Aggregation and Composition
One of my biggest bêtes noires in modeling is aggregation. It's easy to explain glibly: Aggregation is the part-of relationship. It's like saying that a car has an engine and wheels as its parts. This sounds good, but the difficult thing is considering what the difference is between aggregation and association.
In the pre-UML days, people were usually rather vague on what was aggregation and what was association. Whether vague or not, they were always inconsistent with everyone else. As a result, many modelers think that aggregation is important, although for different reasons. So the UML included aggregation, but with hardly any semantics. As Jim Rumbaugh says, "Think of it as a modeling placebo" (Rumbaugh, Jacobson, ...
Get UML Distilled: A Brief Guide to the Standard Object Modeling Language, Second Edition now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
