UpCasting

[slepax]

Hey all,

Here is a problem:
I have tree classes, A and B, also known that B extends A.

now...

A a = new A();
B b = (B)a;




The code above throws a ClassCastException, why ?

I've been working for couple of years with C++ and I'm pretty sure this kind of casting is possible, why not in Java ?

If I'm right, and this kind of casting cannot be done, what's the big idea behind OOP in Java ?


Thanks,
Ronen.

[Gritsch]

Hi,

it doesnt work. If you say B extends A


class B extends A




so I think you only can :


B b = new b();
A a = (A) b;




Thats OOP! You cant cast A to B. Because A doesnt know anything from B.

A child can behave like a parent, but a parent doesnt know anything about their childs.

Martin