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class cating

 
Greenhorn
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is it not possible to downcast a class???
Eg:-
class Animal{
//some code
}
class Dog extends Animal{
//some code
}
public class Test{
public static void main(String[] args)
{
Animal a=new Animal();
Dog d=(Dog)a;
}}
 
Greenhorn
Posts: 18
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Animal a=new Dog();
Dog d=(Dog)a;

the above represents a downcasting.And the code you have supplied will give classcastexception.This is clearly explained in page no-113(k&b).
 
Ranch Hand
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You sure can downcast from a parent type to a child type. Now, whether the casting operation will succed will depend on the runtime type of the object being cast.

So in your example, the code will compile but you'll get a ClassCastException at runtime.

Try the following code sample



You may want to read about casting from the JLS
http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.5
http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#25379

HTH
Ashish Hareet
[ September 04, 2008: Message edited by: Ashish Hareet ]
 
chennuru viswanath
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thanks for the reply....
 
Java Cowboy
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In the lines above, you have an Animal object, and by casting it to a Dog object, you tell the compiler: "Look, I have this object here, and you should treat it as an instance of class Dog".

This can only work if the object really is a Dog object. If the object you're casting isn't a Dog object, you'll get a ClassCastException when you run the code. Casting does not magically convert one type of object to a different type of object.
 
Ranch Hand
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can we check whether the object is of type dog and then downcast using instance of
 
Ashish Hareet
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Originally posted by Rajeev Ja:
can we check whether the object is of type dog and then downcast using instance of



Absolutely, in the code sample from the original post you can do the following



which will prevent the ClassCastException from being thrown.

HTH
Ashish Hareet
[ September 04, 2008: Message edited by: Ashish Hareet ]
 
Ranch Hand
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Here you can refer the Dog object to the Animal reference and using instanceof, at runtime check whether the instance is actually of type Dog.
instanceof is typically used in polymorphism, where at runtime the actual type of object is checked.
If you do:

The if condition would always be false since an Animal object dosen't satisfy the IS-A condition for Dog.
[ September 04, 2008: Message edited by: Somnath Paul ]
 
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