Interface Future<V>
Cancellation of a Future need not abruptly terminate its computation. Method cancel causes isCancelled() to return true unless already isDone(); in either case isDone() subsequently reports true. This suppresses execution by an ExecutorService if not already started. There are several options for suppressing unnecessary computation or unblocking a running Future that will not generate a result. When task bodies are simple and short, no special attention is warranted. Computational methods in Future-aware code bodies (for example ForkJoinTask, FutureTask) may inspect their own isDone() status before or while engaging in expensive computations. In blocking I/O or communication contexts, the optional mayInterruptIfRunning argument of cancel may be used to support conventions that tasks should unblock and exit when Thread.interrupted, whether checked inside a task body or as a response to an InterruptedException. It is still preferable to additionally check isDone() status when possible to avoid unintended effects of other uses of Thread.interrupt().
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; } } } 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<>(task); executor.execute(future);
Memory consistency effects: Actions taken by the asynchronous computation happen-before actions following the corresponding Future.get() in another thread.
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Nested Class Summary
Nested Classes -
Method Summary
All MethodsInstance MethodsAbstract MethodsDefault MethodsModifier and TypeMethodDescriptionbooleancancel(boolean mayInterruptIfRunning)Attempts to cancel execution of this task.default ThrowableReturns the exception thrown by the task, without waiting.get()Waits if necessary for the computation to complete, and then retrieves its result.Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.booleanReturns true if this task was cancelled before it completed normally.booleanisDone()Returns true if this task completed.default VReturns the computed result, without waiting.default Future.Statestate()Returns the computation state.
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Method Details
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cancel
boolean cancel(boolean mayInterruptIfRunning)Attempts to cancel execution of this task. This method has no effect if the task is already completed or cancelled, or could not be cancelled for some other reason. Otherwise, if 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 (when known by the implementation) is interrupted in an attempt to stop the task.Parameters: mayInterruptIfRunning - true if the thread executing this task should be interrupted (if the thread is known to the implementation); otherwise, in-progress tasks are allowed to complete Returns: false if the task could not be cancelled, typically because it has already completed; true otherwise. If two or more threads cause a task to be cancelled, then at least one of them returns true. Implementations may provide stronger guarantees.The return value from this method does not necessarily indicate whether the task is now cancelled; use isCancelled().
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isCancelled
boolean isCancelled()Returns true if this task was cancelled before it completed normally.Returns: true if this task was cancelled before it completed -
isDone
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 -
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 -
get
V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutExceptionWaits 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 -
resultNow
Returns the computed result, without waiting.Implementation Requirements: The default implementation invokes isDone() to test if the task has completed. If done, it invokes get() to obtain the result. Returns: the computed result Throws: IllegalStateException - if the task has not completed or the task did not complete with a result Since: 19This method is for cases where the caller knows that the task has already completed successfully, for example when filtering a stream of Future objects for the successful tasks and using a mapping operation to obtain a stream of results.
Copy results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList(); -
exceptionNow
Returns the exception thrown by the task, without waiting.Implementation Requirements: The default implementation invokes isDone() to test if the task has completed. If done and not cancelled, it invokes get() and catches the ExecutionException to obtain the exception. Returns: the exception thrown by the task Throws: IllegalStateException - if the task has not completed, the task completed normally, or the task was cancelled Since: 19This method is for cases where the caller knows that the task has already completed with an exception.
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state
Returns the computation state.Implementation Requirements: The default implementation uses isDone(), isCancelled(), and get() to determine the state. Returns: the computation state Since: 19
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