我想使用并行流api来读取redis，获取所有项目，并随机抽取10%的项目。但我发现它有时会 random.nextInt(itemCount) 不接受0作为输入。这使我很困惑。
是多线程问题吗？我不知道该怎么办。
代码如下：

private Timer timer = new Timer(); // I start the job in a timer.
private Random rand = new Random();
private List<String> ids = getIds(); // get redis keys.
private ForkJoinPool updateThreadsPool = new ForkJoinPool(32);
timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {

                ForkJoinTask<List<List<Item>>> asyncTask = updateThreadsPool.submit(() -> {
                      return ids.parallelStream().map(id -> getFromRedis(id)).collect(Collectors.toList());
                });

                int totalNum = 0;
                List<Item> totalItems = new ArrayList<>();
                try {
                    totalNum = asyncTask.get().stream().peek(totalItems::addAll)
                         .reduce(0, (num, list) -> num + list.size(), Integer::sum);
                } catch (Exception e) { // do something }

                int randIndex = rand.nextInt(totalNum); // this is where it throws exception.
                // says that totalNum is zero. 
                // I'm pretty sure totalItems is not empty, because I print log for that.
                // I tried use int totalNum = totalItems.size(), but I got the same problem.
            }
        }, 1000L, 1000L
);