CompletionService 按任务执行结果的顺序封装

将已经完成的任务的结果 与 新的异步任务的生成 进行解耦。
最先执行完成的任务的结果放到最前面，可以对先出结果的任务的结果进行处理

    public static void main(String[] args) throws Exception {
        //创建一个线程池
        ExecutorService executorService = new ThreadPoolExecutor(4, 10, 10, TimeUnit.SECONDS,
                new LinkedBlockingQueue<>(20), new ThreadPoolExecutor.AbortPolicy());
        //创建一个CompletionService对象，通过该对象来提交任务，任务的结果依次会放到一个阻塞队列中
        CompletionService<Integer> completionService = new ExecutorCompletionService<>(executorService);
        IntStream.range(0, 10).forEach(i -> {
            completionService.submit(() -> {
                Thread.sleep((long) (Math.random() * 1000));
                System.out.println(Thread.currentThread().getName());
                return i * i;
            });
        });
        for (int i = 0; i < 10; ++i) {
            int result = completionService.take().get();
            System.out.println(result);
        }
        executorService.shutdown();
    }