public class Demo_Print {
  public static void main(String[] args) {
    Print p = new Print();
    new Thread() {
      public void run() {
        while (true) {
          p.print1();
        }
      };
    }.start();

    new Thread() {
      public void run() {
        while (true) {
          p.print2();
        }
      };
    }.start();
  }
}

class Print {
  int flag = 1;

  public synchronized void print1() {
    if (flag != 1) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("你");
    System.out.print("好");
    System.out.print("嗎????????????");
    System.out.println();

    flag = 2;
    this.notify();
  }

  public synchronized void print2() {
    if (flag != 2) {
      try {
        this.wait();
      } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }
    }
    System.out.print("我");
    System.out.print("好");
    System.out.println();

    flag = 1;
    this.notify();
  }
}