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Re: question about casting and inheritance

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On 1/9/2014 1:48 PM, marc.at.compass@gmail.com wrote:
> 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 there'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 {
> 	String uniqueMethodForB(){
> 		return ("uniqueMethodForB");
> 	}
> }
>
> public class LearnJava{
>      public static void main(String[] args) throws Exception {
> 		LearnJava LJ = new LearnJava();
> 		A a = new A();
> 		B b = new B();
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
> 		a=b;
> 		b=(B)a;
> 		System.out.println("=============");
> 		System.out.println(a.uniqueMethodForA());
> 		System.out.println(b.uniqueMethodForB());
>     }
> }
> outputs:
> uniqueMethodForA
> uniqueMethodForB
> =============
> 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 first place at all!
> Why is it the line b=(B)a; doesn't give me a ClassCastException during runtime?

     Because "a" refers to the object originally pointed to by "b",
that is, to the object created by "new B()".  That object is a B
object, so converting its A-type reference "a" to a B-type reference
is no problem.   For contrast, try

	A a = new A();
	B b = new B();
	...
	A a2 = b;
	B b2 = (B) a;  // ClassCastException here

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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question about casting and inheritance marc.at.compass@gmail.com - 2014-01-09 10:48 -0800
  Re: question about casting and inheritance leif.roar@invalid.com (Leif Roar Moldskred) - 2014-01-09 13:03 -0600
  Re: question about casting and inheritance Eric Sosman <esosman@comcast-dot-net.invalid> - 2014-01-09 14:07 -0500
  Re: question about casting and inheritance markspace <markspace@nospam.nospam> - 2014-01-09 13:00 -0800
  Re: question about casting and inheritance Ricardo Palomaes <rpmdisguise-java@nowhere.com> - 2014-01-09 23:24 +0100
    Re: question about casting and inheritance Ricardo Palomares <rpmdisguise-java@nowhere.com> - 2014-01-12 12:20 +0100
  Re: question about casting and inheritance Patricia Shanahan <pats@acm.org> - 2014-01-10 03:04 -0800
    Re: question about casting and inheritance Arved Sandstrom <asandstrom2@eastlink.ca> - 2014-01-11 09:38 -0400
  Re: question about casting and inheritance Roedy Green <see_website@mindprod.com.invalid> - 2014-01-10 16:36 -0800
  Re: question about casting and inheritance marc.at.compass@gmail.com - 2014-01-11 08:01 -0800

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