java.util.concurrent
Class SynchronousQueue<E>

java.lang.Object
  java.util.AbstractCollection
      java.util.AbstractQueue
          java.util.concurrent.SynchronousQueue

All Implemented Interfaces:

BlockingQueue, Collection, java.lang.Iterable, Queue, java.io.Serializable

public class SynchronousQueue<E>

extends AbstractQueue<E>

implements BlockingQueue<E>, java.io.Serializable

A blocking queue in which each put must wait for a take, and vice versa. A synchronous queue does not have any internal capacity - in particular it does not have a capacity of one. You cannot peek at a synchronous queue because an element is only present when you try to take it; you cannot add 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 thread is trying to add to the queue; if there are no queued threads then no element is being added and the head is 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 synch up with an object running in another thread in order to hand it some information, event, or task.

This class implements all of the optional methods of the Collection and Iterator interfaces.

Since:

1.5

Author:

Doug Lea

See Also:

Serialized Form

Constructor Summary

SynchronousQueue()
Creates a SynchronousQueue.

Method Summary

void	clear() Does nothing.
boolean	contains(java.lang.Object o) Always returns false.
boolean	containsAll(Collection<?> c) Returns false unless given collection is empty.
boolean	isEmpty() Always returns true.
Iterator<E>	iterator() Returns an empty iterator in which hasNext always returns false.
boolean	offer(E o) Inserts the specified element into this queue, if another thread is waiting to receive it.
boolean	offer(E o, 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() Retrieves and removes the head of this queue, if another thread is currently making an element available.
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.
void	put(E o) Adds the specified element to this queue, waiting if necessary for another thread to receive it.
int	remainingCapacity() Always returns zero.
boolean	remove(java.lang.Object o) Always returns false.
boolean	removeAll(Collection<?> c) Always returns false.
boolean	retainAll(Collection<?> c) Always returns false.
int	size() Always returns zero.
E	take() Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.
java.lang.Object[]	toArray() Returns a zero-length array.
<T> T[]	toArray(T[] a) Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.

Methods inherited from class java.util.AbstractQueue

add, element, remove

Methods inherited from class java.util.AbstractCollection

addAll, toString

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.concurrent.BlockingQueue

add, addAll

Methods inherited from interface java.util.Queue

element, remove

Methods inherited from interface java.util.Collection

equals, hashCode

Constructor Detail

SynchronousQueue

public SynchronousQueue()

Creates a SynchronousQueue.

Method Detail

put

public void put(E o)
         throws java.lang.InterruptedException

Adds the specified element to this queue, waiting if necessary for another thread to receive it.

Specified by:

put in interface BlockingQueue

Parameters:

o - the element to add

Throws:

java.lang.InterruptedException - if interrupted while waiting.

java.lang.NullPointerException - if the specified element is null.

offer

public boolean offer(E o,
                     long timeout,
                     TimeUnit unit)
              throws java.lang.InterruptedException

Inserts the specified element into this queue, waiting if necessary up to the specified wait time for another thread to receive it.

Specified by:

offer in interface BlockingQueue

Parameters:

o - the element to add

timeout - how long to wait before giving up, in units of unit

unit - a TimeUnit determining how to interpret the timeout parameter

Returns:

true if successful, or false if the specified waiting time elapses before a taker appears.

Throws:

java.lang.InterruptedException - if interrupted while waiting.

java.lang.NullPointerException - if the specified element is null.

take

public E take()
       throws java.lang.InterruptedException

Retrieves and removes the head of this queue, waiting if necessary for another thread to insert it.

Specified by:

take in interface BlockingQueue

Returns:

the head of this queue

Throws:

java.lang.InterruptedException - if interrupted while waiting.

poll

public E poll(long timeout,
              TimeUnit unit)
       throws java.lang.InterruptedException

Retrieves and removes the head of this queue, waiting if necessary up to the specified wait time, for another thread to insert it.

Specified by:

poll in interface BlockingQueue

Parameters:

timeout - how long to wait before giving up, in units of unit

unit - a TimeUnit determining how to interpret the timeout parameter

Returns:

the head of this queue, or null if the specified waiting time elapses before an element is present.

Throws:

java.lang.InterruptedException - if interrupted while waiting.

offer

public boolean offer(E o)

Inserts the specified element into this queue, if another thread is waiting to receive it.

Specified by:

offer in interface BlockingQueue

Parameters:

o - the element to add.

Returns:

true if it was possible to add the element to this queue, else false

Throws:

java.lang.NullPointerException - if the specified element is null

poll

public E poll()

Retrieves and removes the head of this queue, if another thread is currently making an element available.

Specified by:

poll in interface Queue

Returns:

the head of this queue, or null if no element is available.

isEmpty

public boolean isEmpty()

Always returns true. A SynchronousQueue has no internal capacity.

Specified by:

isEmpty in interface Collection

Overrides:

isEmpty in class AbstractCollection

Returns:

true

size

public int size()

Always returns zero. A SynchronousQueue has no internal capacity.

Specified by:

size in interface Collection

Specified by:

size in class AbstractCollection

Returns:

zero.

remainingCapacity

public int remainingCapacity()

Always returns zero. A SynchronousQueue has no internal capacity.

Specified by:

remainingCapacity in interface BlockingQueue

Returns:

zero.

clear

public void clear()

Does nothing. A SynchronousQueue has no internal capacity.

Specified by:

clear in interface Collection

Overrides:

clear in class AbstractQueue

contains

public boolean contains(java.lang.Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

contains in interface Collection

Overrides:

contains in class AbstractCollection

Parameters:

o - the element

Returns:

false

remove

public boolean remove(java.lang.Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

remove in interface Collection

Overrides:

remove in class AbstractCollection

Parameters:

o - the element to remove

Returns:

false

containsAll

public boolean containsAll(Collection<?> c)

Returns false unless given collection is empty. A SynchronousQueue has no internal capacity.

Specified by:

containsAll in interface Collection

Overrides:

containsAll in class AbstractCollection

Parameters:

c - the collection

Returns:

false unless given collection is empty

See Also:

AbstractCollection.contains(Object)

removeAll

public boolean removeAll(Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

removeAll in interface Collection

Overrides:

removeAll in class AbstractCollection

Parameters:

c - the collection

Returns:

false

See Also:

AbstractCollection.remove(Object), AbstractCollection.contains(Object)

retainAll

public boolean retainAll(Collection<?> c)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

retainAll in interface Collection

Overrides:

retainAll in class AbstractCollection

Parameters:

c - the collection

Returns:

false

See Also:

AbstractCollection.remove(Object), AbstractCollection.contains(Object)

peek

public E peek()

Always returns null. A SynchronousQueue does not return elements unless actively waited on.

Specified by:

peek in interface Queue

Returns:

null

iterator

public Iterator<E> iterator()

Returns an empty iterator in which hasNext always returns false.

Specified by:

iterator in interface Collection

Specified by:

iterator in class AbstractCollection

Returns:

an empty iterator

toArray

public java.lang.Object[] toArray()

Returns a zero-length array.

Specified by:

toArray in interface Collection

Overrides:

toArray in class AbstractCollection

Returns:

a zero-length array

toArray

public <T> T[] toArray(T[] a)

Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.

Specified by:

toArray in interface Collection

Overrides:

toArray in class AbstractCollection

Parameters:

a - the array into which the elements of the collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.

Returns:

the specified array