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 example, here is a task-based program for computing Factorials:

 
 import java.util.concurrent.RecursiveTask;
 import java.math.BigInteger;
 public class Factorial {
   static class FactorialTask extends RecursiveTask<BigInteger> {
     private final int from, to;
     FactorialTask(int from, int to) { this.from = from; this.to = to; }
     protected BigInteger compute() {
       int range = to - from;
       if (range == 0) {                       // base case
         return BigInteger.valueOf(from);
       } else if (range == 1) {                // too small to parallelize
         return BigInteger.valueOf(from).multiply(BigInteger.valueOf(to));
       } else {                                // split in half
         int mid = from + range / 2;
         FactorialTask leftTask = new FactorialTask(from, mid);
         leftTask.fork();         // perform about half the work locally
         return new FactorialTask(mid + 1, to).compute()
                .multiply(leftTask.join());
       }
     }
   }
   static BigInteger factorial(int n) { // uses ForkJoinPool.commonPool()
     return (n <= 1) ? BigInteger.ONE : new FactorialTask(1, n).invoke();
   }
   public static void main(String[] args) {
     System.out.println(factorial(Integer.parseInt(args[0])));
   }
 }

Since:

1.7

Author:

Doug Lea

See Also:

• Serialized Form

Nested Class Summary

Nested classes/interfaces declared in interface java.util.concurrent.Future

Future.State

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, adaptInterruptible, 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, quietlyJoin, quietlyJoinUninterruptibly, reinitialize, setForkJoinTaskTag, tryUnfork

Methods declared in class java.lang.Object

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

Methods declared in interface java.util.concurrent.Future

exceptionNow, resultNow, state

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