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From: Ricardo Palomaes <rpmdisguise-java@nowhere.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: question about casting and inheritance
Date: Thu, 09 Jan 2014 23:24:06 +0100
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El 09/01/14 19:48, marc.at.compass@gmail.com escribi=F3:
> As you see variable a points to an instance of A and not to (an
> instance of) B.


In fact, variable "a" is a reference to an object of class A. Since
class B extends A, an object of class B is also an object of class A
(it is "just a more specific type of A"). Thus, when you do:

a =3D b;

a is referencing the same object than b, ie., an object of class B.
While accessing that object from "a", it only can behave as a class A
object, and you couldn't do a.uniqueMethodForB(). However, as soon as
you access the object from b again, you can access again the methods
exclusive of a class B object. Note that, during this sequence:

a =3D b;
b =3D (B) a;

the object itself has not been touched nor modified at all; you only
have modified the references "a" and "b" (and "b" has been modified
only to leave exactly as it was).


> I effectively say "treat a as if it is b". I can
> even call a method on b that is unique for B. But a didn't point to
> an instance of B in the first place at all!


Yes, it did. At least, since you did a =3D b. :-)



> Why is it the line b=3D(B)a; doesn't give me a ClassCastException
> during runtime?


Because a holds a reference to a class A object that happens also to
be a class B object, so it is not wrong to reference as such.

HTH