Error when using new ForkJoin feature

[Tim McGuire]

Hi, I was experimenting with the new Fork/Join "task stealing" feature described at http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/RecursiveAction.html

Here is my implementation of the IncremementTask from that page:

import java.util.concurrent.RecursiveAction;

class IncrementTask extends RecursiveAction {
	public static void main(String[] args) {
		long[] xArray = new long[100_000];
		for(int x=0; x < 100_000; x++){
			xArray[x] = x;
		}
		int lo = 0;
		int hi = 99_999;
		IncrementTask incrementTask = new IncrementTask(xArray,lo,hi);
		incrementTask.compute();
	}

   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() {
	   int THRESHOLD = 50_000;
     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));
     }
   }
 }

This code generates the following error and I don't understand why:

Exception in thread "main" java.lang.ClassCastException: java.lang.Thread cannot be cast to java.util.concurrent.ForkJoinWorkerThread
        at java.util.concurrent.ForkJoinTask.fork(ForkJoinTask.java:622)
        at java.util.concurrent.ForkJoinTask.invokeAll(ForkJoinTask.java:684)
        at IncrementTask.compute(IncrementTask.java:27)
        at IncrementTask.main(IncrementTask.java:12)

beyond that, how can a programmer begin to estimate the thresholds that would be reasonable for breaking off tasks in this way?

[Jesper de Jong]

You're creating a RecursiveAction but it looks like you're not really using the fork-join framework.

You have to create a ForkJoinPool and let it execute your tasks, in line 12 of main(), instead of calling compute() directly.

Here's an example.

[Tim McGuire]

Thank you . I changed my main method to look like this and it works:

	public static void main(String[] args) {
		long[] xArray = new long[100_000];
		for(int x=0; x < 100_000; x++){
			xArray[x] = x;
		}
		int lo = 0;
		int hi = 99_999;
		System.out.println(xArray[0]);
		 ForkJoinPool pool = new ForkJoinPool();
        pool.invoke(new IncrementTask(xArray,lo, hi));
		//IncrementTask incrementTask = new IncrementTask(xArray,lo,hi);
		//incrementTask.compute();
	}