public class AtomicIntegerFileldUpdaterDemo implements Runnable {
static Candidate tom;
static Candidate peter;
public static AtomicIntegerFieldUpdater scoreUpdater = AtomicIntegerFieldUpdater.newUpdater(Candidate.class, “score”);

@Override
public void run() {
    for (int i = 0; i < 10000; i++) {
        tom.score++;
        scoreUpdater.getAndIncrement(peter);
    }
}

@SneakyThrows
public static void main(String[] args) {
    AtomicIntegerFileldUpdaterDemo r = new AtomicIntegerFileldUpdaterDemo();
    tom = new Candidate();
    peter = new Candidate();
    /*Thread thread = new Thread(r);
    Thread thread2 = new Thread(r);
    thread.start();
    thread2.start();
    thread.join();
    thread2.join();*/

    /*使用for循环每次都是打印20000*/
    for (int i = 0; i < 2; i++) {
        Thread s=new Thread(r);
        s.start();
        s.join();
    }
    System.out.println("普通变量:"+tom.score);
    System.out.println("升级后的变量:"+peter.score);
}
static class Candidate {
    volatile int score;
}

}
打印：
普通变量:20000
升级后的变量:20000

老师这里在for循环中执行s.start,s.join,多次打印出来的tom和peter的score都是20000。
把两个线程分别start和join就没有问题，是什么原因呢？