java.util.concurrent
Class SynchronousQueue<E>

java.lang.Object
  java.util.AbstractCollection<E>
      java.util.AbstractQueue<E>
          java.util.concurrent.SynchronousQueue<E>

Type Parameters:

E - the type of elements held in this collection

All Implemented Interfaces:

Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, Queue<E>

public class SynchronousQueue<E>
extends AbstractQueue<E>
implements BlockingQueue<E>, 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, not even 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 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. Fairness generally decreases throughput but reduces variability and avoids starvation.

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

This class is a member of the Java Collections Framework.

Since:

1.5

See Also:

Serialized Form

Constructor Summary

SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.

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

Method Summary

void	clear() Does nothing.
boolean	contains(Object o) Always returns false.
boolean	containsAll(Collection<?> c) Returns false unless given collection is empty.
int	drainTo(Collection<? super E> c) Removes all available elements from this queue and adds them into the given collection.
int	drainTo(Collection<? super E> c, int maxElements) Removes at most the given number of available elements from this queue and adds them into the given collection.
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(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.
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, addAll, element, remove

Methods inherited from class java.util.AbstractCollection

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

Methods inherited from interface java.util.Queue

element, remove

Methods inherited from interface java.util.Collection

addAll, equals, hashCode

Constructor Detail

SynchronousQueue

public SynchronousQueue()

Creates a SynchronousQueue with nonfair access policy.

SynchronousQueue

public SynchronousQueue(boolean fair)

Creates a SynchronousQueue with specified fairness policy.

Parameters:

fair - if true, threads contend in FIFO order for access; otherwise the order is unspecified.

Method Detail

put

public void put(E o)
         throws InterruptedException

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

Specified by:

put in interface BlockingQueue<E>

Parameters:

o - the element to add

Throws:

InterruptedException - if interrupted while waiting.

NullPointerException - if the specified element is null.

offer

public boolean offer(E o,
                     long timeout,
                     TimeUnit unit)
              throws 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<E>

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 consumer appears.

Throws:

InterruptedException - if interrupted while waiting.

NullPointerException - if the specified element is null.

take

public E take()
       throws InterruptedException

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

Specified by:

take in interface BlockingQueue<E>

Returns:

the head of this queue

Throws:

InterruptedException - if interrupted while waiting.

poll

public E poll(long timeout,
              TimeUnit unit)
       throws 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<E>

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:

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<E>

Specified by:

offer in interface Queue<E>

Parameters:

o - the element to add.

Returns:

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

Throws:

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<E>

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<E>

Overrides:

isEmpty in class AbstractCollection<E>

Returns:

true

size

public int size()

Always returns zero. A SynchronousQueue has no internal capacity.

Specified by:

size in interface Collection<E>

Specified by:

size in class AbstractCollection<E>

Returns:

zero.

remainingCapacity

public int remainingCapacity()

Always returns zero. A SynchronousQueue has no internal capacity.

Specified by:

remainingCapacity in interface BlockingQueue<E>

Returns:

zero.

clear

public void clear()

Does nothing. A SynchronousQueue has no internal capacity.

Specified by:

clear in interface Collection<E>

Overrides:

clear in class AbstractQueue<E>

contains

public boolean contains(Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

contains in interface Collection<E>

Overrides:

contains in class AbstractCollection<E>

Parameters:

o - the element

Returns:

false

remove

public boolean remove(Object o)

Always returns false. A SynchronousQueue has no internal capacity.

Specified by:

remove in interface Collection<E>

Overrides:

remove in class AbstractCollection<E>

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<E>

Overrides:

containsAll in class AbstractCollection<E>

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<E>

Overrides:

removeAll in class AbstractCollection<E>

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<E>

Overrides:

retainAll in class AbstractCollection<E>

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<E>

Returns:

null

iterator

public Iterator<E> iterator()

Returns an empty iterator in which hasNext always returns false.

Specified by:

iterator in interface Iterable<E>

Specified by:

iterator in interface Collection<E>

Specified by:

iterator in class AbstractCollection<E>

Returns:

an empty iterator

toArray

public Object[] toArray()

Returns a zero-length array.

Specified by:

toArray in interface Collection<E>

Overrides:

toArray in class AbstractCollection<E>

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<E>

Overrides:

toArray in class AbstractCollection<E>

Parameters:

a - the array

Returns:

the specified array

drainTo

public int drainTo(Collection<? super E> c)

Description copied from interface: BlockingQueue

Removes all available elements from this queue and adds them into the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Specified by:

drainTo in interface BlockingQueue<E>

Parameters:

c - the collection to transfer elements into

Returns:

the number of elements transferred.

drainTo

public int drainTo(Collection<? super E> c,
                   int maxElements)

Description copied from interface: BlockingQueue

Removes at most the given number of available elements from this queue and adds them into the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

Specified by:

drainTo in interface BlockingQueue<E>

Parameters:

c - the collection to transfer elements into

maxElements - the maximum number of elements to transfer

Returns:

the number of elements transferred.