public interface Executor {

    /**

     * Executes the given command at some time in the future.  The command

     * may execute in a new thread, in a pooled thread, or in the calling

     * thread, at the discretion of the {@code Executor} implementation.

     *

     * @param command the runnable task

     * @throws RejectedExecutionException if this task cannot be

     * accepted for execution

     * @throws NullPointerException if command is null

     */

    void execute(Runnable command);

}

public interface ExecutorService extends Executor {

    /**

     * Initiates an orderly shutdown in which previously submitted

     * tasks are executed, but no new tasks will be accepted.

     * Invocation has no additional effect if already shut down.

     *

     * <p>This method does not wait for previously submitted tasks to

     * complete execution.  Use {@link #awaitTermination awaitTermination}

     * to do that.

     *

     * @throws SecurityException if a security manager exists and

     *         shutting down this ExecutorService may manipulate

     *         threads that the caller is not permitted to modify

     *         because it does not hold {@link

     *         java.lang.RuntimePermission}{@code ("modifyThread")},

     *         or the security manager's {@code checkAccess} method

     *         denies access.

     */

    void shutdown();

    /**

     * Attempts to stop all actively executing tasks, halts the

     * processing of waiting tasks, and returns a list of the tasks

     * that were awaiting execution.

     *

     * <p>This method does not wait for actively executing tasks to

     * terminate.  Use {@link #awaitTermination awaitTermination} to

     * do that.

     *

     * <p>There are no guarantees beyond best-effort attempts to stop

     * processing actively executing tasks.  For example, typical

     * implementations will cancel via {@link Thread#interrupt}, so any

     * task that fails to respond to interrupts may never terminate.

     *

     * @return list of tasks that never commenced execution

     * @throws SecurityException if a security manager exists and

     *         shutting down this ExecutorService may manipulate

     *         threads that the caller is not permitted to modify

     *         because it does not hold {@link

     *         java.lang.RuntimePermission}{@code ("modifyThread")},

     *         or the security manager's {@code checkAccess} method

     *         denies access.

     */

    List<Runnable> shutdownNow();

    /**

     * Returns {@code true} if this executor has been shut down.

     *

     * @return {@code true} if this executor has been shut down

     */

    boolean isShutdown();

    /**

     * Returns {@code true} if all tasks have completed following shut down.

     * Note that {@code isTerminated} is never {@code true} unless

     * either {@code shutdown} or {@code shutdownNow} was called first.

     *

     * @return {@code true} if all tasks have completed following shut down

     */

    boolean isTerminated();

    /**

     * Blocks until all tasks have completed execution after a shutdown

     * request, or the timeout occurs, or the current thread is

     * interrupted, whichever happens first.

     *

     * @param timeout the maximum time to wait

     * @param unit the time unit of the timeout argument

     * @return {@code true} if this executor terminated and

     *         {@code false} if the timeout elapsed before termination

     * @throws InterruptedException if interrupted while waiting

     */

    boolean awaitTermination(long timeout, TimeUnit unit)

        throws InterruptedException;

    /**

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

     * Future representing the pending results of the task. The

     * Future's {@code get} method will return the task's result upon

     * successful completion.

     *

     * <p>

     * If you would like to immediately block waiting

     * for a task, you can use constructions of the form

     * {@code result = exec.submit(aCallable).get();}

     *

     * <p>Note: The {@link Executors} class includes a set of methods

     * that can convert some other common closure-like objects,

     * for example, {@link java.security.PrivilegedAction} to

     * {@link Callable} form so they can be submitted.

     *

     * @param task the task to submit

     * @param <T> the type of the task's result

     * @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

     */

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

    /**

     * Submits a Runnable task for execution and returns a Future

     * representing that task. The Future's {@code get} method will

     * return the given result upon successful completion.

     *

     * @param task the task to submit

     * @param result the result to return

     * @param <T> the type of the result

     * @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

     */

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

    /**

     * Submits a Runnable task for execution and returns a Future

     * representing that task. The Future's {@code get} method will

     * return {@code null} upon <em>successful</em> completion.

