Re: ClassCast Exception

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 22 Nov 2006 15:24:02 GMT

Message-ID:

<mYZ8h.2663$1s6.1152@newsread2.news.pas.earthlink.net>

ba.hons wrote:

Hello,

i wonder if anyone can help or advise me on why am getting a ClassCast
Exception

I have a class called A (for instance) which implements another class
(B)

I have also created a new class called C which implements A

Now i have a method that i can not control which returns an instance of
A, but i need to convert this instance when it is returned to my new
type C which just has an extra method.

I cant just add the extra method to class A as i cant modify this code.

i tried to cast A to C like this

C test = (C)objectOfTypeA;

and then i get a classcast exception?

Anyone any ideas?

Thanks

Adam

The A returned by the other method is NOT a C, and Java won't let you
pretend it is one. However, you could write a C subclass, D, whose
constructor takes an A as parameter.

D would remember the A it was passed, and implement all the common B
methods by calling the corresponding method in that A. It would have to
find its own way of implementing the added method, based on A's methods,
but if that cannot be done your problem is impossible anyway.

You could then write:

C test = new D(objectOfTypeA);

That said, it is not at all clear that this is really the best approach.
You may get more useful answers if you describe the problem at the next
level up. Why do you want this particular class extension structure?
What problem is it solving?

Patricia