August 1999
Intermediate to advanced
224 pages
4h 25m
English
book
UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition
by Martin Fowler, Kendall Scott
Content preview from UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition
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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.
Shep is a Border Collie.
A Border Collie is a Dog.
Dogs are Animals.
A Border Collie is a Breed.
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 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.