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A versatile Java(TM) library that implements many useful container and utility classes.
There is a newer version: 1.1.12

/*
 * de.unkrig.commons - A general-purpose Java class library
 *
 * Copyright (c) 2011, Arno Unkrig
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the
 * following conditions are met:
 *
 *    1. Redistributions of source code must retain the above copyright notice, this list of conditions and the
 *       following disclaimer.
 *    2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the
 *       following disclaimer in the documentation and/or other materials provided with the distribution.
 *    3. The name of the author may not be used to endorse or promote products derived from this software without
 *       specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
 * THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

package de.unkrig.commons.util;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.LinkedBlockingQueue;

import de.unkrig.commons.lang.protocol.Consumer;
import de.unkrig.commons.lang.protocol.ConsumerWhichThrows;
import de.unkrig.commons.lang.protocol.Producer;
import de.unkrig.commons.lang.protocol.RunnableWhichThrows;

/**
 * Collects the subjects produced by several producers and forwards them to one consumer in the right order. This is
 * useful for parallelized tasks from which the results must be collected and be brought into the original order.
 *
 * @param   The type of the subjects
 * @param  The exception that the producers may throw
 * @see #run()
 */
public
class WyeConsumer implements RunnableWhichThrows {

    private final ConsumerWhichThrows target;
    private final List>        producers = new ArrayList>();

    /**
     * @see #run()
     */
    public
    WyeConsumer(ConsumerWhichThrows target) {
        this.target = target;
    }

    /**
     * @see #run()
     */
    public Consumer
    newConsumer(int capacity) {

        final LinkedBlockingQueue queue = new LinkedBlockingQueue(capacity);

        this.producers.add(new Producer() {

            @Override public T
            produce() {
                for (;;) {
                    try {
                        return queue.take();
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                    }
                }
            }
        });

        return new Consumer() {

            @Override public void
            consume(T subject) { queue.add(subject); }
        };
    }

    /**
     * Copies the subjects that are sent to the {@link Consumer} that was returned by the first call to {@link
     * #newConsumer(int)} up to, but not including, the first {@code null} reference, to the {@code target} consumer,
     * then the objects that are sent to the {@link Consumer} that was returned by the second call to {@link
     * #newConsumer(int)}, and so forth.
     */
    @Override public void
    run() throws EX {

        for (Producer producer : this.producers) {

            for (;;) {
                T subject = producer.produce();
                if (subject == null) break;
                this.target.consume(subject);
            }
        }
    }
}