I got the following:

class A{  
    int foo;  
}  
class B extends A{  
    public void bar();  
}

I got a instance of A and want to convert it to an instance of B without losing the reference to the variable foo.

For example:

A a = new A();  
a.foo = 2;  
B b = a; <-- what I want to do.  
//use b  
b.foo = 3;  
//a.foo should now be 3

Thanks for any help!