public class JoinTest {     public static void main(String[] args) {    // TODO Auto-generated method stub    Thread t1 = new ThreadA();//利用多态实例化一个A线程    Thread t2 = new ThreadB(t1);//利用多态实例化一个B线程    t1.start();    t2.start();   }    }    /*继承于Thread的子类*/  class ThreadA extends Thread {   public void run() {    for (int i = 1; i <= 20; i++) {     System.out.println(i + "  ￥￥￥");     try {      Thread.sleep(1000);     } catch (InterruptedException e) {     }    }   }  }    /*继承于Thread的子类*/  class ThreadB extends Thread {   private Thread t;     public ThreadB() {   }     public ThreadB(Thread t) {    this.t = t;   }     public void run() {    for (int i = 1; i <= 20; i++) {     System.out.println(i + "  ###");     try {      Thread.sleep(1000);      if (i == 10) {       t.join();// join 使这两个线程进入串行状态（也就是说t突然插了进来，必须等t执行完毕之后，才会让B线程继续执行）      }     } catch (InterruptedException e) {     }    }   }  }