question about casting and inheritance

Hello,
I'm preparing for Java 7 OCA exam. I have a q about casting..
I learned you can cast a variable of a class to another object only if ther=
e's a hierarchical relation between the variable's class _and_ the object's=
 class you cast to. And also, the varibla should point to an object og the =
class you're casting to or you get a ClassCastException during runtime.
So I made up this example:

class A {
    String uniqueMethodForA(){
        return ("uniqueMethodForA");
    }
}

class B extends A {
    String uniqueMethodForB(){
        return ("uniqueMethodForB");
    }
}

public class LearnJava{
    public static void main(String[] args) throws Exception {
        LearnJava LJ = new LearnJava();
        A a = new A();
        B b = new B();
        System.out.println(a.uniqueMethodForA());
        System.out.println(b.uniqueMethodForB());
        a=b;
        b=(B)a;
        System.out.println("=============");
        System.out.println(a.uniqueMethodForA());
        System.out.println(b.uniqueMethodForB());
   }
}
outputs:
uniqueMethodForA
uniqueMethodForB
=============
uniqueMethodForA
uniqueMethodForB

As you see variable a points to an instance of A and not to (an instance of=
) B. I effectively say "treat a as if it is b". I can even call a method on=
 b that is unique for B. But a didn't point to an instance of B in the firs=
t place at all!
Why is it the line b=(B)a; doesn't give me a ClassCastException during ru=
ntime?