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Date: Thu, 9 Jan 2014 10:48:34 -0800 (PST)
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Subject: question about casting and inheritance
From: marc.at.compass@gmail.com
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Xref: csiph.com comp.lang.java.programmer:27262

Hello,
I'm preparing for Java 7 OCA exam. I have a q about casting..
I learned you can cast a variable of a class to another object only if ther=
e's a hierarchical relation between the variable's class _and_ the object's=
 class you cast to. And also, the varibla should point to an object og the =
class you're casting to or you get a ClassCastException during runtime.
So I made up this example:

class A {
	String uniqueMethodForA(){
		return ("uniqueMethodForA");
	}
}

class B extends A {=20
	String uniqueMethodForB(){
		return ("uniqueMethodForB");
	}
}

public class LearnJava{
    public static void main(String[] args) throws Exception {
		LearnJava LJ =3D new LearnJava();
		A a =3D new A();
		B b =3D new B();
		System.out.println(a.uniqueMethodForA());
		System.out.println(b.uniqueMethodForB());
		a=3Db;
		b=3D(B)a;
		System.out.println("=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D");
		System.out.println(a.uniqueMethodForA());
		System.out.println(b.uniqueMethodForB());
   }
}
outputs:
uniqueMethodForA
uniqueMethodForB
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
uniqueMethodForA
uniqueMethodForB

As you see variable a points to an instance of A and not to (an instance of=
) B. 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 firs=
t place at all!
Why is it the line b=3D(B)a; doesn't give me a ClassCastException during ru=
ntime?