Java; casting base class to derived class

because if B extends C, it means B is a C and not C is a B.

rethink what you are trying to do.

The existing answers are fine in terms of an abstract argument, but I'd like to make a more concrete one. Suppose you could do that. Then this code would have to compile and run:

// Hypothetical code
Object object = new Object();
InputStream stream = (InputStream) object; // No exception allowed?
int firstByte = stream.read();

Where exactly would the implementation of the read method come from? It's abstract in InputStream. Where would it get the data from? It simply isn't appropriate to treat a bare java.lang.Object as an InputStream. It's much better for the cast to throw an exception.

In my experience it's tricky to get "parallel class hierarchies" like the one you're describing to work. You may find that generics help, but it can get hairy very quickly.