public class

SynchronousQueue

extends AbstractQueue<E>
implements Serializable BlockingQueue<E>
java.lang.Object
   ↳ java.util.AbstractCollection<E>
     ↳ java.util.AbstractQueue<E>
       ↳ java.util.concurrent.SynchronousQueue<E>

Class Overview

A 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.

Summary

Public Constructors
SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.
SynchronousQueue(boolean fair)
Creates a SynchronousQueue with the specified fairness policy.
Public Methods
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 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.
<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.
Object[] toArray()
Returns a zero-length array.
[Expand]
Inherited Methods
From class java.util.AbstractQueue
From class java.util.AbstractCollection
From class java.lang.Object
From interface java.lang.Iterable
From interface java.util.Collection
From interface java.util.Queue
From interface java.util.concurrent.BlockingQueue

Public Constructors

public SynchronousQueue ()

Since: API Level 1

Creates a SynchronousQueue with nonfair access policy.

public SynchronousQueue (boolean fair)

Since: API Level 1

Creates a SynchronousQueue with the specified fairness policy.

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

Public Methods

public void clear ()

Since: API Level 1

Does nothing. A SynchronousQueue has no internal capacity.

public boolean contains (Object o)

Since: API Level 1

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o the element
Returns
  • false

public boolean containsAll (Collection<?> c)

Since: API Level 1

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

Parameters
c the collection
Returns
  • false unless given collection is empty

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

Since: API Level 1

Removes at most the given number of available elements from this queue and adds them to 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.

Parameters
c the collection to transfer elements into
maxElements the maximum number of elements to transfer
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

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

Since: API Level 1

Removes all available elements from this queue and adds them to 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.

Parameters
c the collection to transfer elements into
Returns
  • the number of elements transferred
Throws
UnsupportedOperationException
ClassCastException
NullPointerException
IllegalArgumentException

public boolean isEmpty ()

Since: API Level 1

Always returns true. A SynchronousQueue has no internal capacity.

Returns
  • true

public Iterator<E> iterator ()

Since: API Level 1

Returns an empty iterator in which hasNext always returns false.

Returns
  • an empty iterator

public boolean offer (E e)

Since: API Level 1

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

Parameters
e the element to add
Returns
  • true if the element was added to this queue, else false
Throws
NullPointerException if the specified element is null

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

Since: API Level 1

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

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
NullPointerException

public E peek ()

Since: API Level 1

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

Returns
  • null

public E poll ()

Since: API Level 1

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

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

public E poll (long timeout, TimeUnit unit)

Since: API Level 1

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

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

public void put (E o)

Since: API Level 1

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

Parameters
o the element to add
Throws
InterruptedException
NullPointerException

public int remainingCapacity ()

Since: API Level 1

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
  • zero.

public boolean remove (Object o)

Since: API Level 1

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
o the element to remove
Returns
  • false

public boolean removeAll (Collection<?> c)

Since: API Level 1

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c the collection
Returns
  • false

public boolean retainAll (Collection<?> c)

Since: API Level 1

Always returns false. A SynchronousQueue has no internal capacity.

Parameters
c the collection
Returns
  • false

public int size ()

Since: API Level 1

Always returns zero. A SynchronousQueue has no internal capacity.

Returns
  • zero.

public E take ()

Since: API Level 1

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

Returns
  • the head of this queue
Throws
InterruptedException

public T[] toArray (T[] a)

Since: API Level 1

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

Parameters
a the array
Returns
  • the specified array
Throws
NullPointerException if the specified array is null

public Object[] toArray ()

Since: API Level 1

Returns a zero-length array.

Returns
  • a zero-length array