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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?

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  • 1
    You can always call super.compareTo() from the subclass. Commented Jan 9, 2013 at 16:43
  • 1
    The methods are dynamic bound to the instance. bobo is a PolarBear therefore always the compareTo(...) method of PolarBear is called. Why would you want something else to be called? Commented Jan 9, 2013 at 16:44
  • 2
    That should be implements Comparable<Animal> by the way Commented Jan 9, 2013 at 16:45

4 Answers 4

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7

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.

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3 Comments

The method needs to have super.compareTo? What if I want to have 2 compareTo methods, one in Animal, one in PolarBear that compare different things? Wouldn't putting super.compareTo in PolarBear.compareTo's method body lose that purpose?
It depends. Since you are overriding the method in PolarBear, you are, by calling super.compareTo, extending the functionality from the parent class. You don't have to do it. You won't be able to access the Animal's compareTo directly from a PolarBear instance.
@user1901074 - most places that deal with comparisons in the java libraries allow you to provide a Comparator class that can compare the objects instead on relying on the objects being comparable. this is normally how you'd handle scenarios where you have several comparison implementations.
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compareTo and inheritence is generally a tricky business. in theory, having subclasses to a Comparable concrete superclass breaks the comparable contract. for example - suppose i have a superclass A that compares to other A's by checking fields a1 and a1. now lets add a subclass B that has an extra field b1. anA.compareTo(aB) would compare by a1 and a2 while ((A)aB).compareTo(anA) would not do the same
as to your question - you can call the Animal version of compareTo only from the subclass itself, by doing something like

super.compareTo(<something>)

but not from "outside", as you've overridden the method. it is your responsibility to maintain the same "contract" to the outside world as the method you've just overridden.

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1

You can always call super.compareTo(arg0) from within your PolarBear to invoke the logic placed in Animal.compareTo.

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1 Comment

You can only use super inside an object. You are doing that in a main function (which is an static function and does not belong to any object). You have to put the super call inside your PolarBear's compareTo method
0

It's very simple. the underlying object is of PolarBear class and you are changing the interface (Class that is used to hold the reference of the object) of the object every time.

JVM check at compile time whether a method in the interface (Class which is used to hold the reference of the object) exist or not but binding takes place at run time because Child (PolearBear) has that method so it will always run that.

use super.compareTo() in order to call the parent method.

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