Casting

[devi nachimuthu]

The following defines an object hierarchy.
class class1{}
class class2 extends class1 {}
class class3 extends class2 {}
1. when is the following method valid?
public class3 castMe(class1 ref) {
return (class3)ref;
}
a) always
b) never
c)if and only if ref references an class3 object
d) if and only if ref references an class2 object
The given answer is c. I cannot understand why it is c. Can some body explain...?
Thanks

[Sachin Kombrabail]

There can be three ways in which castMe can be called. Let us see how each will react.
1. If you pass an instance of C1 then the method will try to cast into C3 this will give you a ClassCastException since java does not support downcasting.
2. If you pass an instance of C2 then the same message as in C1.
3. If you pass an instance of C3 that will go through.
So (c) is the only answer for your question.

[devi nachimuthu]

I agree with you sachin, but how can i pass C1 to C3. There is no relation between Class1 and Class3, Class3 is inheriting from Class2. So how can we pass C1 to C3...?