Question

A Person can either be a Pianist, which has a .play() method, either a Singer, which has a .sing() method, either both a Pianist and a Singer, in the latter case he would be able to .sing() and also to .play()

What would be a possible design for this situation?

Answer 1

Sounds like homework to me.

I would suggest you want to use encapsulation and capability objects. In this case, make the instruments the performer knows how to play be capabilities.

You would have something like this:

class Performer extends Person {
     List<Capability> capabilities;
}

// Perhaps call it 'Skills'
interface Capability {
     public void play();
}

class Pianist implements Capability {
     public void play() {
         // ....
     }
}

class Singer implements Capability {
     public void play() {
         sing();
     }

     public void sing() {
         // ....
     }
}