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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/publicdomain/zero/1.0/
 */

package jsr166y;
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 {@link #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.
 * {@linkplain #tryTransfer(Object) Non-blocking} and
 * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of
 * {@code tryTransfer} are also available.
 * A {@code TransferQueue} may also be queried, via {@link
 * #hasWaitingConsumer}, whether there are any threads waiting for
 * items, which is a converse analogy to a {@code peek} operation.
 *
 * 

Like other blocking queues, a {@code 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 {@link SynchronousQueue}, {@code put} * and {@code transfer} are effectively synonymous. * *

This interface is a member of the * * Java Collections Framework. * * @since 1.7 * @author Doug Lea * @param the type of elements held in this collection */ public interface TransferQueue extends BlockingQueue { /** * 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 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * 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); /** * 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 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else waits until the element is received by a consumer. * * @param e the element to transfer * @throws InterruptedException if interrupted while waiting, * in which case the element is not left 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; /** * 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 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), * else waits until the element is received by a consumer, * returning {@code false} if the specified wait time elapses * before the element can be transferred. * * @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 left enqueued * @throws InterruptedException if interrupted while waiting, * in which case the element is not left 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 receive an element via {@link #take} or * timed {@link #poll(long,TimeUnit) 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 * receive elements via {@link #take} or timed * {@link #poll(long,TimeUnit) 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 receive elements */ int getWaitingConsumerCount(); }





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