Class RecursiveAction

Sample Usages. Here is a sketch of a ForkJoin sort that sorts a given long[] array:

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

 class IncrementTask extends RecursiveAction {
   final long[] array; final int lo; final int 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;
   }
 }