I have a question about java polymorphism

Question

Why is the final output "WuffRingding" instead of "RingdingRingding"?

package ubung;


 class Hund{
    public Hund(){
    }

    public String bellen(){
        return "Wuff";
    }

    public String spielen(Hund h){
        return "Wuff" + h.bellen();
    }


}

class Fuchs extends Hund{

    public Fuchs(){

    }

    public String bellen(){
        return "Ringding";
    }

    public String spielen(Fuchs f){
        return "Ringding"+ f.bellen();
    }

}



public class park {

    public static void main(String[] args){
        Hund bello = new Hund();
        Fuchs foxi = new Fuchs();
        Hund hybrid = new Fuchs();

        System.out.println(hybrid.spielen(foxi));


    }
}

Why is the final output "WuffRingding" instead of "RingdingRingding"? Why is the final output "WuffRingding" instead of "RingdingRingding"?

Answer 1

The output is WuffRingding since the method public String spielen(Hund h) is overloaded in the child class Fuchs to public String spielen(Fuchs f). Java calls the method of the type and not of the actual instance when a method is overloaded.

So the type used here is Hund for the hybrid variable and since the method to be called is decided at compile time for overloaded methods the overloaded method of the Hund class is invoked althogh it is pointing to the Fuchs instance.

Now you supplied a instance of Fuchs as a parameter to the spielen(Hund h) which clearly takes the type Hund as a parameter, this worked as Hund is a parent of Fuchs and can hold Fuchs reference, this combined with runtime/dynamic polymorphism the bellen method of the Fuchs class is called instead of Hund as at runtime the type of the instance is inspected and it was actually pointing to an instance of Fuchs.

TLDR;

• Invoked spielen on a Hund reference, calls spielen of Hund class as it is overloaded.
• Passed an instance of Fuchs so at runtime the method bellen of Fuchs is called instead of Hund due to dynamic method dispatch / runtime polymorphism.