Why do we need downcasting in Java? What is the use of it?

Question

Why do we need downcasting in Java? What is the use of it?

Answer 1

here's an example of a valid downcast.
class Dog {}
class Dogma extends Dog {}
class Dogmatic extends Dog {}
Dog dogwood = new Dogma(); // a Dogma object is upcast to a Dog
Dogma bush = (Dogma) dogwood; // a Dogma object is downcast from a Dog
You cannot upcast a Dogmatic to a Dog and then downcast it to a Dogma. You also cannot downcast a Dog to a Dogma. In the working example, above, the dogwood references a Dogma, so you can downcast the Dogma.
and one more example
If you have a class hierarchy where class B extends class A (B is a specialization of A) you can use B in the place of A since B supports all the operations that B does. This is called polymorphism.
When you use an object of type B in place of A you actually do a up-cast, meaning you make the object of type B look like an object of type A (a less specific class type).
What you are asking about is down-casting which is making an object be of a more specific type, i.e. transforming an object of type A to type B. Is is only allowed if the object was actually created as a type B.
This is allowed (line 2 is a down-cast):
A a = new B();
B b = (B)a;