     *

     * @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<?> submit(Runnable task);

    /**

     * Executes the given tasks, returning a list of Futures holding

     * their status and results when all complete.

     * {@link Future#isDone} is {@code true} for each

     * element of the returned list.

     * Note that a <em>completed</em> task could have

     * terminated either normally or by throwing an exception.

     * The results of this method are undefined if the given

     * collection is modified while this operation is in progress.

     *

     * @param tasks the collection of tasks

     * @param <T> the type of the values returned from the tasks

     * @return a list of Futures representing the tasks, in the same

     *         sequential order as produced by the iterator for the

     *         given task list, each of which has completed

     * @throws InterruptedException if interrupted while waiting, in

     *         which case unfinished tasks are cancelled

     * @throws NullPointerException if tasks or any of its elements are {@code null}

     * @throws RejectedExecutionException if any task cannot be

     *         scheduled for execution

     */

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)

        throws InterruptedException;

    /**

     * Executes the given tasks, returning a list of Futures holding

     * their status and results

     * when all complete or the timeout expires, whichever happens first.

     * {@link Future#isDone} is {@code true} for each

     * element of the returned list.

     * Upon return, tasks that have not completed are cancelled.

     * Note that a <em>completed</em> task could have

     * terminated either normally or by throwing an exception.

     * The results of this method are undefined if the given

     * collection is modified while this operation is in progress.

     *

     * @param tasks the collection of tasks

     * @param timeout the maximum time to wait

     * @param unit the time unit of the timeout argument

     * @param <T> the type of the values returned from the tasks

     * @return a list of Futures representing the tasks, in the same

     *         sequential order as produced by the iterator for the

     *         given task list. If the operation did not time out,

     *         each task will have completed. If it did time out, some

     *         of these tasks will not have completed.

     * @throws InterruptedException if interrupted while waiting, in

     *         which case unfinished tasks are cancelled

     * @throws NullPointerException if tasks, any of its elements, or

     *         unit are {@code null}

     * @throws RejectedExecutionException if any task cannot be scheduled

     *         for execution

     */

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,

                                  long timeout, TimeUnit unit)

        throws InterruptedException;

    /**

     * Executes the given tasks, returning the result

     * of one that has completed successfully (i.e., without throwing

     * an exception), if any do. Upon normal or exceptional return,

     * tasks that have not completed are cancelled.

     * The results of this method are undefined if the given

     * collection is modified while this operation is in progress.

     *

     * @param tasks the collection of tasks

     * @param <T> the type of the values returned from the tasks

     * @return the result returned by one of the tasks

     * @throws InterruptedException if interrupted while waiting

     * @throws NullPointerException if tasks or any element task

     *         subject to execution is {@code null}

     * @throws IllegalArgumentException if tasks is empty

     * @throws ExecutionException if no task successfully completes

     * @throws RejectedExecutionException if tasks cannot be scheduled

     *         for execution

     */

    <T> T invokeAny(Collection<? extends Callable<T>> tasks)

        throws InterruptedException, ExecutionException;

    /**

     * Executes the given tasks, returning the result

     * of one that has completed successfully (i.e., without throwing

     * an exception), if any do before the given timeout elapses.

     * Upon normal or exceptional return, tasks that have not

     * completed are cancelled.

     * The results of this method are undefined if the given

     * collection is modified while this operation is in progress.

     *

     * @param tasks the collection of tasks

     * @param timeout the maximum time to wait

     * @param unit the time unit of the timeout argument

     * @param <T> the type of the values returned from the tasks

     * @return the result returned by one of the tasks

     * @throws InterruptedException if interrupted while waiting

     * @throws NullPointerException if tasks, or unit, or any element

     *         task subject to execution is {@code null}

     * @throws TimeoutException if the given timeout elapses before

     *         any task successfully completes

     * @throws ExecutionException if no task successfully completes

     * @throws RejectedExecutionException if tasks cannot be scheduled

     *         for execution

     */

    <T> T invokeAny(Collection<? extends Callable<T>> tasks,

                    long timeout, TimeUnit unit)

        throws InterruptedException, ExecutionException, TimeoutException;

}