Casting a supertype to a subtype

[Santiago Bravo]

Hi All,

Question taked from Devaka's exam simulator:

1. class A{}
2. class B extends A{}
3. class C extends B{}
4. class D extends C{
5.       public static void main(String args[]){
6.              C c=new D();
7.              B b=(B)c;
8.              D d=(D)b;
9.              Object ob=new A();
10.             A a=(A)ob;
11.      }
12. }
 

The answer is that the code compiles fine.

Why does line 8 not give a runtime exception? Is it due to the reference 'b' acually referring to a C object?

If I change line 7 to

    
Bb b=new Bb();

this will give a runtime exception - ClassCastException.



Thanks

[Ernest Friedman-Hill]

Originally posted by Santiago Bravo:

Why does line 8 not give a runtime exception? Is it due to the reference 'b' acually referring to a C object?

Look at line 6: 'b' actually holds a 'D' object!

If b contained a C, then this would indeed throw an exception.

A cast never changes the class of an object. You can never cast an object to a type that it does not have. The only time you can cast a supertype to a subtype and have it work properly is if the superclass reference is actually referring to an instance of that particular subtype.