What is the difference between up-casting and down-casting with respect to class variable

Question

What is the difference between up-casting and down-casting with respect to class variable?

For example in the following program class Animal contains only one method

Answer 1

I know this question asked quite long time ago but for the new users of this question. Please read this article where contains complete description on upcasting, downcasting and use of instanceof operator

• There's no need to upcast manually, it happens on its own:

  Mammal m = (Mammal)new Cat(); equals to Mammal m = new Cat();

• But downcasting must always be done manually:

  Cat c1 = new Cat();      
  Animal a = c1;      //automatic upcasting to Animal
  Cat c2 = (Cat) a;    //manual downcasting back to a Cat
  

Why is that so, that upcasting is automatical, but downcasting must be manual? Well, you see, upcasting can never fail. But if you have a group of different Animals and want to downcast them all to a Cat, then there's a chance, that some of these Animals are actually Dogs, and process fails, by throwing ClassCastException. This is where is should introduce an useful feature called "instanceof", which tests if an object is instance of some Class.

 Cat c1 = new Cat();         
    Animal a = c1;       //upcasting to Animal
    if(a instanceof Cat){ // testing if the Animal is a Cat
        System.out.println("It's a Cat! Now i can safely downcast it to a Cat, without a fear of failure.");        
        Cat c2 = (Cat)a;
    }

For more information please read this article