Why Downcasting throws Exception?

Question

In java:

Base b = new Base();
Derived d = (Derived)b; 

throws ClassCastException. Why? Why downcasting throws Exception<

Answer 1

A derived class inherits the behavior from its super class. Hence, casting a sub-class object to a super-class reference works since the derived class object is capable of fulfilling the contract defined by the super-class.

On the other hand, a super-class (by the very way you define the classes) clearly doesn't implement most of the methods present in the sub-class. Well, that's why you extended the super-class in the first place - to extend its implementation.

So, casting a super-class object to a sub-class type is an inherently unsafe operation because the base class object cannot fulfill its sub-class' contract completely.