N00b query: Why would anyone ever want to define their own Exception class?

Question

I've been reading thru my Java textbook for the past couple hours.
Exceptions are a wonderful thing. I already found several instances where I could've implemented try/cacth in my earlier programs.
Anyway, getting back to the point. My question is, can someone give me a realistic situation/example where a custom Expcetion class is REQUIRED? (the key word here is "required"!)
I can see why someone would want to have his own Exception class..... e.getMessage() as custom error messages are SO DAMN COOL!!!!!! :P
hehe
But seriously, if you are making intermidiate/advanced Java programs, would you ever REQUIRE to make your own Exception class? Afterall, even a custom made Exception class always "extends" from a pre-defined Java class, right?
Let me make this a bit more clear... public class CustomException extends IOException{  }Now, if I am making a try/catch statement, I can simply say
try{
throw new CustomException;
catch (IOException e) { }
Now as you can see, the CustomException was caught by the catch claus, because IOException is the superclass of CustomException. So, in other words, the whole CustomException thingy didnt do anything useful.
I know I know, I am so naive. Enlighten me >.<

Answer 1

Sure. Say you want to have a system where you want to include a custom error code which maps to some internationalized error messages. You would create an Exception subclass with a field to hold that value separate from the normal "default" message. Then you could throw that exception in all your code. Other code can catch it as a plain Exception if they want and use the "default" message, which is okay if they don't really care about the error code.
I don't think you are ever "required" to make your own exceptions. I have done so, but I don't often. It depends. See, there are plenty of Exception subclasses in the standard packages, and most of them cover many of the things you need. So more often if I'm throwing an exception, I'll be using the already existing ones, like IllegalArgumentException or IOException (whatever is relevant to the code).
Yes, you can do what you did below with CustomException. However the reason you might do that is cuz you really want to do this:
try {
   // call some code that may throw IOException from some standard IO package
   // or may throw CustomException from some of my methods...
} catch (CustomException ce) {
   // handle cusotm exception
} catch (IOException ioe) {
   // handle IO exception
}Cuz you may want to differentiate between your exceptions vs. IOExceptions that might be thrown from some java.io class.
Usually when you use an exception class it's a named class that relates to some condition. It may hold additional information besides the standard message, but I think most of the time it's just the class name which describes the problem. And if there isn't one that describes the problem that you're code might encounter, then create a subclass.