问题，中间线程还没走完，最后的线程已经完成

public class Problem {
    public static void main(String[] args){
        //问题，中间线程还没走完，最后的线程已经完成
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + "同学离开");
            }, String.valueOf(i)).start();
        }
        System.out.println(Thread.currentThread().getName() + "班长锁门");
    }
}

解决方案，用 CountDownLatch 减少计数

public class Solution {
    public static void main(String[] args) throws InterruptedException {
        //创建CountDownLatch，设置初始值
        CountDownLatch countDownLatch = new CountDownLatch(10);
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + "同学离开");
                //计数减一
                countDownLatch.countDown();
            }, String.valueOf(i)).start();
        }
        //等待
        countDownLatch.await();
        System.out.println(Thread.currentThread().getName() + "班长锁门");
    }
}