RecursiveTask (Java SE 17 & JDK 17)
Description: declaration: module: java.base, package: java.util.concurrent, class: RecursiveTask
Keywords: java.util.concurrent.RecursiveTask class

Module java.base
Package java.util.concurrent

# Class RecursiveTask<V>

java.lang.Object
java.util.concurrent.ForkJoinTask<V>
java.util.concurrent.RecursiveTask<V>

All Implemented Interfaces:
  `Serializable`, `Future<V>`

public abstract class RecursiveTask<V> extends ForkJoinTask<V>
A recursive result-bearing `ForkJoinTask`.

For a classic example, here is a task computing Fibonacci numbers:

     class Fibonacci extends RecursiveTask<Integer> {
       final int n;
       Fibonacci(int n) { this.n = n; }
       protected Integer compute() {
         if (n <= 1)
           return n;
         Fibonacci f1 = new Fibonacci(n - 1);
         f1.fork();
         Fibonacci f2 = new Fibonacci(n - 2);
         return f2.compute() + f1.join();
       }
     }

However, besides being a dumb way to compute Fibonacci functions (there is a simple fast linear algorithm that you'd use in practice), this is likely to perform poorly because the smallest subtasks are too small to be worthwhile splitting up. Instead, as is the case for nearly all fork/join applications, you'd pick some minimum granularity size (for example 10 here) for which you always sequentially solve rather than subdividing.

Since:
  1.7
See Also:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `RecursiveTask()`
  Constructor for subclasses to call.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected abstract V`
  `compute()`
  The main computation performed by this task.
  `protected final boolean`
  `exec()`
  Implements execution conventions for RecursiveTask.
  `final V`
  `getRawResult()`
  Returns the result that would be returned by `ForkJoinTask.join()`, even if this task completed abnormally, or `null` if this task is not known to have been completed.
  `protected final void`
  `setRawResult(V value)`
  Forces the given value to be returned as a result.

  ### Methods declared in class java.util.concurrent.ForkJoinTask

  `adapt, adapt, adapt, cancel, compareAndSetForkJoinTaskTag, complete, completeExceptionally, fork, get, get, getException, getForkJoinTaskTag, getPool, getQueuedTaskCount, getSurplusQueuedTaskCount, helpQuiesce, inForkJoinPool, invoke, invokeAll, invokeAll, invokeAll, isCancelled, isCompletedAbnormally, isCompletedNormally, isDone, join, peekNextLocalTask, pollNextLocalTask, pollSubmission, pollTask, quietlyComplete, quietlyInvoke, quietlyJoin, reinitialize, setForkJoinTaskTag, tryUnfork`

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

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

- ## Constructor Details

  - ### RecursiveTask

    public RecursiveTask()
    Constructor for subclasses to call.

- ## Method Details

  - ### compute

    protected abstract V compute()
    The main computation performed by this task.
    Returns:
      the result of the computation

  - ### getRawResult

    public final V getRawResult()
    Description copied from class: `ForkJoinTask`
    Returns the result that would be returned by `ForkJoinTask.join()`, even if this task completed abnormally, or `null` if this task is not known to have been completed. This method is designed to aid debugging, as well as to support extensions. Its use in any other context is discouraged.
    Specified by:
      `getRawResult` in class `ForkJoinTask<V>`
    Returns:
      the result, or `null` if not completed

  - ### setRawResult

    protected final void setRawResult(V value)
    Description copied from class: `ForkJoinTask`
    Forces the given value to be returned as a result. This method is designed to support extensions, and should not in general be called otherwise.
    Specified by:
      `setRawResult` in class `ForkJoinTask<V>`
    Parameters:
      `value` - the value

  - ### exec

    protected final boolean exec()
    Implements execution conventions for RecursiveTask.
    Specified by:
      `exec` in class `ForkJoinTask<V>`
    Returns:
      `true` if this task is known to have completed normally