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Added in API level 1

Future

public interface Future

java.util.concurrent.Future<V>
Known Indirect Subclasses


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:

Summary

Public methods

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

cancel

Added in API level 1
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 boolean: true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete
Returns
boolean false if the task could not be cancelled, typically because it has already completed normally; true otherwise

get

Added in API level 1
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 long: the maximum time to wait
unit TimeUnit: the time unit of the timeout argument
Returns
V 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

get

Added in API level 1
V get ()

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

Returns
V 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

isCancelled

Added in API level 1
boolean isCancelled ()

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

Returns
boolean true if this task was cancelled before it completed

isDone

Added in API level 1
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
boolean true if this task completed
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