How can This cast fail?

[Amit Batra]

class A {}
class B extends A {}
class C extends A {}
public class Testee {
    public static void main(String[] args) {
        A x = new A();
        B y = new B();
        C z = new C();
        
    y=(A)y;   //How can this fail?


    }
}

Y can be cast to an A as (A)y, and at runtime it would be Y that was actually being assigned to Y. so shldnt the above work? How can I understand this? Ive been having alot of problems with this lately?

[Vaibhav Chauhan]

If you are in misconception that narrowing of a primitive data type is same as upcasting of subclass to superclass you are wrong.

you can't cast a subclass class reference to a superclass always. See the following code:

A b=new B();
A y=(A)b;

only in above case you can do casting to a superclass object because actually b is a reference of class A, however it is refering to B class object.

[Vaibhav Chauhan]

when you write:

y=(A)y;

you are assigning A class object to its subclass reference. The rule says that you can assign subclass object to its superclass reference but not the other way around.

[Barry Gaunt]

y is a B. You can assign a B to y or you can assign a subclass of B to y.
You cannot assign a super class of B to y.

From the Java Language Specification:

4.12.2 Variables of Reference Type
A variable of a class type T can hold a null reference or a reference to an instance of class T or of any class that is a subclass of T

[Arul Prasad]

hi Barry

y is a B. You can assign a B to y or you can assign a subclass of B to y.
You cannot assign a super class of B to y.

Hope Y is a B was wrong in terms of your explanation. I hope it was B is a Y .

For example :

MyFrame is a Frame.

so

  class MyFrame extends Frame
{
}
 

So Frame reference variable can hold Frame , MyFrame , is a MyFrame (subclass) or null

[Barry Gaunt]

B y = new B();

The above declares y to be of type B. So y is a B. The actual object assigned to y is a B.

[mansi upadeo]

first of all y=(A)y will give you a compile time error,
you are converting a subclass ref to superclass type and storing the reference
in a variable y which is of type subclass.here the compiler will find
a type mismatch and complain of incompatible types.
but if you changeit to
x=(A)y
it will compile as now you are assinging it to a superclass reference
variable.
this will also work at runtime.ucan check it by applying the instaceof operator
as y instanceof x which holds true and thus it is a valid cast.

[Arul Prasad]

hi Barry

thanks , now i got it , so your telling about the object of the particular type. but i wrongly thought it as a Inheritance Relations "is a" type of relation.