Module java.base
Package java.util.concurrent

Interface Future<V>

  • Type Parameters:
    V - The result type returned by this Future'sget method
    All Known Subinterfaces:
    RunnableFuture<V>,RunnableScheduledFuture<V>,ScheduledFuture<V>
    All Known Implementing Classes:
    CompletableFuture,CountedCompleter,ForkJoinTask,FutureTask,RecursiveAction,RecursiveTask,SwingWorker
    public interfaceFuture<V>
    AFuture 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 methodget when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by thecancel 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 aFuture for the sake of cancellability but not provide a usable result, you can declare types of the formFuture<?> and returnnull as a result of the underlying task.

    Sample Usage (Note that the following classes are all made-up.)

     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; }   } }
    TheFutureTask class is an implementation ofFuture that implementsRunnable, and so may be executed by anExecutor. For example, the above construction withsubmit could be replaced by:
     FutureTask<String> future = new FutureTask<>(task); executor.execute(future);

    Memory consistency effects: Actions taken by the asynchronous computationhappen-before actions following the correspondingFuture.get() in another thread.

    Since:
    1.5
    See Also:
    FutureTask,Executor

    • Method Summary

      All Methods Instance Methods Abstract Methods 
      Modifier and TypeMethodDescription
      booleancancel​(boolean mayInterruptIfRunning)
      Attempts to cancel execution of this task.
      Vget()
      Waits if necessary for the computation to complete, and then retrieves its result.
      Vget​(long timeout,TimeUnit unit)
      Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
      booleanisCancelled()
      Returnstrue if this task was cancelled before it completed normally.
      booleanisDone()
      Returnstrue if this task completed.

    • Method Detail

      • cancel

        boolean cancel​(boolean mayInterruptIfRunning)
        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 whencancel is called, this task should never run. If the task has already started, then themayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

        After this method returns, subsequent calls toisDone() will always returntrue. Subsequent calls toisCancelled() will always returntrue if this method returnedtrue.

        Parameters:
        mayInterruptIfRunning -true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete
        Returns:
        false if the task could not be cancelled, typically because it has already completed normally;true otherwise

      • isCancelled

        boolean isCancelled()
        Returnstrue if this task was cancelled before it completed normally.
        Returns:
        true if this task was cancelled before it completed

      • isDone

        boolean isDone()
        Returnstrue if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will returntrue.
        Returns:
        true if this task completed