casting

[vini singh]

class A{}
class B extends A{}
class C extends A{}

final class D extends B{
public void doMethod(){
A a=new B();
B b=new D();
C c=new C();
D d=null;
//Insert Here
}
}


d=(D)(B)a //how can thisline cause classcastexception

[Ruben Soto]

It will compile fine, but it will issue a ClassCastException, because a refers to a B object, and B is not a D (it is the other way around.)

The first cast is OK, it is the second cast that is the problem:
(D)(B)a

[Jor Cheung]

(B)a is valid since a is an object of class B. However (D)(B)a is not valid since you can not cast an object B to class D as you can not "cast down".

[Yuan Du]

Hi,

It will alway throw ClassCastException if i try to down cast a object.

But why trying to down cast a null object, it doesn't throw NullPointException in Runtime.

eg.
Object o = null;
Integer i = (Integer)o;

Could you please explain it?

Thanks,

Yuan

[Ruben Soto]

Hi Yuan,

The null literal is a special literal of the null type, which denotes that the reference it is assigned to doesn't point to any object.
A reference of any type can be assigned null, and that is why downcasting is working in that sense. Normally, for downcasting you need to consider the actual type of the object involved. But for a null reference there is no object pointed to, so you can cast a null reference to any type.

[Yuan Du]

Thank you Ruben!

Yuan

[Ruben Soto]

No problemo! Just remember that you don't cast objects, but references. Upcasting always works, but for downcasting you have to make sure that the actual object's type satisfies the Is-A relationship for the type you are attempting to downcast the reference pointing to that object to.