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Framework for running background processes and keeping track with their states.
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package org.amplecode.cave.process.queue;

/*
 * Copyright (c) 2008, the original author or authors.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * 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.
 *     * Neither the name of the AmpleCode project nor the names of its
 *       contributors may be used to endorse or promote products derived from
 *       this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "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 COPYRIGHT OWNER OR CONTRIBUTORS 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.
 */

import java.util.Collection;

/**
 * Process queue. The process positions in the queue are collectively defined by
 * each process' {@link ProcessQueueConstraints}. The {@link #getHead()} method
 * is the interesting method. The others should be self-describing.
 * 
 * @see #getHead()
 * @see ProcessQueueConstraints
 * 
 * @author Torgeir Lorange Ostby
 * @version $Id: ProcessQueue.java 58 2008-02-02 14:38:01Z torgeilo $
 */
public interface ProcessQueue
{
    void add( P process );

    void remove( P process );

    /**
     * Returns the head processes. The head processes are collectively defined
     * by each process' {@link ProcessQueueConstraints}. It returns an empty
     * collection if and only if {@link #isEmpty()} returns true. Head will only
     * change when one of the queue altering methods are called ({@link
     * #add(ProcessQueueConstraints)}, {@link #remove(ProcessQueueConstraints)},
     * and {@link #clear()}). Remove processes from the head to make new
     * processes advance in the queue.
     */
    Collection
 getHead();

    /**
     * Returns all processes in no specific order.
     */
    Collection getAll();

    int size();

    boolean isEmpty();

    void clear();
}