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Doubt about ClassCastException (was: Doubt)

 
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Can any one explain when class cast exception will occur with respect to down casting
[ May 14, 2007: Message edited by: Barry Gaunt ]
 
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ClassCastException:
Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.
 
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hi John,
Can you give an example of legal downcasting?
thanks!
 
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class Sample
{
public static void main(String[] args)
{
Object obj = new String("hello");
String str = (String) obj;
.....
}
}

Regards
Kumaran J
 
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Basically the ClassCastException arises when the JVM discovers that you have broken a promise that you have made to the compiler when you made the downcast.
 
sravanthi pulukuri
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Can you elucidate it with an example??
 
debasmita pattnayak
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hi kumaran,
thanks for the reply... i got my answer.

 
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Originally posted by sravanthi pulukuri:
Can you elucidate it with an example??



Hi,

See the code to get it in better way:



Note: Compiler does not know the runtime type, because object creation
and its reference assignment is matter of runtime. Compiler only
wants that what assignment you are doing is castable. It can't see what
object the ref variable is pointing to. K&B says, here compiler relies on
us, in fact it has no other alternative except relying on us here.

But JVM see that we are trying to assign Animal object to Dog ref variable
hence ClassCastExeption. Dog is Animal for sure but Animal is not required
to be Dog only (It can be Horse, Cow, Buffalo etc)

By the way John Stone gave nice example with String and Object,
and very similar question I faced in exam too.



Thanks,
 
Barry Gaunt
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I blatently steal Chandra's example and make it more explicit:


[ May 14, 2007: Message edited by: Barry Gaunt ]
 
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