主要用于并行计算中，和 MapReduce 原理类似，都是把大的计算任务拆分成多个小任务并行计算。

public class ForkJoinExample extends RecursiveTask<Integer> {
    private final int threhold = 5;
    private int first;
    private int last;

    public ForkJoinExample(int first, int last) {
        this.first = first;
        this.last = last;
    }

    @Override
    protected Integer compute() {
        int result = 0;
        if (last - first <= threhold) {
            // 任务足够小则直接计算
            for (int i = first; i <= last; i++) {
                result += i;
            }
        } else {
            // 拆分成小任务
            int middle = first + (last - first) / 2;
            ForkJoinExample leftTask = new ForkJoinExample(first, middle);
            ForkJoinExample rightTask = new ForkJoinExample(middle + 1, last);
            leftTask.fork();
            rightTask.fork();
            result = leftTask.join() + rightTask.join();
        }
        return result;
    }
}

public static void main(String[] args) throws ExecutionException, InterruptedException {
    ForkJoinExample example = new ForkJoinExample(1, 10000);
    ForkJoinPool forkJoinPool = new ForkJoinPool();
    Future result = forkJoinPool.submit(example);
    System.out.println(result.get());
}