Added in API level 1

Summary: Ctors | Methods | Protected Methods | Inherited Methods

FutureTask

public class FutureTask
extends Object implements RunnableFuture<V>

java.lang.Object
↳	java.util.concurrent.FutureTask<V>

A cancellable asynchronous computation. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. The result can only be retrieved when the computation has completed; the get methods will block if the computation has not yet completed. Once the computation has completed, the computation cannot be restarted or cancelled (unless the computation is invoked using runAndReset()).

A FutureTask can be used to wrap a Callable or Runnable object. Because FutureTask implements Runnable, a FutureTask can be submitted to an Executor for execution.

In addition to serving as a standalone class, this class provides protected functionality that may be useful when creating customized task classes.

Summary

Public constructors
`FutureTask(Runnable runnable, V result)` Creates a `FutureTask` that will, upon running, execute the given `Runnable`, and arrange that `get` will return the given result on successful completion.
`FutureTask(Callable<V> callable)` Creates a `FutureTask` that will, upon running, execute the given `Callable`.

Public methods
`boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`Throwable`	`exceptionNow()` Returns the exception thrown by the task, without waiting.
`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.
`V`	`get()` Waits if necessary for the computation to complete, and then retrieves its result.
`boolean`	`isCancelled()` Returns `true` if this task was cancelled before it completed normally.
`boolean`	`isDone()` Returns `true` if this task completed.
`V`	`resultNow()` Returns the computed result, without waiting.
`void`	`run()` Sets this Future to the result of its computation unless it has been cancelled.
`Future.State`	`state()`
`String`	`toString()` Returns a string representation of this FutureTask.

Protected methods
`void`	`done()` Protected method invoked when this task transitions to state `isDone` (whether normally or via cancellation).
`boolean`	`runAndReset()` Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled.
`void`	`set(V v)` Sets the result of this future to the given value unless this future has already been set or has been cancelled.
`void`	`setException(Throwable t)` Causes this future to report an `ExecutionException` with the given throwable as its cause, unless this future has already been set or has been cancelled.

Inherited methods

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.concurrent.RunnableFuture


      run()

Sets this Future to the result of its computation unless it has been cancelled.

From interface


        
          java.util.concurrent.Future

`abstract boolean`	`cancel(boolean mayInterruptIfRunning)` Attempts to cancel execution of this task.
`default Throwable`	`exceptionNow()` Returns the exception thrown by the task, without waiting.
`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.
`default V`	`resultNow()` Returns the computed result, without waiting.
`default Future.State`	`state()`

From interface


        
          java.lang.Runnable


      run()

When an object implementing interface Runnable is used to create a thread, starting the thread causes the object's run method to be called in that separately executing thread.

Public constructors

FutureTask

Added in API level 1

public FutureTask (Runnable runnable, 
                V result)

Creates a FutureTask that will, upon running, execute the given Runnable, and arrange that get will return the given result on successful completion.

Parameters
`runnable`	`Runnable`: the runnable task
`result`	`V`: the result to return on successful completion. If you don't need a particular result, consider using constructions of the form: `Future<?> f = new FutureTask<Void>(runnable, null)`

Throws
`NullPointerException`	if the runnable is null

FutureTask

Added in API level 1

public FutureTask (Callable<V> callable)

Creates a FutureTask that will, upon running, execute the given Callable.

Parameters
`callable`	`Callable`: the callable task

Throws
`NullPointerException`	if the callable is null

Public methods

cancel

Added in API level 1

public 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.

The return value from this method does not necessarily indicate whether the task is now cancelled; use isCancelled().

Parameters
`mayInterruptIfRunning`	`boolean`: `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
`boolean`	`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.

exceptionNow

Added in API level 36

public Throwable exceptionNow ()

Returns the exception thrown by the task, without waiting.

This method is for cases where the caller knows that the task has already completed with an exception.

Returns
`Throwable`	the exception thrown by the task

get

Added in API level 1

public 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`
`ExecutionException`
`InterruptedException`
`TimeoutException`

get

Added in API level 1

public V get ()

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

Returns
`V`	the computed result

Throws
`CancellationException`
`ExecutionException`
`InterruptedException`

isCancelled

Added in API level 1

public 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

public 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

resultNow

Added in API level 36

public V resultNow ()

Returns the computed result, without waiting.

This 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. lang=java : results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList();

Returns
`V`	the computed result

run

Added in API level 1

public void run ()

Sets this Future to the result of its computation unless it has been cancelled.

state

Added in API level 36

public Future.State state ()

Returns
`Future.State`	the computation state

toString

Added in API level 1

public String toString ()

Returns a string representation of this FutureTask.

Implementation Requirements:

The default implementation returns a string identifying this FutureTask, as well as its completion state. The state, in brackets, contains one of the strings "Completed Normally", "Completed Exceptionally", "Cancelled", or "Not completed".

Returns
`String`	a string representation of this FutureTask

Protected methods

done

Added in API level 1

protected void done ()

Protected method invoked when this task transitions to state isDone (whether normally or via cancellation). The default implementation does nothing. Subclasses may override this method to invoke completion callbacks or perform bookkeeping. Note that you can query status inside the implementation of this method to determine whether this task has been cancelled.

runAndReset

Added in API level 1

protected boolean runAndReset ()

Executes the computation without setting its result, and then resets this future to initial state, failing to do so if the computation encounters an exception or is cancelled. This is designed for use with tasks that intrinsically execute more than once.

Returns
`boolean`	`true` if successfully run and reset

set

Added in API level 1

protected void set (V v)

Sets the result of this future to the given value unless this future has already been set or has been cancelled.

This method is invoked internally by the run() method upon successful completion of the computation.

Parameters
`v`	`V`: the value

setException

Added in API level 1

protected void setException (Throwable t)

Causes this future to report an ExecutionException with the given throwable as its cause, unless this future has already been set or has been cancelled.

This method is invoked internally by the run() method upon failure of the computation.

Parameters
`t`	`Throwable`: the cause of failure