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Future.javaAPI DocJava SE 5 API5065Fri Aug 26 14:57:26 BST 2005java.util.concurrent

Future

public interface Future
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 {@link 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<String>(new Callable<String>() {
public String call() {
return searcher.search(target);
}});
executor.execute(future);
see
FutureTask
see
Executor
since
1.5
author
Doug Lea
param
The result type returned by this Future's get method

Fields Summary
Constructors Summary
Methods Summary
public booleancancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task. This attempt will fail if the task has already completed, 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.

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

public Vget()
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

public Vget(long timeout, java.util.concurrent.TimeUnit unit)
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

public booleanisCancelled()
Returns true if this task was cancelled before it completed normally.

return
true if task was cancelled before it completed

public booleanisDone()
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.

return
true if this task completed.