java.util.concurrent

Class SynchronousQueue<E>

Implemented Interfaces:
BlockingQueue<E>, Collection<E>, Iterable<E>, Queue<E>, Serializable

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

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.

This class is a member of the ../../../../technotes/guides/collections/index.html"> 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 the specified fairness policy.

Method Summary

T[] toArray(T[] a)
Sets the zeroeth element of the specified array to null (if the array has non-zero length) and returns it.
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(E> c)
int
drainTo(E> c, int maxElements)
boolean
isEmpty()
Always returns true.
Iterator
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.
Object[]
toArray()
Returns a zero-length array.

Methods inherited from class java.util.AbstractQueue<E>

add, addAll, clear, element, remove

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

SynchronousQueue

public SynchronousQueue()
Creates a SynchronousQueue with nonfair access policy.

SynchronousQueue

public SynchronousQueue(boolean fair)
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.

Method Details

T[] toArray

public  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:
T[] toArray in interface Collection<E>
Overrides:
T[] toArray in interface AbstractCollection<E>
Parameters:
a - the array
Returns:
the specified array
Throws:
NullPointerException - if the specified array is null

clear

public void clear()
Does nothing. A SynchronousQueue has no internal capacity.
Specified by:
clear in interface Collection<E>
Overrides:
clear in interface AbstractQueue<E>

contains

public boolean contains(Object o)
Always returns false. A SynchronousQueue has no internal capacity.
Specified by:
contains in interface BlockingQueue<E>
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
Parameters:
o - the element
Returns:
false

containsAll

public boolean containsAll(Collection c)
Returns false unless the given collection is empty. A SynchronousQueue has no internal capacity.
Specified by:
containsAll in interface Collection<E>
Overrides:
containsAll in interface AbstractCollection<E>
Parameters:
c - the collection
Returns:
false unless given collection is empty

drainTo

public int drainTo(E> c)
Specified by:
drainTo in interface BlockingQueue<E>

drainTo

public int drainTo(E> c,
                   int maxElements)
Specified by:
drainTo in interface BlockingQueue<E>

isEmpty

public boolean isEmpty()
Always returns true. A SynchronousQueue has no internal capacity.
Specified by:
isEmpty in interface Collection<E>
Overrides:
isEmpty in interface AbstractCollection<E>
Returns:
true

iterator

public Iterator iterator()
Returns an empty iterator in which hasNext always returns false.
Specified by:
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an empty iterator

offer

public boolean offer(E e)
Inserts the specified element into this queue, if another thread is waiting to receive it.
Specified by:
offer in interface BlockingQueue<E>
offer in interface Queue<E>
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

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>
Returns:
true if successful, or false if the specified waiting time elapses before a consumer appears.

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

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.

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

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>

remainingCapacity

public int remainingCapacity()
Always returns zero. A SynchronousQueue has no internal capacity.
Specified by:
remainingCapacity in interface BlockingQueue<E>
Returns:
zero.

remove

public boolean remove(Object o)
Always returns false. A SynchronousQueue has no internal capacity.
Specified by:
remove in interface BlockingQueue<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
Parameters:
o - the element to remove
Returns:
false

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 interface AbstractCollection<E>
Parameters:
c - the collection
Returns:
false

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 interface AbstractCollection<E>
Parameters:
c - the collection
Returns:
false

size

public int size()
Always returns zero. A SynchronousQueue has no internal capacity.
Specified by:
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>
Returns:
zero.

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

toArray

public Object[] toArray()
Returns a zero-length array.
Specified by:
toArray in interface Collection<E>
Overrides:
toArray in interface AbstractCollection<E>
Returns:
a zero-length array