Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.java.programmer > #9138

Re: Why can't I downcast in the following code

Show all headers | View raw

Chad wrote:
> Given the following...
> 
> class X {}
> class Y extends X {}
> class Z extends X {}
> 
> public class Main {
> 
>     public static void main(String[] args) {
>         X x = new X();
>         Y y = new Y();
>         Z z = new Z();
> 
>         Y o6 = (Y) x; //<--error
>     }
> 
> }
> 
> I get this error...
> 
> run:
> Exception in thread "main" java.lang.ClassCastException: X cannot be
> cast to Y
>         at Main.main(Main.java:12)
> Java Result: 1
> BUILD SUCCESSFUL (total time: 0 seconds)
> 
> 
> I thought I could cast X to Y since they inherited. Why doesn't it
> work in this case?

Inheritance, as you have discovered, does not automatically allow casting.

Upcasting works as you describe; it's downcasting that is tricky.  (In Java, an "upcast" is from an inheriting or descendant type to an ancestor type, a "downcast" is from a base or ancestor type to a descendant type.)

To understand this, remember that inheritance (both the 'extends' and the 'implements' variety) represents /is-a/.  A descendant thingie /is-an/ ancestor thingie, so in your example, 

> class X {}
> class Y extends X {}
> class Z extends X {}

an instance of a 'Y' /is-an/ 'X', but not necessarily would every 'X' /be-a/ 'Y'.
An instance of a 'Z' /is-an/ 'X', but not necessarily would every 'X' /be-a/ 'Z'.

In no way can you say a 'Y' /is-a/ 'Z', nor that a 'Z' /is-a/ 'Y'.  Neither one is the parent of the other.

Which is exactly why downcasting is tricky.  If you cast an 'X' down to a 'Y', that might actually be a 'Z' you're trying to cast.  Oops - 'ClassCastException'.  You cannot cast a 'Z' to a 'Y'.  

In your case it's worse.  You already know that 'x' /is-not-a/ 'Y'.  It's a plain old 'X', with nothing of a 'Y' about it.  So when you try to cast it down to a 'Y', kaboom!

And by the way, you don't get an error for that, you get an exception for that.

-- 
Lew

Back to comp.lang.java.programmer | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

Why can't I downcast in the following code Chad <cdalten@gmail.com> - 2011-10-23 18:22 -0700
  Re: Why can't I downcast in the following code Chad <cdalten@gmail.com> - 2011-10-23 18:47 -0700
    Re: Why can't I downcast in the following code Travers Naran <tnaran@gmail.com> - 2011-10-24 07:57 -0700
  Re: Why can't I downcast in the following code Roedy Green <see_website@mindprod.com.invalid> - 2011-10-23 18:50 -0700
    Re: Why can't I downcast in the following code Chad <cdalten@gmail.com> - 2011-10-23 18:57 -0700
    Re: Why can't I downcast in the following code Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-10-23 19:00 -0700
      Re: Why can't I downcast in the following code Lew <lewbloch@gmail.com> - 2011-10-23 23:22 -0700
        Re: Why can't I downcast in the following code Lew <lewbloch@gmail.com> - 2011-10-23 23:27 -0700
  Re: Why can't I downcast in the following code Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-10-23 19:03 -0700
  Re: Why can't I downcast in the following code Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-10-23 23:16 -0400
    Re: Why can't I downcast in the following code Donkey Hottie <donkey@fredriksson.dy.fi> - 2011-10-24 07:19 +0300
  Re: Why can't I downcast in the following code Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-10-24 00:08 -0500
  Re: Why can't I downcast in the following code Lew <lewbloch@gmail.com> - 2011-10-23 23:17 -0700
    Re: Why can't I downcast in the following code Lew <lewbloch@gmail.com> - 2011-10-23 23:19 -0700

csiph-web