I need clarification regarding REFERENCE TYPES and CASTING.

Question

Hello all,
I'm taking a course on the fundamental of JAVA. Everything's been going smoothly until I slammed into the the concept of CASTING and REFERENCE TYPES. Flat--out == I DON'T GET IT?
I'm having trouble with...
CONVERTING REFERENCE TYPES
CASTING BETWEEN REFERENCE TYPES
WORKING WITH REFERENCE TYPES
I understand what's happening from an academic vantage point. I just don't understand why you'd want to convert REFERENCE TYPES? What would be an application of such an exercise?
1. What IS a REFERENCE TYPE -- exactly?
a. what are we referencing?
b. type? type of what??
for example... why would you want to do a widening conversion, a conversion of the hierarchy tree?
I understand the concept of OBJECTS, CLASSES, METHODS and CONSTRUCTORS so far...
I think it's the terminology that's screwing my up.
Thanks,
Alex

Answer 1

ok... wow, thanks J.
So--in a nutshell-- we're making it so that different
objects:
ie,. ford(), chevy(), honda(), lotus() and
dealers()... so and so forth()...
all share the resources(for lack of a better word) of
the Auto Class? because all of those auto brand
objects and one redically different object can be
unrelated, correct?Um, yes and no.
I just ran with the example you had, but that probably included too many concepts and they got muddied up.
Yes, Chevy, Ford etc. all share the characteristics of Auto, since they're all subclasses. But that's just inheritance, and has nothing to do with casting.
A "reference type" can loosely be described as a variable that refers to an object. (Constrasted with "primitive types" which are int, char, float, etc. and don't refer to objects--they just hold values.)
Casting just tells the compiler that even though as far as it knows you only have a reference to some superclass, the object that reference points to will in fact be an instance of a subclass, and so treat it as such (e.g., we can now call methods that the subclass has that the superclass lacks).
(You can also cast primitives, but one thing at a time.)
So let's say you have class A (which extends object) and B extends A.
A a = new B();
B b = a; // won't compile. compiler sees the "A a" on the left of the =, not "new B()" on the right.
B b = (B)a; // works because we're telling the compiler, "Dude, I'm seriously. This is a B.
Note that if we had done new A() instead of new B(), it would still compile--the compiler would trust us. But at runtime, we'd get a ClassCastException, since we wouldn't actually have a B object.
/**folks, I'm a web designer that has to learn Java
so that I can perform my duties as a JSP author here
at work. I tried to learn JSP sans Java and that was
a simple exercise in ignorance.-- it's really hard
without understanding the root concepts of Java and
for that matter, C. Concepts like "polymorphism,
inheritance, object references... are completely
foreign to me. **/It's a rather big leap from web designing to OO concepts. Take your time, and don't be discouraged if you feel completely confused. It's a prerequisite. :-)