 |
protected boolean tryAcquire(int ignore) { |
290 |
return getState() != 0; |
291 |
} |
292 |
|
293 |
/** |
294 |
* Implements AQS base release to signal if state changed |
295 |
*/ |
296 |
protected boolean tryRelease(int newState) { |
297 |
return compareAndSetState(0, newState); |
298 |
} |
299 |
|
300 |
/** |
301 |
* Takes item and nulls out field (for sake of GC) |
302 |
*/ |
303 |
private Object extract() { |
304 |
Object x = item; |
305 |
item = null; |
306 |
return x; |
307 |
} |
308 |
|
309 |
/** |
310 |
* Tries to cancel on interrupt; if so rethrowing, |
311 |
* else setting interrupt state |
312 |
*/ |
313 |
private void checkCancellationOnInterrupt(InterruptedException ie) |
314 |
throws InterruptedException { |
315 |
if (release(CANCEL)) |
316 |
throw ie; |
317 |
Thread.currentThread().interrupt(); |
318 |
} |
319 |
|
320 |
/** |
321 |
* Fills in the slot created by the consumer and signal consumer to |
322 |
* continue. |
323 |
*/ |
324 |
boolean setItem(Object x) { |
325 |
item = x; // can place in slot even if cancelled |
326 |
return release(ACK); |
327 |
} |
328 |
|
329 |
/** |
330 |
* Removes item from slot created by producer and signal producer |
331 |
* to continue. |
332 |
*/ |
333 |
Object getItem() { |
334 |
return (release(ACK))? extract() : null; |
335 |
} |
336 |
|
337 |
/** |
338 |
* Waits for a consumer to take item placed by producer. |
339 |
*/ |
340 |
void waitForTake() throws InterruptedException { |
341 |
try { |
342 |
acquireInterruptibly(0); |
343 |
} catch (InterruptedException ie) { |
344 |
checkCancellationOnInterrupt(ie); |
345 |
} |
346 |
} |
347 |
|
348 |
/** |
349 |
* Waits for a producer to put item placed by consumer. |
350 |
*/ |
351 |
Object waitForPut() throws InterruptedException { |
352 |
try { |
353 |
acquireInterruptibly(0); |
354 |
} catch (InterruptedException ie) { |
355 |
checkCancellationOnInterrupt(ie); |
356 |
} |
357 |
return extract(); |
358 |
} |
359 |
|
360 |
/** |
361 |
* Waits for a consumer to take item placed by producer or time out. |
362 |
*/ |
363 |
boolean waitForTake(long nanos) throws InterruptedException { |
364 |
try { |
365 |
if (!tryAcquireNanos(0, nanos) && |
366 |
release(CANCEL)) |
367 |
return false; |
368 |
} catch (InterruptedException ie) { |
369 |
checkCancellationOnInterrupt(ie); |
370 |
} |
371 |
return true; |
372 |
} |
373 |
|
374 |
/** |
375 |
* Waits for a producer to put item placed by consumer, or time out. |
376 |
*/ |
377 |
Object waitForPut(long nanos) throws InterruptedException { |
378 |
try { |
379 |
if (!tryAcquireNanos(0, nanos) && |
380 |
release(CANCEL)) |
381 |
return null; |
382 |
} catch (InterruptedException ie) { |
383 |
checkCancellationOnInterrupt(ie); |
384 |
} |
385 |
return extract(); |
386 |
} |
387 |
} |
388 |
|
389 |
/** |
390 |
* Adds the specified element to this queue, waiting if necessary for |
391 |
* another thread to receive it. |
392 |
* |
393 |
* @throws InterruptedException {@inheritDoc} |
394 |
* @throws NullPointerException {@inheritDoc} |
395 |
*/ |
396 |
public void put(E e) throws InterruptedException { |
397 |
if (e == null) throw new NullPointerException(); |
398 |
final ReentrantLock qlock = this.qlock; |
399 |
|
400 |
for (;;) { |
401 |
Node node; |
402 |
boolean mustWait; |
403 |
if (Thread.interrupted()) throw new InterruptedException(); |
404 |
qlock.lock(); |
405 |
try { |
406 |
node = waitingConsumers.deq(); |
407 |
if ( (mustWait = (node == null)) ) |
408 |
node = waitingProducers.enq(e); |
409 |
} finally { |
410 |
qlock.unlock(); |
411 |
} |
412 |
|
413 |
if (mustWait) { |
414 |
try { |
415 |
node.waitForTake(); |
