Question

To conceptually understand inheritance, interfaces & polymorphism, I created a couple of classes to test things. The result I am getting is not the one I expected. The code is as follows:

public class Animal implements Comparable{
//Animal constructor & methods
public int compareTo(Object arg0){System.out.println("Animal.compareTo");}
}

public class Bear extends Animal{
//Bear constructor & methods
}

public class PolarBear extends Bear implements Comparable{
//PolarBear constructor & methods
public int compareTo(Object arg0) {System.out.println("PolarBear.compareTo");

I don’t get any error saying that I can’t implement Comparable again. But in my testing, I can’t get back to Animal’s compareTo method once I create a PolarBear, even though PolarBear should inherit the Animal’s compareTo. I tested the following:

Animal bobo = new PolarBear();
bobo.compareTo(null);
((PolarBear) bobo).compareTo(null);
((Animal)bobo).compareTo(null);
((Comparable) bobo).compareTo(null);    

And each of them printed:

 PolarBear.compareTo

Is there any way to access the Animal‘s compareTo? Logically, wouldn’t I want the ability to be able to compare the animal qualities of a PolarBear, as well as the PolarBear qualities?

Answer 1

It doesn’t matter that you are casting your bobo object to PolarBear, Animal, or Comparable, the run-time type of the object will always be PolarBear since you instantiated it as

Animal bobo = new PolarBear();

Therefore whenever you call .compareTo(wtv) it’ll call the PolarBear implementation.

This is what Polymorphism is. Read up on late binding as that’s how polymorphishm is implemented in java.

If you want to call the compareTo(wtv) method of your parent classes, then you have to do super.compareTo(wtv) inside any other subclass’ methods.