Polymorphism in java: Why do we set parent reference to child object?

Question

I want to understand the use-case of setting a parent reference to a child object. Example: Dog class extends Animal class. (No interfaces, mind it) I would normally create an object of Dog like this:

Dog obj = new Dog();

Now, since Dog is a subclass of Animal it already has access to all of Animal's methods and variables. Then, what difference does this make:

Animal obj = new Dog(); 

Please provide a proper use-case with an code snippet of its use. No theoretical articles about 'Polymorphism' or 'Coding to interfaces' please!

Code:

public class Polymorphism {
    public static void main(String[] args){
        Animal obj1 = new Dog();
        Dog obj2 = new Dog();
        obj1.shout(); //output is bark..
        obj2.shout(); //output is bark..        
    }   
}

class Animal{
    public void shout(){
        System.out.println("Parent animal's shout");
    }       
}

class Dog extends Animal{
    public void shout(){
        System.out.println("bark..");
    }
}

class Lion extends Animal{
    public void shout(){
        System.out.println("roar..");
    }
}

class Horse extends Animal{
    public void shout(){
        System.out.println("neigh");
    }
}

Output is the same for both the cases. Then why do we set parent reference to child object?

Answer 1

This is an implementation of a principle which says -

Program to an interface, not to an implementation.

As an example, if you design a method to accept a reference of type Animal, then in future you can easily pass an= Cat implementation to it (provided of course that the Cat is a sub-type of Animal.

Which means -

public void doSomethingWithAnimal(Animal animal) {
    // perform some action with/on animal
}

is much more flexible than -

public void doSomethingWithAnimal(Dog d) {
    // your code
}

because for the first method, you can easily do something like -

doSomethingWithAnimal(new Cat());

if you ever decide to create new Cat type, inheriting from Animal.