Doubt about ClassCastException (was: Doubt)

[sravanthi pulukuri]

Can any one explain when class cast exception will occur with respect to down casting
[ May 14, 2007: Message edited by: Barry Gaunt ]

[John Stone]

class bum {
public static void main (String[] arg) {
String s = (String) new Object();
}
}

ClassCastException:
Thrown to indicate that the code has attempted to cast an object to a subclass of which it is not an instance.

[debasmita pattnayak]

hi John,
Can you give an example of legal downcasting?
thanks!

[Kumaran Jeyabalan]

class Sample
{
public static void main(String[] args)
{
Object obj = new String("hello");
String str = (String) obj;
.....
}
}

Regards
Kumaran J

[Barry Gaunt]

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]

Can you elucidate it with an example??

[debasmita pattnayak]

hi kumaran,
thanks for the reply... i got my answer.

[Chandra Bhatt]

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

Hi,

See the code to get it in better way:

class Animal {}
 
class Dog extends Animal {}
 
class TestCasting {
    public static void main(String... args) {
        Animal ani = new Animal();
        //See that the Animal ref variable ani refers to the object of  
         //Animal
       Dog d = (Dog) ani;
       /*
         Compiler is happy because you have casted ani reference to Dog
         Animal and Dog come in the same class hierarchy, so no problem
         for now but fails at runtime because the runtime type of 
         ani is Animal and Dog ref variable can't hold Animal object
       */
     
    }
}

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]

I blatently steal Chandra's example and make it more explicit:

class Animal {}
class Dog extends Animal {}
class Cat extends Animal {}
 
class TestCasting {
    public static void main(String... args) {
 
       Animal ani = new Cat();
 
       Dog d = (Dog) ani; // promise the compiler that ani refers to a Dog.
                          // JVM will complain about broken promise
                          // by throwing a ClassCastException because
                          // ani really refers to a Cat
    }
 
}    

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