UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition

Classification and Generalization

I often hear people talk about subtyping as the "is a" relationship. I urge you to beware of that way of thinking. The problem is that the phrase "is a" can mean different things.

Consider the following phrases.

1. Shep is a Border Collie.

2. A Border Collie is a Dog.

3. Dogs are Animals.

4. A Border Collie is a Breed.

5. Dog is a Species.

Now try combining the phrases. If I combine phrases 1 and 2, I get "Shep is a Dog"; 2 and 3 taken together yield "Border Collies are Animals." And 1 plus 2 plus 3 gives me "Shep is an Animal." So far, so good. Now try 1 and 4: "Shep is a Breed." The combination of 2 and 5 is "A Border Collie is a Species." These are not so good.

Why can I combine some of these phrases and not others? The reason ...