To understand the UML, you need to form a conceptual model of the language, and this requires
learning three major elements: the UML's basic building blocks, the rules that dictate how those
building blocks may be put together, and some common mechanisms that apply throughout the
UML. Once you have grasped these ideas, you will be able to read UML models and create some
basic ones. As you gain more experience in applying the UML, you can build on this conceptual
model, using more advanced features of the language.
Building Blocks of the UML
The vocabulary of the UML encompasses three kinds of building blocks:
Things are the abstractions that are first-class citizens in a model; relationships tie these things
together; diagrams group interesting collections of things.
Things in the UML
There are four kinds of things in the UML:
These things are the basic object-oriented building blocks of the UML. You use them to write
Structural things are the nouns of UML models. These are the mostly static parts of a model,
representing elements that are either conceptual or physical. Collectively, the structural things
are called classifiers.
A class is a description of a set of objects that share the same attributes, operations, relationships,
and semantics. A class implements one or more interfaces. Graphically, a class is rendered as a
rectangle, usually including its name, attributes, and operations, as in Figure 2-1.
Figure 2-1. Classes