SynchronousQueue

public class SynchronousQueue extends AbstractQueue<E>
implements BlockingQueue<E> Serializable

A {@linkplain BlockingQueue blocking queue} in which each insert operation must wait for a corresponding remove operation by another thread, and vice versa. A synchronous queue does not have any internal capacity, not even a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to remove it; you cannot insert an element (using any method) unless another thread is trying to remove it; you cannot iterate as there is nothing to iterate. The head of the queue is the element that the first queued inserting thread is trying to add to the queue; if there is no such queued thread then no element is available for removal and poll() will return null. For purposes of other Collection methods (for example contains), a SynchronousQueue acts as an empty collection. This queue does not permit null elements.

Synchronous queues are similar to rendezvous channels used in CSP and Ada. They are well suited for handoff designs, in which an object running in one thread must sync up with an object running in another thread in order to hand it some information, event, or task.

This class supports an optional fairness policy for ordering waiting producer and consumer threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness set to true grants threads access in FIFO order.

This class and its iterator implement all of the optional methods of the Collection and Iterator interfaces.

Public Constructor Summary

SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
SynchronousQueue(boolean fair)
Creates a SynchronousQueue with the specified fairness policy.

Public Method Summary

void
clear()
Does nothing.
boolean
contains(Object o)
Always returns false.
boolean
containsAll(Collection<?> c)
Returns false unless the given collection is empty.
int
drainTo(Collection<? super E> c, int maxElements)
Removes at most the given number of available elements from this queue and adds them to the given collection.
int
drainTo(Collection<? super E> c)
Removes all available elements from this queue and adds them to the given collection.
boolean
isEmpty()
Always returns true.
Iterator<E>
iterator()
Returns an empty iterator in which hasNext always returns false.
boolean
offer(E e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
boolean
offer(E e, long timeout, TimeUnit unit)
Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.
E
peek()
Always returns null.
E
poll(long timeout, TimeUnit unit)
Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.
E
poll()
Retrieves and removes the head of this queue, if another thread is currently making an element available.
void
put(E e)
Adds the specified element to this queue, waiting if necessary for another thread to receive it.
int
remainingCapacity()
Always returns zero.
boolean
remove(Object o)
Always returns false.
boolean
removeAll(Collection<?> c)
Always returns false.
boolean
retainAll(Collection<?> c)
Always returns false.
int
size()
Always returns zero.
Spliterator<E>
spliterator()
Returns an empty spliterator in which calls to trySplit() always return null.
E
take()
Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
<T> T[]
toArray(T[] a)
Sets the zeroth element of the specified array to null (if the array has non-zero length) and returns it.
Object[]
toArray()
Returns a zero-length array.
String
toString()
Always returns "[]".

Inherited Method Summary