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added in API level 21

TransferQueue

public interface TransferQueue
implements BlockingQueue<E>

java.util.concurrent.TransferQueue<E>


A BlockingQueue in which producers may wait for consumers to receive elements. A TransferQueue may be useful for example in message passing applications in which producers sometimes (using method transfer(E)) await receipt of elements by consumers invoking take or poll, while at other times enqueue elements (via method put) without waiting for receipt. Non-blocking and time-out versions of tryTransfer are also available. A TransferQueue may also be queried, via hasWaitingConsumer(), whether there are any threads waiting for items, which is a converse analogy to a peek operation.

Like other blocking queues, a TransferQueue may be capacity bounded. If so, an attempted transfer operation may initially block waiting for available space, and/or subsequently block waiting for reception by a consumer. Note that in a queue with zero capacity, such as SynchronousQueue, put and transfer are effectively synonymous.

Summary

Public methods

abstract int getWaitingConsumerCount()

Returns an estimate of the number of consumers waiting to receive elements via BlockingQueue.take() or timed poll.

abstract boolean hasWaitingConsumer()

Returns true if there is at least one consumer waiting to receive an element via BlockingQueue.take() or timed poll.

abstract void transfer(E e)

Transfers the element to a consumer, waiting if necessary to do so.

abstract boolean tryTransfer(E e)

Transfers the element to a waiting consumer immediately, if possible.

abstract boolean tryTransfer(E e, long timeout, TimeUnit unit)

Transfers the element to a consumer if it is possible to do so before the timeout elapses.

Inherited methods

Public methods

getWaitingConsumerCount

added in API level 21
public abstract int getWaitingConsumerCount ()

Returns an estimate of the number of consumers waiting to receive elements via BlockingQueue.take() or timed poll. The return value is an approximation of a momentary state of affairs, that may be inaccurate if consumers have completed or given up waiting. The value may be useful for monitoring and heuristics, but not for synchronization control. Implementations of this method are likely to be noticeably slower than those for hasWaitingConsumer().

Returns
int the number of consumers waiting to receive elements

hasWaitingConsumer

added in API level 21
public abstract boolean hasWaitingConsumer ()

Returns true if there is at least one consumer waiting to receive an element via BlockingQueue.take() or timed poll. The return value represents a momentary state of affairs.

Returns
boolean true if there is at least one waiting consumer

transfer

added in API level 21
public abstract void transfer (E e)

Transfers the element to a consumer, waiting if necessary to do so.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in BlockingQueue.take() or timed poll), else waits until the element is received by a consumer.

Parameters
e E: the element to transfer

Throws
InterruptedException if interrupted while waiting, in which case the element is not left enqueued
ClassCastException if the class of the specified element prevents it from being added to this queue
NullPointerException if the specified element is null
IllegalArgumentException if some property of the specified element prevents it from being added to this queue

tryTransfer

added in API level 21
public abstract boolean tryTransfer (E e)

Transfers the element to a waiting consumer immediately, if possible.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in BlockingQueue.take() or timed poll), otherwise returning false without enqueuing the element.

Parameters
e E: the element to transfer

Returns
boolean true if the element was transferred, else false

Throws
ClassCastException if the class of the specified element prevents it from being added to this queue
NullPointerException if the specified element is null
IllegalArgumentException if some property of the specified element prevents it from being added to this queue

tryTransfer

added in API level 21
public abstract boolean tryTransfer (E e, 
                long timeout, 
                TimeUnit unit)

Transfers the element to a consumer if it is possible to do so before the timeout elapses.

More precisely, transfers the specified element immediately if there exists a consumer already waiting to receive it (in BlockingQueue.take() or timed poll), else waits until the element is received by a consumer, returning false if the specified wait time elapses before the element can be transferred.

Parameters
e E: the element to transfer

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

unit TimeUnit: a TimeUnit determining how to interpret the timeout parameter

Returns
boolean true if successful, or false if the specified waiting time elapses before completion, in which case the element is not left enqueued

Throws
InterruptedException if interrupted while waiting, in which case the element is not left enqueued
ClassCastException if the class of the specified element prevents it from being added to this queue
NullPointerException if the specified element is null
IllegalArgumentException if some property of the specified element prevents it from being added to this queue