[20160804]synchronized

 class Timer{
    private static int num;

    void add(String name){
      //synchronized (this){
        num++;
        try{  Thread.sleep(1);  }
        catch(InterruptedException e){};
        System.out.println(name+"  you are the  "+num+"  user to use timer!");
      //}
    }
 }

 public class TestSync implements Runnable{
     Timer timer = new Timer();

     public static void main(String[] args) {
        TestSync t = new TestSync();

        Thread t1 = new Thread(t,"t1");
        Thread t2 = new Thread(t,"t2");

        t1.start();
        t2.start();
     }

     public void run(){
        timer.add(Thread.currentThread().getName());
     }
 }