Question about Java polymorphism and casting

Question

I have a class C. Class E extends it.

E e = new E();
C c = new C();

Why is

e = (E) c;

Upon further review

Answer 1

the int/double is unrelated; that is a conversion, not a cast - there is no relationship between int and double.

Re the question; a type's object is fixed at creation. An object that is a C is not (and can never be) an E. However, you can treat an E as a C, since inheritance represents "is a". For example:

E e = new E();
C c = e;

Here we still only have one object - simply that the c variable thinks of it as a C, so won't expose methods specific to E (even though the object is an E).

If we then add:

E secondE = (E) c;

This is a type check; again, we haven't changed the object, but to put c into an E variable requires us to prove to the compiler/runtime that it really is an E. We didn't need this in the first example as it can already prove that any E is also a C.

Likewise, with the getClass() - all the cast does is change how the compiler thinks of the object; you haven't changes the object itself. It is still a K.

You need to separate variables from objects. The cast is talking about the variables; they don't change the object.