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How can This cast fail?

 
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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?
 
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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
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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.
 
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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

 
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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


So Frame reference variable can hold Frame , MyFrame , is a MyFrame (subclass) or null
 
Barry Gaunt
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The above declares y to be of type B. So y is a B. The actual object assigned to y is a B.
 
Greenhorn
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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
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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.



 
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