Difference between an instance of a class and a class representing an instance already?

Question

I use Java as an example but this is more of a general OOP design related question.

Lets take the IOExceptions in Java as an example. Why is there a cla

Answer 1

Following comments are on the condition where sub-classes do not actually extend the functionality of their super class.

From Oracle doc:

Signals that an I/O exception of some sort has occurred. This class is the general class of exceptions produced by failed or interrupted I/O operations.

It says IOException is a general exception. If we have a cause enum:

enum cause{
    FileNotFound, CharacterCoding, ...;
}

We will not be able to throw an IOException if the cause in our custom code is not included in the enum. In another word, it makes IOException more specific instead of general.

Assuming we are not programming a library, and the functionality of class Dog below is specific in our business requirement:

enum DogBreed {
    Bulldog, Poodle;
}

class Dog {
    DogBreed dogBreed;
    public Dog(DogBreed dogBreed) {
       this.dogBreed = dogBreed;
    }
}

Personally I think it is good to use enum because it simplifies the class structure (less classes).