Class diagrams contain associations, and object diagrams contain links. Both associations and links represent relationships. Associations represent relationships between classes; links represent relationships between objects. The next few sections discuss the UML’s representation of associations and links in detail.
As discussed in Chapter 2, an association defines a type of link and is a general relationship between classes. For example, the project management system involves various general relationships, including manage, lead, execute, input, and output between projects, managers, teams, work products, requirements, and systems. Consider, for example, how a project manager leads a team.
A binary association relates two classes. For example, one binary relationship in the project management system is between individual workers and their units of work, and another binary relationship is between individual workers and their work products.
In a UML class diagram, a binary association is shown as a solid-line path connecting the two related classes. A binary association may be labeled with a name. The name is usually read from left to right and top to bottom; otherwise, it may have a small black solid triangle next to it where the point of the triangle indicates the direction in which to read the name, but the arrow is purely descriptive, and the name of the association should be understood by the classes it relates.