Question

I wonder why it is allowed to have different type of object reference?
For example;

Animal cow = new Cow();

Can you please give an example where it is useful to use different type of object reference?

Edit:Cow extends Animal

Answer 1

This is at the heart of polymorphism and abstraction. For example, it means that I can write:

public void handleData(InputStream input) {
    ...
}

… and handle any kind of input stream, whether that’s from a file, network, in-memory etc. Or likewise, if you’ve got a List<String>, you can ask for element 0 of it regardless of the implementation, etc.

The ability to treat an instance of a subclass as an instance of a superclass is called Liskov’s Substitution Principle. It allows for loose coupling and code reuse.

Also read the Polymorphism part of the Java tutorial for more information.