Polymorphism on simple example

Question

I think I'm starting to understand this topic, but not completely. Can somebody explain to me on this example:

public class Solution
{
    public static void main(String[] args)
    {
        Cow cow = new Whale();

        System.out.println(cow.getName());
    }

    public static class Cow
    {
        public String getName()
        {
            return "Im cow";
        }
    }

    public static class Whale extends Cow
    {
        public String getName() {
            return "Im whale";
        }
    }
}

what is the difference when is called this:

Cow cow = new Whale();
System.out.println(cow.getName());

and this:

Whale whale = new Whale();

System.out.println(whale.getName());

I will have the same output, but in what cases or maybe when we should call the methods for the Cow class, and when form Whale class. Sorry if I gave too stupid or too simple example. I hope you undeerstood what I wanted to say. Thanks in advance.

Answer 1

It's good question to understand Polymorphism. I am not sure that Whale should extends Cow ;), but I can show you a bit of different structure:

public class Solution {
    public static void main(String[] args) {
        Animal animal1 = new Cow();
        Animal animal2 = new Whale();

        ArrayList<Animal> myAnimals = new ArrayList<Animal>();
        myAnimals.add(animal1);
        myAnimals.add(animal2);
        //show all my animals
        for (Animal animal : arrayList) {
            System.out.println(animal.getName());

        }


    }


    public static class Animal {
        public String getName() {
            return "Im general animal";
        }
    }

    public static class Cow extends Animal {
        public String getName() {
            return "Im cow";
        }
    }

    public static class Whale extends Animal {
        public String getName() {
            return "Im whale";
        }
    }
}

In above example I created Animal class which is extended by Cow and Whale. I can create list of my animals and display they names. In your example is no difference to make a object for Cow cow = new Whale(); and Whale whale = new Whale(); Is it clear for you?

Answer 2

When using Cow cow = new Whale(), the object is created like a Whale but only has access to Cow methods.

For example, if you add to your Whale class the method:

public int getDeepestDive() {
  // some code
}

This code will work:

Whale whale = new Whale();
System.out.println(whale.getDeepestDive());

This code will tell you that Cow doesn't have a method called getDeepestDive():

Cow whale = new Whale();
System.out.println(whale.getDeepestDive());