Module java.base
Package java.util.concurrent

# Class RecursiveAction

java.lang.Object
java.util.concurrent.ForkJoinTask<Void>
java.util.concurrent.RecursiveAction

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

public abstract class RecursiveAction extends ForkJoinTask<Void>
A recursive resultless `ForkJoinTask`. This class establishes conventions to parameterize resultless actions as `Void` `ForkJoinTask`s. Because `null` is the only valid value of type `Void`, methods such as `join` always return `null` upon completion.

Sample Usages. Here is a simple but complete ForkJoin sort that sorts a given `long[]` array:

     static class SortTask extends RecursiveAction {
       final long[] array; final int lo, hi;
       SortTask(long[] array, int lo, int hi) {
         this.array = array; this.lo = lo; this.hi = hi;
       }
       SortTask(long[] array) { this(array, 0, array.length); }
       protected void compute() {
         if (hi - lo < THRESHOLD)
           sortSequentially(lo, hi);
         else {
           int mid = (lo + hi) >>> 1;
           invokeAll(new SortTask(array, lo, mid),
                     new SortTask(array, mid, hi));
           merge(lo, mid, hi);
         }
       }
       // implementation details follow:
       static final int THRESHOLD = 1000;
       void sortSequentially(int lo, int hi) {
         Arrays.sort(array, lo, hi);
       }
       void merge(int lo, int mid, int hi) {
         long[] buf = Arrays.copyOfRange(array, lo, mid);
         for (int i = 0, j = lo, k = mid; i < buf.length; j++)
           array[j] = (k == hi || buf[i] < array[k]) ?
             buf[i++] : array[k++];
       }
     }

You could then sort `anArray` by creating `new SortTask(anArray)` and invoking it in a ForkJoinPool. As a more concrete simple example, the following task increments each element of an array:

     class IncrementTask extends RecursiveAction {
       final long[] array; final int lo, hi;
       IncrementTask(long[] array, int lo, int hi) {
         this.array = array; this.lo = lo; this.hi = hi;
       }
       protected void compute() {
         if (hi - lo < THRESHOLD) {
           for (int i = lo; i < hi; ++i)
             array[i]++;
         }
         else {
           int mid = (lo + hi) >>> 1;
           invokeAll(new IncrementTask(array, lo, mid),
                     new IncrementTask(array, mid, hi));
         }
       }
     }

The following example illustrates some refinements and idioms that may lead to better performance: RecursiveActions need not be fully recursive, so long as they maintain the basic divide-and-conquer approach. Here is a class that sums the squares of each element of a double array, by subdividing out only the right-hand-sides of repeated divisions by two, and keeping track of them with a chain of `next` references. It uses a dynamic threshold based on method `getSurplusQueuedTaskCount`, but counterbalances potential excess partitioning by directly performing leaf actions on unstolen tasks rather than further subdividing.

     double sumOfSquares(ForkJoinPool pool, double[] array) {
       int n = array.length;
       Applyer a = new Applyer(array, 0, n, null);
       pool.invoke(a);
       return a.result;
     }

     class Applyer extends RecursiveAction {
       final double[] array;
       final int lo, hi;
       double result;
       Applyer next; // keeps track of right-hand-side tasks
       Applyer(double[] array, int lo, int hi, Applyer next) {
         this.array = array; this.lo = lo; this.hi = hi;
         this.next = next;
       }

       double atLeaf(int l, int h) {
         double sum = 0;
         for (int i = l; i < h; ++i) // perform leftmost base step
           sum += array[i] * array[i];
         return sum;
       }

       protected void compute() {
         int l = lo;
         int h = hi;
         Applyer right = null;
         while (h - l > 1 && getSurplusQueuedTaskCount() <= 3) {
           int mid = (l + h) >>> 1;
           right = new Applyer(array, mid, h, right);
           right.fork();
           h = mid;
         }
         double sum = atLeaf(l, h);
         while (right != null) {
           if (right.tryUnfork()) // directly calculate if not stolen
             sum += right.atLeaf(right.lo, right.hi);
           else {
             right.join();
             sum += right.result;
           }
           right = right.next;
         }
         result = sum;
       }
     }

Since:
  1.7
See Also:
  - Serialized Form

- ## Constructor Summary

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

- ## Method Summary

  Modifier and Type
  Method
  Description
  `protected abstract void`
  `compute()`
  The main computation performed by this task.
  `protected final boolean`
  `exec()`
  Implements execution conventions for RecursiveActions.
  `final Void`
  `getRawResult()`
  Always returns `null`.
  `protected final void`
  `setRawResult(Void mustBeNull)`
  Requires null completion value.

  ### 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

  - ### RecursiveAction

    public RecursiveAction()
    Constructor for subclasses to call.

- ## Method Details

  - ### compute

    protected abstract void compute()
    The main computation performed by this task.

  - ### getRawResult

    public final Void getRawResult()
    Always returns `null`.
    Specified by:
      `getRawResult` in class `ForkJoinTask<Void>`
    Returns:
      `null` always

  - ### setRawResult

    protected final void setRawResult(Void mustBeNull)
    Requires null completion value.
    Specified by:
      `setRawResult` in class `ForkJoinTask<Void>`
    Parameters:
      `mustBeNull` - the value

  - ### exec

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