416 |
return; |
417 |
} catch (InterruptedException ex) { |
418 |
unlinkCancelledProducer(node); |
419 |
throw ex; |
420 |
} |
421 |
} |
422 |
|
423 |
else if (node.setItem(e)) |
424 |
return; |
425 |
|
426 |
// else consumer cancelled, so retry |
427 |
} |
428 |
} |
429 |
|
430 |
/** |
431 |
* Inserts the specified element into this queue, waiting if necessary |
432 |
* up to the specified wait time for another thread to receive it. |
433 |
* |
434 |
* @return <tt>true</tt> if successful, or <tt>false</tt> if the |
435 |
* specified waiting time elapses before a consumer appears. |
436 |
* @throws InterruptedException {@inheritDoc} |
437 |
* @throws NullPointerException {@inheritDoc} |
438 |
*/ |
439 |
public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException { |
440 |
if (e == null) throw new NullPointerException(); |
441 |
long nanos = unit.toNanos(timeout); |
442 |
final ReentrantLock qlock = this.qlock; |
443 |
for (;;) { |
444 |
Node node; |
445 |
boolean mustWait; |
446 |
if (Thread.interrupted()) throw new InterruptedException(); |
447 |
qlock.lock(); |
448 |
try { |
449 |
node = waitingConsumers.deq(); |
450 |
if ( (mustWait = (node == null)) ) |
451 |
node = waitingProducers.enq(e); |
452 |
} finally { |
453 |
qlock.unlock(); |
454 |
} |
455 |
|
456 |
if (mustWait) { |
457 |
try { |
458 |
boolean x = node.waitForTake(nanos); |
459 |
if (!x) |
460 |
unlinkCancelledProducer(node); |
461 |
return x; |
462 |
} catch (InterruptedException ex) { |
463 |
unlinkCancelledProducer(node); |
464 |
throw ex; |
465 |
} |
466 |
} |
467 |
|
468 |
else if (node.setItem(e)) |
469 |
return true; |
470 |
|
471 |
// else consumer cancelled, so retry |
472 |
} |
473 |
} |
474 |
|
475 |
/** |
476 |
* Retrieves and removes the head of this queue, waiting if necessary |
477 |
* for another thread to insert it. |
478 |
* |
479 |
* @return the head of this queue |
480 |
* @throws InterruptedException {@inheritDoc} |
481 |
*/ |
482 |
public E take() throws InterruptedException { |
483 |
final ReentrantLock qlock = this.qlock; |
484 |
for (;;) { |
485 |
Node node; |
486 |
boolean mustWait; |
487 |
|
488 |
if (Thread.interrupted()) throw new InterruptedException(); |
489 |
qlock.lock(); |
490 |
try { |
491 |
node = waitingProducers.deq(); |
492 |
if ( (mustWait = (node == null)) ) |
493 |
node = waitingConsumers.enq(null); |
494 |
} finally { |
495 |
qlock.unlock(); |
496 |
} |
497 |
|
498 |
if (mustWait) { |
499 |
try { |
500 |
Object x = node.waitForPut(); |
501 |
return (E)x; |
502 |
} catch (InterruptedException ex) { |
503 |
unlinkCancelledConsumer(node); |
504 |
throw ex; |
505 |
} |
506 |
} |
507 |
else { |
508 |
Object x = node.getItem(); |
509 |
if (x != null) |
510 |
return (E)x; |
511 |
// else cancelled, so retry |
512 |
} |
513 |
} |
514 |
} |
515 |
|
516 |
/** |
517 |
* Retrieves and removes the head of this queue, waiting |
518 |
* if necessary up to the specified wait time, for another thread |
519 |
* to insert it. |
520 |
* |
521 |
* @return the head of this queue, or <tt>null</tt> if the |
522 |
* specified waiting time elapses before an element is present. |
523 |
* @throws InterruptedException {@inheritDoc} |
524 |
*/ |
525 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
526 |
long nanos = unit.toNanos(timeout); |
527 |
final ReentrantLock qlock = this.qlock; |
528 |
|
529 |
for (;;) { |
530 |
Node node; |
531 |
boolean mustWait; |
532 |
|
533 |
if (Thread.interrupted()) throw new InterruptedException(); |
534 |
qlock.lock(); |
535 |
try { |
536 |
node = waitingProducers.deq(); |
537 |
if ( (mustWait = (node == null)) ) |
538 |
