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/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package scala.concurrent.forkjoin;
import java.util.concurrent.*;
/**
* A {@link BlockingQueue} in which producers may wait for consumers
* to receive elements. A {@code TransferQueue} may be useful for
* example in message passing applications in which producers
* sometimes (using method {@code transfer}) await receipt of
* elements by consumers invoking {@code take} or {@code poll},
* while at other times enqueue elements (via method {@code put})
* without waiting for receipt. Non-blocking and time-out versions of
* {@code tryTransfer} are also available. A TransferQueue may also
* be queried via {@code hasWaitingConsumer} whether there are any
* threads waiting for items, which is a converse analogy to a
* {@code peek} operation.
*
* <p>Like any {@code BlockingQueue}, a {@code TransferQueue} may be
* capacity bounded. If so, an attempted {@code 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 {@link SynchronousQueue},
* {@code put} and {@code transfer} are effectively synonymous.
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.7
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public interface TransferQueue<E> extends BlockingQueue<E> {
/**
* Transfers the specified element if there exists a consumer
* already waiting to receive it, otherwise returning {@code false}
* without enqueuing the element.
*
* @param e the element to transfer
* @return {@code true} if the element was transferred, else
* {@code false}
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean tryTransfer(E e);
/**
* Inserts the specified element into this queue, waiting if
* necessary for space to become available and the element to be
* dequeued by a consumer invoking {@code take} or {@code poll}.
*
* @param e the element to transfer
* @throws InterruptedException if interrupted while waiting,
* in which case the element is not enqueued.
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
void transfer(E e) throws InterruptedException;
/**
* Inserts the specified element into this queue, waiting up to
* the specified wait time if necessary for space to become
* available and the element to be dequeued by a consumer invoking
* {@code take} or {@code poll}.
*
* @param e the element to transfer
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @return {@code true} if successful, or {@code false} if
* the specified waiting time elapses before completion,
* in which case the element is not enqueued.
* @throws InterruptedException if interrupted while waiting,
* in which case the element is not enqueued.
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this queue
* @throws NullPointerException if the specified element is null
* @throws IllegalArgumentException if some property of the specified
* element prevents it from being added to this queue
*/
boolean tryTransfer(E e, long timeout, TimeUnit unit)
throws InterruptedException;
/**
* Returns {@code true} if there is at least one consumer waiting
* to dequeue an element via {@code take} or {@code poll}.
* The return value represents a momentary state of affairs.
*
* @return {@code true} if there is at least one waiting consumer
*/
boolean hasWaitingConsumer();
/**
* Returns an estimate of the number of consumers waiting to
* dequeue elements via {@code take} or {@code 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
* {@link #hasWaitingConsumer}.
*
* @return the number of consumers waiting to dequeue elements
*/
int getWaitingConsumerCount();
}
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