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Date: Fri, 10 Jan 2014 03:04:43 -0800
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: question about casting and inheritance
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On 1/9/2014 10:48 AM, 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?
>

There are two ways of thinking about Java that, in combination, make
this type of issue very simple and clear:

1. Distinguish between the type of an expression and the class of an object.

2. Every non-null reference expression is a pointer to an object whose
class is matches the expression's type, or extends it, or implements it.

Every expression has a type that is determined at compile time. For a
variable or parameter the type is directly declared. The type of
variable a is A, and the type of variable b is B. Other expressions have
types that can be determined from the nature of the expression and the
types of its operands. The type of "(B)a" is B.

Each object has a class that is determined by how the object is created,
through clone() or new. The class of the object created by "new B()" is
B, and remains B regardless of what variables or expressions point to it.

Applying this thinking to the code above, "a=b" assigns to variable a
the value of b, which is a pointer to an object of class B. Because B
extends A, the compiler knows that variable b either is null or is a
pointer to an object that variable a is allowed to point to, so no cast
is needed.

"b=a" would be rejected at compile time because variable a can point to
objects that variable b cannot point to.

"b=(B)a" is accepted by the compiler because variable a can point to an
object that variable b can also point to. It would cause a
ClassCastException at run time if, in fact, variable a pointed to an
object variable b cannot. It points to an object that was created by
"new B()", an object whose class is B, so the cast is valid.

Patricia