Recently, the subject of patterns has become one of the hottest topics in object-oriented development. The most well-known patterns are the design patterns of the Gang of Four (Gamma, E., R. Helm, R. Johnson, and J. Vlissides, Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley, 1994), fundamental implementation patterns that are widely useful in object-oriented implementations. An important part of most OO systems is the domain model—those classes which model the underlying business that the software is supporting. We can see patterns in domain models too, patterns that often recur in surprising places. I will provide a short introduction to some analysis patterns that I have come across.

I'm not going to do this by surveying the field, or discussing theory—patterns don't really lend themselves to that. Rather, I'll show you some examples of patterns and illustrate them with UML diagrams (Fowler, M., UML Distilled: Applying the Standard Object Modeling Language, Addison-Wesley, 1997) and Java interface declarations.

REPRESENTING QUANTITIES

Imagine you are writing a computerized medical record system for a hospital. You have to record several measurements about your patients, such as their height, weight, and blood-glucose level. You might choose to model this using numeric attributes (see Figure 1 and Listing 1).

This approach is quite common, but may present several problems. One major problem is with using numbers.