Solve a computing task with the ForkJoin framework.

MyForkJoinPool.java

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;

/**
 * Solve a computing task with the ForkJoin framework.
 */
public class MyForkJoinPool {

    private static final int NTHREADS = 10;

    public static void main(String[] args) {
        ForkJoinPool pool = new ForkJoinPool(NTHREADS);
        Long sum = pool.invoke(new SumTask(0l, 1000000l));
        System.out.printf("Final sum: %d%n", sum);
    }
}

class SumTask extends RecursiveTask<Long> {

    private static final int THRESHOLD = 10;
    private Long from;
    private Long to;

    public SumTask(Long from, Long to) {
        this.from = from;
        this.to = to;
    }

    @Override
    public Long compute() {
        Long sum = 0l;
        if ((to-from) <= THRESHOLD) {
            System.out.printf("%s computing directly: %d-%d%n", getName(), from, to);
            for (long i = from; i <= to; i++) {
                sum += i;
            }
        } else {
            long mid = (from+to)/2;
            // fork
            System.out.printf("%s forking lower half: %d-%d%n", getName(), from, mid);
            SumTask lower = new SumTask(from, mid);
            lower.fork();
            // compute
            System.out.printf("%s computing higher half: %d-%d%n", getName(), mid+1, to);
            SumTask higher = new SumTask(mid+1, to);
            Long higherRes = higher.compute();
            // join
            sum = lower.join() + higherRes;
        }
        System.out.printf("%s returning sum: %d%n", getName(), sum);
        return sum;
    }

    private String getName() {
        return Thread.currentThread().getName();
    }
}