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down cast only throw exception

jacob deiter
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Joined: Apr 02, 2008
Posts: 576
what problem with this code?

I have class

but I got class cast exception,

it is a down cast only then why it throw exception
Miklos Szeles
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Joined: Oct 21, 2008
Posts: 142
Hi Jacob,

You try to cast an Object instance to an abc instance which is not possible.
Maybe you wanted to write

Anyway it is advised to use uppercase starting letters for class names.
jacob deiter
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Joined: Apr 02, 2008
Posts: 576
Polymorphism support down casting,

My code is as follow

But I got casting exception why, is it valid casting or not ?

Please explain the reaon for this also
Miklos Szeles
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Joined: Oct 21, 2008
Posts: 142
No, you misunderstood something. I advice you to reread the subject.
Just think about it logically:
Class B extends A. It means that B has everything which A has plus it can have some other methods and fields. So whenever you need an A instance you can use a B instance since it contains everything what is needed for the A instance, but you can't use an A instance as a B instance.
An A type reference can hold an A instance and a B instance. If the reference contains a B instance then you can cast your reference to B. In your example the reference contains an A instance(not a B) so you can't cast it to B.

Jarred Olson
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Joined: Jul 31, 2009
Posts: 37
You can use the instanceof operator to to see if the object you're trying to cast is castable (is that a word) to the type you want. It will prevent you from getting the ClassCastException.
Siva Masilamani
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Joined: Sep 19, 2008
Posts: 385
Simple point to remember is

Super class reference variable can point to Sub class instance not vice versa.

Hoe this anwers your question.


Failure is not an option.
I agree. Here's the link: http://aspose.com/file-tools
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