Future

AI-generated Key Takeaways

• Future represents the result of an asynchronous computation, allowing you to check completion status, wait for completion, and retrieve the result.

• The get method retrieves the result, blocking if necessary until the computation is finished.

• Tasks can be cancelled using the cancel method, preventing execution or interrupting an in-progress task.

• Future provides methods like isCancelled and isDone to check the task's status.

• Implementations like FutureTask can be executed by an Executor, enabling asynchronous task management.

public interface Future

Known Indirect Subclasses CompletableFuture<T>, CountedCompleter<T>, ForkJoinTask<V>, FutureTask<V>, RecursiveAction, RecursiveTask<V>, RunnableFuture<V>, RunnableScheduledFuture<V>, ScheduledFuture<V> CompletableFuture<T> A Future that may be explicitly completed (setting its value and status), and may be used as a CompletionStage, supporting dependent functions and actions that trigger upon its completion.  CountedCompleter<T> A ForkJoinTask with a completion action performed when triggered and there are no remaining pending actions.  ForkJoinTask<V> Abstract base class for tasks that run within a ForkJoinPool.  FutureTask<V> A cancellable asynchronous computation.  RecursiveAction A recursive resultless ForkJoinTask.  RecursiveTask<V> A recursive result-bearing ForkJoinTask.  RunnableFuture<V> A Future that is Runnable.  RunnableScheduledFuture<V> A ScheduledFuture that is Runnable.  ScheduledFuture<V> A delayed result-bearing action that can be cancelled.

A 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 get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the 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 Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future<?> and return null 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(final String target)
       throws InterruptedException {
     Future<String> future
       = executor.submit(new Callable<String>() {
         public String call() {
             return searcher.search(target);
         }});
     displayOtherThings(); // do other things while searching
     try {
       displayText(future.get()); // use future
     } catch (ExecutionException ex) { cleanup(); return; }
   }
 }

The FutureTask class is an implementation of Future that implements Runnable, and so may be executed by an Executor. For example, the above construction with submit could be replaced by:

 FutureTask<String> future =
   new FutureTask<>(new Callable<String>() {
     public String call() {
       return searcher.search(target);
   }});
 executor.execute(future);

Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.

See Also

• FutureTask
• Executor

Public Method Summary

abstract boolean	cancel(boolean mayInterruptIfRunning) Attempts to cancel execution of this task.
abstract V	get(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.
abstract V	get() Waits if necessary for the computation to complete, and then retrieves its result.
abstract boolean	isCancelled() Returns true if this task was cancelled before it completed normally.
abstract boolean	isDone() Returns true if this task completed.

Public Methods

public abstract 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 when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

After this method returns, subsequent calls to isDone() will always return true. Subsequent calls to isCancelled() will always return true if this method returned true.

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

public abstract V get (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.

Parameters

timeout	the maximum time to wait
unit	the time unit of the timeout argument

Returns

• the computed result

Throws

CancellationException	if the computation was cancelled
ExecutionException	if the computation threw an exception
InterruptedException	if the current thread was interrupted while waiting
TimeoutException	if the wait timed out

public abstract V get ()

Waits if necessary for the computation to complete, and then retrieves its result.

Returns

• the computed result

Throws

CancellationException	if the computation was cancelled
ExecutionException	if the computation threw an exception
InterruptedException	if the current thread was interrupted while waiting

public abstract boolean isCancelled ()

Returns true if this task was cancelled before it completed normally.

Returns

• true if this task was cancelled before it completed

public abstract boolean isDone ()

Returns 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 true.

Returns

• true if this task completed