Java多线程runnable

class wantSum{
    private int sum; 
    public void add(int n){
        sum+=n;       
        System.out.println("sum="+sum);        
     }
}   
class Cus implements Runnable{
    private wantSum a=new wantSum();   
    public void run(){
        synchronized(a){
            for(int x=0;x<3;x++)    
           {
              a.add(100); 
            }     
        }    
    }     
 }  
public class wantSumDemo{
    public static void main(String []args){
        Cus b=new Cus();     
        Thread t1=new Thread(b);      
        Thread t2=new Thread(b);     
        t1.start();    
        t2.start();    
      }        
}