node = waitingConsumers.enq(null); |
539 |
} finally { |
540 |
qlock.unlock(); |
541 |
} |
542 |
|
543 |
if (mustWait) { |
544 |
try { |
545 |
Object x = node.waitForPut(nanos); |
546 |
if (x == null) |
547 |
unlinkCancelledConsumer(node); |
548 |
return (E)x; |
549 |
} catch (InterruptedException ex) { |
550 |
unlinkCancelledConsumer(node); |
551 |
throw ex; |
552 |
} |
553 |
} |
554 |
else { |
555 |
Object x = node.getItem(); |
556 |
if (x != null) |
557 |
return (E)x; |
558 |
// else cancelled, so retry |
559 |
} |
560 |
} |
561 |
} |
562 |
|
563 |
// Untimed nonblocking versions |
564 |
|
565 |
/** |
566 |
* Inserts the specified element into this queue, if another thread is |
567 |
* waiting to receive it. |
568 |
* |
569 |
* @param e the element to add |
570 |
* @return <tt>true</tt> if the element was added to this queue, else |
571 |
* <tt>false</tt> |
572 |
* @throws NullPointerException if the specified element is null |
573 |
*/ |
574 |
public boolean offer(E e) { |
575 |
if (e == null) throw new NullPointerException(); |
576 |
final ReentrantLock qlock = this.qlock; |
577 |
|
578 |
for (;;) { |
579 |
Node node; |
580 |
qlock.lock(); |
581 |
try { |
582 |
node = waitingConsumers.deq(); |
583 |
} finally { |
584 |
qlock.unlock(); |
585 |
} |
586 |
if (node == null) |
587 |
return false; |
588 |
|
589 |
else if (node.setItem(e)) |
590 |
return true; |
591 |
// else retry |
592 |
} |
593 |
} |
594 |
|
595 |
/** |
596 |
* Retrieves and removes the head of this queue, if another thread |
597 |
* is currently making an element available. |
598 |
* |
599 |
* @return the head of this queue, or <tt>null</tt> if no |
600 |
* element is available. |
601 |
*/ |
602 |
public E poll() { |
603 |
final ReentrantLock qlock = this.qlock; |
604 |
for (;;) { |
605 |
Node node; |
606 |
qlock.lock(); |
607 |
try { |
608 |
node = waitingProducers.deq(); |
609 |
} finally { |
610 |
qlock.unlock(); |
611 |
} |
612 |
if (node == null) |
613 |
return null; |
614 |
|
615 |
else { |
616 |
Object x = node.getItem(); |
617 |
if (x != null) |
618 |
return (E)x; |
619 |
// else retry |
620 |
} |
621 |
} |
622 |
} |
623 |
|
624 |
/** |
625 |
* Always returns <tt>true</tt>. |
626 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
627 |
* |
628 |
* @return <tt>true</tt> |
629 |
*/ |
630 |
public boolean isEmpty() { |
631 |
return true; |
632 |
} |
633 |
|
634 |
/** |
635 |
* Always returns zero. |
636 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
637 |
* |
638 |
* @return zero |
639 |
*/ |
640 |
public int size() { |
641 |
return 0; |
642 |
} |
643 |
|
644 |
/** |
645 |
* Always returns zero. |
646 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
647 |
* |
648 |
* @return zero |
649 |
*/ |
650 |
public int remainingCapacity() { |
651 |
return 0; |
652 |
} |
653 |
|
654 |
/** |
655 |
* Does nothing. |
656 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
657 |
*/ |
658 |
public void clear() {} |
659 |
|
660 |
/** |
661 |
* Always returns <tt>false</tt>. |
662 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
663 |
* |
664 |
* @param o object to be checked for containment in this queue |
665 |
* @return <tt>false</tt> |
666 |
*/ |
667 |
public boolean contains(Object o) { |
668 |
return false; |
669 |
} |
670 |
|
671 |
/** |
672 |
* Always returns <tt>false</tt>. |
673 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
674 |
* |
675 |
* @param o the element to remove |
676 |
* @return <tt>false</tt> |
677 |
*/ |
678 |
public boolean remove(Object o) { |
679 |
return false; |
680 |
} |
681 |
|
682 |
/** |
683 |
* Returns <tt>false</tt> unless the given collection is empty. |
