Overloading inside interface in Java -

i need implements various update methods observers in project, overload object argument specific type. instead of using

public void update (observable o, object arg0){    if(arg0 instanceof class1) ....    else if (arg0 instanceof class2) .... 

or switch (which 'c' way of working), thought of using interface extends observer , overloads update method, this

public interface extendedobserver extends observer{    public void update (observable obj, class1 cls);    public void update (observable obj, class2 cls); } 

and implement observers. every different observer use specific update method.

i wanted know if it's correct way of working, because

1. i don't think overloading method inside interface correct
2. i still have add unused update methods inside every class implements interface.

• there no problem in overloading method inside interface
• an interface contract mandatory implementing classes implement. if not want classes implement method. there no need include in interface well.
• so can write overloaded method in classes needed. or make abstract class implement interface , write overloaded method in it.
• this way not end of exposing unnecessary method contract other classes