这样就能正确编译（程序本身没有问题，关于复述的加减乘除）
public class Complex{
                       int rp,ip;
    public Complex(int rp,int ip){
                       this.rp=rp;
         this.ip=ip;
        }
    public Complex add(Complex a,Complex b){
        return new Complex(a.rp+b.rp,a.ip+b.ip);
    }
    public Complex sub(Complex a,Complex b){
        return new Complex(a.rp-b.rp,a.ip-b.ip);
    }
    public Complex mul(Complex a,Complex b){
                 Complex c=new Complex(a.rp*b.rp,a.rp*b.ip);
        Complex d=new Complex((-1)*(a.ip*b.ip),a.ip*b.rp);    
        return add(c,d);
    }
    public Complex div(Complex a,Complex b){
                 Complex c=mul(a,b);
        Complex d=mul(b,b);
        return new Complex(c.rp/d.rp,c.ip/d.rp);
    }
    public String toString(){
         return rp+"+"+ip+"i";
    }
    public static void main(String[] args)throws Exception {
          Com com=new Com();
        Complex complex=new Complex(3,4);
               Complex a=new Complex(3,4);
        Complex b=new Complex(5,6);
        System.out.println(complex.add(a,b));
         System.out.println(complex);
    }
}
*********************************************************************************
这样就不能编译
*********************************************************************************        
public class Complex{
                       int rp,ip;
    public Complex(int rp,int ip){
                       this.rp=rp;
         this.ip=ip;
        }
    public Complex add(Complex a,Complex b){
        return new Complex(a.rp+b.rp,a.ip+b.ip);
    }
    public Complex sub(Complex a,Complex b){
        return new Complex(a.rp-b.rp,a.ip-b.ip);
    }
    public Complex mul(Complex a,Complex b){
                 Complex c=new Complex(a.rp*b.rp,a.rp*b.ip);
        Complex d=new Complex((-1)*(a.ip*b.ip),a.ip*b.rp);    
        return add(c,d);
    }
    public Complex div(Complex a,Complex b){
                 Complex c=mul(a,b);
        Complex d=mul(b,b);
        return new Complex(c.rp/d.rp,c.ip/d.rp);
    }
    public String toString(){
         return rp+"+"+ip+"i";
    }
    


}        
************************************************
public class Com{
public static void main(String[] args)throws Exception {
          Com com=new Com();
        Complex complex=new Complex(3,4);
               Complex a=new Complex(3,4);
        Complex b=new Complex(5,6);
        System.out.println(complex.add(a,b));
         System.out.println(complex);
        }
}        
***********************************************    
这是 怎么回事啊 ？？？？