684 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
685 |
* |
686 |
* @param c the collection |
687 |
* @return <tt>false</tt> unless the given collection is empty |
688 |
* @throws NullPointerException if the specified collection is null |
689 |
*/ |
690 |
public boolean containsAll(Collection<?> c) { |
691 |
return c.isEmpty(); |
692 |
} |
693 |
|
694 |
/** |
695 |
* Always returns <tt>false</tt>. |
696 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
697 |
* |
698 |
* @param c the collection |
699 |
* @return <tt>false</tt> |
700 |
*/ |
701 |
public boolean removeAll(Collection<?> c) { |
702 |
return false; |
703 |
} |
704 |
|
705 |
/** |
706 |
* Always returns <tt>false</tt>. |
707 |
* A <tt>SynchronousQueue</tt> has no internal capacity. |
708 |
* |
709 |
* @param c the collection |
710 |
* @return <tt>false</tt> |
711 |
*/ |
712 |
public boolean retainAll(Collection<?> c) { |
713 |
return false; |
714 |
} |
715 |
|
716 |
/** |
717 |
* Always returns <tt>null</tt>. |
718 |
* A <tt>SynchronousQueue</tt> does not return elements |
719 |
* unless actively waited on. |
720 |
* |
721 |
* @return <tt>null</tt> |
722 |
*/ |
723 |
public E peek() { |
724 |
return null; |
725 |
} |
726 |
|
727 |
|
728 |
static class EmptyIterator<E> implements Iterator<E> { |
729 |
public boolean hasNext() { |
730 |
return false; |
731 |
} |
732 |
public E next() { |
733 |
throw new NoSuchElementException(); |
734 |
} |
735 |
public void remove() { |
736 |
throw new IllegalStateException(); |
737 |
} |
738 |
} |
739 |
|
740 |
/** |
741 |
* Returns an empty iterator in which <tt>hasNext</tt> always returns |
742 |
* <tt>false</tt>. |
743 |
* |
744 |
* @return an empty iterator |
745 |
*/ |
746 |
public Iterator<E> iterator() { |
747 |
return new EmptyIterator<E>(); |
748 |
} |
749 |
|
750 |
|
751 |
/** |
752 |
* Returns a zero-length array. |
753 |
* @return a zero-length array |
754 |
*/ |
755 |
public Object[] toArray() { |
756 |
return new Object[0]; |
757 |
} |
758 |
|
759 |
/** |
760 |
* Sets the zeroeth element of the specified array to <tt>null</tt> |
761 |
* (if the array has non-zero length) and returns it. |
762 |
* |
763 |
* @param a the array |
764 |
* @return the specified array |
765 |
* @throws NullPointerException if the specified array is null |
766 |
*/ |
767 |
public <T> T[] toArray(T[] a) { |
768 |
if (a.length > 0) |
769 |
a[0] = null; |
770 |
return a; |
771 |
} |
772 |
|
773 |
/** |
774 |
* @throws UnsupportedOperationException {@inheritDoc} |
775 |
* @throws ClassCastException {@inheritDoc} |
776 |
* @throws NullPointerException {@inheritDoc} |
777 |
* @throws IllegalArgumentException {@inheritDoc} |
778 |
*/ |
779 |
public int drainTo(Collection<? super E> c) { |
780 |
if (c == null) |
781 |
throw new NullPointerException(); |
782 |
if (c == this) |
783 |
throw new IllegalArgumentException(); |
784 |
int n = 0; |
785 |
E e; |
786 |
while ( (e = poll()) != null) { |
787 |
c.add(e); |
788 |
++n; |
789 |
} |
790 |
return n; |
791 |
} |
792 |
|
793 |
/** |
794 |
* @throws UnsupportedOperationException {@inheritDoc} |
795 |
* @throws ClassCastException {@inheritDoc} |
796 |
* @throws NullPointerException {@inheritDoc} |
797 |
* @throws IllegalArgumentException {@inheritDoc} |
798 |
*/ |
799 |
public int drainTo(Collection<? super E> c, int maxElements) { |
800 |
if (c == null) |
801 |
throw new NullPointerException(); |
802 |
if (c == this) |
803 |
throw new IllegalArgumentException(); |
804 |
int n = 0; |
805 |
E e; |
806 |
while (n < maxElements && (e = poll()) != null) { |
807 |
c.add(e); |
808 |
++n; |
809 |
} |
810 |
return n; |
811 |
} |
812 |
} |