有大佬可以告诉我下面代码为啥debug跟run结果不一致吗。

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Test test = new Test();
        Thread thread1 = new Thread(() -> {
            try {
                System.out.println(test.change());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Thread thread2 = new Thread(() -> {
            try {
                System.out.println(test.change());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        thread1.start();
        TimeUnit.MILLISECONDS.sleep(8);
        thread2.start();
    }
}

public class Test {
    private volatile int a = 0;
    public int change() throws InterruptedException {
        while (a == 0){
            TimeUnit.MILLISECONDS.sleep(10);
            a++;

        }
        return a;
    }

}