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.

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.

Nested and Inner Type Summary

Modifier and Type	Class and Description
public static enum	Future.State Represents the computation state.

Method Summary

Modifier and Type	Method and Description
public boolean	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. cancel(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 mayInterruptIfRunning) Attempts to cancel execution of this task.
public default Throwable	Returns: the exception thrown by the task exceptionNow() Returns the exception thrown by the task, without waiting.
public V	Returns: the computed result get() Waits if necessary for the computation to complete, and then retrieves its result.
public V	Returns: the computed result get(long the maximum time to wait timeout, TimeUnit the time unit of the timeout argument unit) Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
public boolean	Returns: `true` if this task was cancelled before it completed isCancelled() Returns `true` if this task was cancelled before it completed normally.
public boolean	Returns: `true` if this task completed isDone() Returns `true` if this task completed.
public default V	Returns: the computed result resultNow() Returns the computed result, without waiting.
public default Future.State	Returns: the computation state state() Returns the computation state.

Method Detail

cancel back to summary

cancel	back to summary
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.

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 back to summary

exceptionNow	back to summary
public default 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. Implementation Specification 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:Throwable the exception thrown by the task Exceptions IllegalStateException: if the task has not completed, the task completed normally, or the task was cancelled Since 19

public default 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.

Implementation Specification

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:Throwable

the exception thrown by the task

Exceptions

IllegalStateException:: if the task has not completed, the task completed normally, or the task was cancelled

Since

get	back to summary
public V get() throws InterruptedException, ExecutionException Waits if necessary for the computation to complete, and then retrieves its result. Returns:V the computed result Exceptions InterruptedException: if the current thread was interrupted while waiting ExecutionException: if the computation threw an exception CancellationException: if the computation was cancelled

get

back to summary

public V get() throws InterruptedException, ExecutionException

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

Returns:V

the computed result

Exceptions

InterruptedException:: if the current thread was interrupted while waiting
ExecutionException:: if the computation threw an exception
CancellationException:: if the computation was cancelled

get	back to summary
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException 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 Exceptions InterruptedException: if the current thread was interrupted while waiting ExecutionException: if the computation threw an exception TimeoutException: if the wait timed out CancellationException: if the computation was cancelled

get

back to summary

public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException

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

Exceptions

InterruptedException:: if the current thread was interrupted while waiting
ExecutionException:: if the computation threw an exception
TimeoutException:: if the wait timed out
CancellationException:: if the computation was cancelled

isCancelled back to summary

isCancelled	back to summary
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

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 back to summary

isDone	back to summary
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

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 back to summary

resultNow	back to summary
public default 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. `results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList();` results = futures.stream() .filter(f -> f.state() == Future.State.SUCCESS) .map(Future::resultNow) .toList(); Implementation Specification The default implementation invokes `isDone()` to test if the task has completed. If done, it invokes `get()` to obtain the result. Returns:V the computed result Exceptions IllegalStateException: if the task has not completed or the task did not complete with a result Since 19

public default 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.

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

Implementation Specification

The default implementation invokes isDone() to test if the task has completed. If done, it invokes get() to obtain the result.

Returns:V

the computed result

Exceptions

IllegalStateException:: if the task has not completed or the task did not complete with a result

Since

state back to summary

state	back to summary
public default Future.State state() Returns the computation state. Implementation Specification The default implementation uses `isDone()`, `isCancelled()`, and `get()` to determine the state. Returns:Future.State the computation state Since 19

public default Future.State state()

Returns the computation state.

Implementation Specification

The default implementation uses isDone(), isCancelled(), and get() to determine the state.

Returns:Future.State: the computation state
Since: 19

java.util.concurrent back to summary

public final Enum Future.State

Class Inheritance

extends Enum<Future.State>

java.lang.Object
java.lang.Enum
java.util.concurrent.Future.State

Represents the computation state.

Since: 19

Field Summary

Modifier and Type	Field and Description
public static final Future.State	CANCELLED The task was cancelled.
public static final Future.State	FAILED The task completed with an exception.
public static final Future.State	RUNNING The task has not completed.
public static final Future.State	SUCCESS The task completed with a result.

Constructor Summary

Access	Constructor and Description
private	State()

Method Summary

Modifier and Type	Method and Description
public static Future.State	valueOf(String name)
public static Future.State[]	values()

Inherited from java.lang.Enum:: clone compareTo describeConstable equals finalize getDeclaringClass hashCode name ordinal toString valueOf

Field Detail

CANCELLED back to summary

CANCELLED	back to summary
public static final Future.State CANCELLED The task was cancelled. See Also `cancel(boolean)`

public static final Future.State CANCELLED

The task was cancelled.

See Also: cancel(boolean)

FAILED back to summary

FAILED	back to summary
public static final Future.State FAILED The task completed with an exception. See Also `Future#exceptionNow()`

public static final Future.State FAILED

The task completed with an exception.

See Also: Future#exceptionNow()

RUNNING	back to summary
public static final Future.State RUNNING The task has not completed.

SUCCESS back to summary

SUCCESS	back to summary
public static final Future.State SUCCESS The task completed with a result. See Also `Future#resultNow()`

public static final Future.State SUCCESS

The task completed with a result.

See Also: Future#resultNow()

Constructor Detail

State	back to summary
private State()

Method Detail

valueOf	back to summary
public static Future.State valueOf(String name)

values	back to summary
public static Future.State[] values()

public Interface Future<V>

Nested and Inner Type Summary

Method Summary

Method Detail

public final Enum Future.State

Field Summary

Constructor Summary

Method Summary

Field Detail

Constructor Detail

Method Detail