Module java.base
Package java.util.concurrent

# Class FutureTask<V>

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

Type Parameters:
  `V` - The result type returned by this FutureTask's `get` methods

All Implemented Interfaces:
  `Runnable`, `Future<V>`, `RunnableFuture<V>`

public class FutureTask<V> extends Object implements RunnableFuture<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.

Since:
  1.5

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `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`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `cancel(boolean mayInterruptIfRunning)`
  Attempts to cancel execution of this task.
  `protected void`
  `done()`
  Protected method invoked when this task transitions to state `isDone` (whether normally or via cancellation).
  `V`
  `get()`
  Waits if necessary for the computation to complete, and then retrieves its result.
  `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.
  `boolean`
  `isCancelled()`
  Returns `true` if this task was cancelled before it completed normally.
  `boolean`
  `isDone()`
  Returns `true` if this task completed.
  `void`
  `run()`
  Sets this Future to the result of its computation unless it has been cancelled.
  `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.
  `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.
  `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.
  `String`
  `toString()`
  Returns a string representation of this FutureTask.

  ### Methods declared in class java.lang.Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Constructor Details

  - ### FutureTask

    public FutureTask(Callable<V> callable)
    Creates a `FutureTask` that will, upon running, execute the given `Callable`.
    Parameters:
      `callable` - the callable task
    Throws:
      `NullPointerException` - if the callable is null

  - ### FutureTask

    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` - the runnable task
      `result` - 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

- ## Method Details

  - ### isCancelled

    public boolean isCancelled()
    Description copied from interface: `Future`
    Returns `true` if this task was cancelled before it completed normally.
    Specified by:
      `isCancelled` in interface `Future<V>`
    Returns:
      `true` if this task was cancelled before it completed

  - ### isDone

    public boolean isDone()
    Description copied from interface: `Future`
    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`.
    Specified by:
      `isDone` in interface `Future<V>`
    Returns:
      `true` if this task completed

  - ### cancel

    public boolean cancel(boolean mayInterruptIfRunning)
    Description copied from interface: `Future`
    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 `Future.isCancelled()`.

    Specified by:
      `cancel` in interface `Future<V>`
    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.

  - ### get

    public V get() throws InterruptedException, ExecutionException
    Description copied from interface: `Future`
    Waits if necessary for the computation to complete, and then retrieves its result.
    Specified by:
      `get` in interface `Future<V>`
    Returns:
      the computed result
    Throws:
      `CancellationException` - if the computation was cancelled
      `InterruptedException` - if the current thread was interrupted while waiting
      `ExecutionException` - if the computation threw an exception

  - ### get

    public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
    Description copied from interface: `Future`
    Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
    Specified by:
      `get` in interface `Future<V>`
    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
      `InterruptedException` - if the current thread was interrupted while waiting
      `ExecutionException` - if the computation threw an exception
      `TimeoutException` - if the wait timed out

  - ### done

    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.

  - ### set

    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` - the value

  - ### setException

    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` - the cause of failure

  - ### run

    public void run()
    Description copied from interface: `RunnableFuture`
    Sets this Future to the result of its computation unless it has been cancelled.
    Specified by:
      `run` in interface `Runnable`
    Specified by:
      `run` in interface `RunnableFuture<V>`
    See Also:
      - `Thread.run()`

  - ### runAndReset

    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:
      `true` if successfully run and reset

  - ### toString

    public String toString()
    Returns a string representation of this FutureTask.
    Overrides:
      `toString` in class `Object`
    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:
      a string representation of this FutureTask