class Rational{
       int m;
       int n;
       public Ratinoal(int m,int n){this.m=m;this.n=n;}

       /* System.out.println(<Rational obj>) */
       public String toString() {
         return String.valueOf(m) + "/" + String.valueOf(n);;
       }
       public void add(Rational r) {}
       /* alternative
        * public Rational add(Rational r) { return new Rational(...);}
        */
}

public class Rational {
    public Rational add(Rational r) {
        ...
        return this;
    }
}

public final class Rational {
    public Rational add(Rational r) {
        ...
        return new Rational(...);
    }
}