/*

 * 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.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * 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 {@code Future} represents the result of an asynchronous

 * computation.  Methods are provided to check if the computation is

 * complete, to wait for its completion, and to retrieve the result of

 * the computation.  The result can only be retrieved using method

 * {@code get} when the computation has completed, blocking if

 * necessary until it is ready.  Cancellation is performed by the

 * {@code cancel} method.  Additional methods are provided to

 * determine if the task completed normally or was cancelled. Once a

 * computation has completed, the computation cannot be cancelled.

 * If you would like to use a {@code Future} for the sake

 * of cancellability but not provide a usable result, you can

 * declare types of the form {@code Future<?>} and

 * return {@code null} as a result of the underlying task.

 *

 * <p><b>Sample Usage</b> (Note that the following classes are all

 * made-up.)

 *

 * <pre> {@code

 * interface ArchiveSearcher { String search(String target); }

 * class App {

 *   ExecutorService executor = ...

 *   ArchiveSearcher searcher = ...

 *   void showSearch(String target) throws InterruptedException {

 *     Callable<String> task = () -> searcher.search(target);

 *     Future<String> future = executor.submit(task);

 *     displayOtherThings(); // do other things while searching

 *     try {

 *       displayText(future.get()); // use future

 *     } catch (ExecutionException ex) { cleanup(); return; }

 *   }

 * }}</pre>

 *

 * The {@link FutureTask} class is an implementation of {@code Future} that

 * implements {@code Runnable}, and so may be executed by an {@code Executor}.

 * For example, the above construction with {@code submit} could be replaced by:

 * <pre> {@code

 * FutureTask<String> future = new FutureTask<>(task);

 * executor.execute(future);}</pre>

 *

 * <p>Memory consistency effects: Actions taken by the asynchronous computation

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

 * actions following the corresponding {@code Future.get()} in another thread.

 *

 * @see FutureTask

 * @see Executor

 * @since 1.5

 * @author Doug Lea

 * @param <V> The result type returned by this Future's {@code get} method

 */

public interface Future<V> {

    /**

     * Attempts to cancel execution of this task.  This attempt will

     * fail if the task has already completed, has already been cancelled,

     * or could not be cancelled for some other reason. If successful,

     * and this task has not started when {@code cancel} is called,

     * this task should never run.  If the task has already started,

     * then the {@code mayInterruptIfRunning} parameter determines

     * whether the thread executing this task should be interrupted in

     * an attempt to stop the task.

     *

     * <p>After this method returns, subsequent calls to {@link #isDone} will

     * always return {@code true}.  Subsequent calls to {@link #isCancelled}

     * will always return {@code true} if this method returned {@code true}.

     *

     * @param mayInterruptIfRunning {@code true} if the thread executing this

     * task should be interrupted; otherwise, in-progress tasks are allowed

     * to complete

     * @return {@code false} if the task could not be cancelled,

     * typically because it has already completed normally;

     * {@code true} otherwise

     */

    boolean cancel(boolean mayInterruptIfRunning);

    /**

     * Returns {@code true} if this task was cancelled before it completed

     * normally.

     *

     * @return {@code true} if this task was cancelled before it completed

     */

    boolean isCancelled();

    /**

     * Returns {@code true} if this task completed.

     *

     * Completion may be due to normal termination, an exception, or

     * cancellation -- in all of these cases, this method will return

     * {@code true}.

     *

     * @return {@code true} if this task completed

     */

    boolean isDone();

    /**

     * Waits if necessary for the computation to complete, and then

     * retrieves its result.

     *

     * @return the computed result

     * @throws CancellationException if the computation was cancelled

     * @throws ExecutionException if the computation threw an

     * exception

     * @throws InterruptedException if the current thread was interrupted

     * while waiting

     */

    V get() throws InterruptedException, ExecutionException;

    /**

     * Waits if necessary for at most the given time for the computation

     * to complete, and then retrieves its result, if available.

     *

     * @param timeout the maximum time to wait

     * @param unit the time unit of the timeout argument

     * @return the computed result

     * @throws CancellationException if the computation was cancelled

     * @throws ExecutionException if the computation threw an

     * exception

     * @throws InterruptedException if the current thread was interrupted

     * while waiting

     * @throws TimeoutException if the wait timed out

     */

    V get(long timeout, TimeUnit unit)

        throws InterruptedException, ExecutionException, TimeoutException;

}