ForkJoin ​

import java.util.Random;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Create an array of 2000 random numbers:
        long[] array = new long[2000];
        long expectedSum = 0;
        for (int i = 0; i < array.length; i++) {
            array[i] = random();
            expectedSum += array[i];
        }
        System.out.println("Expected sum: " + expectedSum);
        // Fork/Join:
        ForkJoinTask<Long> task = new SumTask(array, 0, array.length);
        long startTime = System.currentTimeMillis();
        Long result = ForkJoinPool.commonPool().invoke(task);
        long endTime = System.currentTimeMillis();
        System.out.println("Fork/join sum: " + result + " in " + (endTime - startTime) + " ms.");
    }

    static Random random = new Random(0);

    static long random() {
        return random.nextInt(10000);
    }
}

class SumTask extends RecursiveTask<Long> {
    static final int THRESHOLD = 500;
    long[] array;
    int start;
    int end;

    SumTask(long[] array, int start, int end) {
        this.array = array;
        this.start = start;
        this.end = end;
    }

    @Override
    protected Long compute() {
        if (end - start <= THRESHOLD) {
            // If the task is small enough, compute directly:
            long sum = 0;
            for (int i = start; i < end; i++) {
                sum += this.array[i];
                // Intentionally slow down the computation:
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                }
            }
            return sum;
        }
        // Task is too large, split it into two:
        int middle = (end + start) / 2;
        System.out.println(String.format("split %d~%d ==> %d~%d, %d~%d", start, end, start, middle, middle, end));
        SumTask subtask1 = new SumTask(this.array, start, middle);
        SumTask subtask2 = new SumTask(this.array, middle, end);
        invokeAll(subtask1, subtask2);
        Long subresult1 = subtask1.join();
        Long subresult2 = subtask2.join();
        Long result = subresult1 + subresult2;
        System.out.println("result = " + subresult1 + " + " + subresult2 + " ==> " + result);
        return result;
    }
}

class SumTask extends RecursiveTask<Long> {
    protected Long compute() {
        // "Split" sub-tasks:
        SumTask subtask1 = new SumTask(...);
        SumTask subtask2 = new SumTask(...);
        // invokeAll runs the two sub-tasks in parallel:
        invokeAll(subtask1, subtask2);
        // Get the results of the sub-tasks:
        Long subresult1 = subtask1.join();
        Long subresult2 = subtask2.join();
        // Combine the results:
        return subresult1 + subresult2;
    }
}