在线程不安全类与写法这一节测试ArrayList时遇到问题，代码如下，和老师的一样，没什么区别：
public class ArrayListExample {
//请求总数
public static int clientTotal = 5000;
//同时并发执行的线程数
public static int threadLocal = 200;

public static List<Integer> list = new ArrayList<>();

public static void main(String[] args) throws InterruptedException{
    //定义线程池
    ExecutorService executorService = Executors.newCachedThreadPool();
    //定义信号量,允许并发的数目为200
    final Semaphore semaphore = new Semaphore(threadLocal);
    //定义CountDownLatch
    final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);

    for (int i = 0; i < clientTotal; i++) {
        final int count = i;
        //将所有请求放入到线程池中执行
        executorService.execute(() ->{
            try {
                semaphore.acquire();
                update(count);
                semaphore.release();
            } catch (InterruptedException e) {
                log.error("InterruptedException",e);
            }
            //线程每执行完一次add操作时，就调用countDownLatch
            countDownLatch.countDown();
        });
    }
    //保证countDownLatch的值为0，即所有的线程都执行完毕
    countDownLatch.await();
    //关闭线程池
    executorService.shutdown();
    log.info("list size:{}",list.size());
}

private static void update(int i){
    list.add(i);
}

}

但是这段代码在运行的时候却偶尔会抛出异常：

当然，有时候也会运行出正确的结果来，想请教一下老师这里为什么数组下标会越界，ArrayList在扩容时还有什么讲究吗？