ForkJoinPool

public class TestForkJoin {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        int[] array = new int[10000];
        IntStream.range(0, 10000)
                .forEach(i -> array[i] = i);
        System.out.println(Arrays.stream(array).sum());
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        ForkJoinTask<Long> submit = forkJoinPool.submit(new LongSum(0, array.length, array));
        System.out.println(submit.get());
    }
}

public class LongSum extends RecursiveTask<Long> {
    static final int SEQUENTIAL_THRESHOLD = 10000;
    int low;
    int high;
    int[] array;
    public LongSum(int low, int high, int[] array) {
        this.low = low;
        this.high = high;
        this.array = array;
    }
    @Override
    protected Long compute() {
        if (high - low <= SEQUENTIAL_THRESHOLD) {
            long sum = 0;
            for (int i = low; i < high; i++) {
                sum += array[i];
            }
            return sum;
        } else {
            int mid = low + (high - low) / 2;
            LongSum left = new LongSum(low, mid, array);
            LongSum right = new LongSum(mid, high, array);
            left.fork();
            right.fork();
            Long leftAns = left.join();
            Long rightAns = right.join();
            return leftAns + rightAns;
        }
    }
}