/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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 java.util.concurrent;

/**

 * A service that decouples the production of new asynchronous tasks

 * from the consumption of the results of completed tasks.  Producers

 * {@code submit} tasks for execution. Consumers {@code take}

 * completed tasks and process their results in the order they

 * complete.  A {@code CompletionService} can for example be used to

 * manage asynchronous I/O, in which tasks that perform reads are

 * submitted in one part of a program or system, and then acted upon

 * in a different part of the program when the reads complete,

 * possibly in a different order than they were requested.

 *

 * <p>Typically, a {@code CompletionService} relies on a separate

 * {@link Executor} to actually execute the tasks, in which case the

 * {@code CompletionService} only manages an internal completion

 * queue. The {@link ExecutorCompletionService} class provides an

 * implementation of this approach.

 *

 * <p>Memory consistency effects: Actions in a thread prior to

 * submitting a task to a {@code CompletionService}

 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

 * actions taken by that task, which in turn <i>happen-before</i>

 * actions following a successful return from the corresponding {@code take()}.

 */

public interface CompletionService<V> {

    /**

     * Submits a value-returning task for execution and returns a Future

     * representing the pending results of the task.  Upon completion,

     * this task may be taken or polled.

     *

     * @param task the task to submit

     * @return a Future representing pending completion of the task

     * @throws RejectedExecutionException if the task cannot be

     *         scheduled for execution

     * @throws NullPointerException if the task is null

     */

    Future<V> submit(Callable<V> task);

    /**

     * Submits a Runnable task for execution and returns a Future

     * representing that task.  Upon completion, this task may be

     * taken or polled.

     *

     * @param task the task to submit

     * @param result the result to return upon successful completion

     * @return a Future representing pending completion of the task,

     *         and whose {@code get()} method will return the given

     *         result value upon completion

     * @throws RejectedExecutionException if the task cannot be

     *         scheduled for execution

     * @throws NullPointerException if the task is null

     */

    Future<V> submit(Runnable task, V result);

    /**

     * Retrieves and removes the Future representing the next

     * completed task, waiting if none are yet present.

     *

     * @return the Future representing the next completed task

     * @throws InterruptedException if interrupted while waiting

     */

    Future<V> take() throws InterruptedException;

    /**

     * Retrieves and removes the Future representing the next

     * completed task, or {@code null} if none are present.

     *

     * @return the Future representing the next completed task, or

     *         {@code null} if none are present

     */

    Future<V> poll();

    /**

     * Retrieves and removes the Future representing the next

     * completed task, waiting if necessary up to the specified wait

     * time if none are yet present.

     *

     * @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 the Future representing the next completed task or

     *         {@code null} if the specified waiting time elapses

     *         before one is present

     * @throws InterruptedException if interrupted while waiting

     */

    Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;

}