If you are building a house, things like walls, doors, windows, cabinets, and lights will form part of your vocabulary. None of these things stands alone, however. Walls connect to other walls. Doors and windows are placed in walls to form openings for people and for light. Cabinets and lights are physically attached to walls and ceilings. You group walls, doors, windows, cabinets, and lights together to form higher-level things, such as rooms.Associations are structural relationships among instances. For example, rooms consist of walls and other things; walls themselves may have embedded doors and windows; pipes may pass through walls.
Generalizations connect generalized classes to more-specialized ones in what is known as subclass/superclass or child/parent relationships. For example, a picture window is a kind of window with very large, fixed panes; a patio window is a kind of window with panes that open side to side.
These three kinds of relationships cover most of the important ways in which things collaborate with one another. Not surprisingly, they also map well to the ways that are provided by most object-oriented programming languages to connect objects.
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Other kinds of relationships, such as realization and refinement, are discussed in Chapter 10 . |
The UML provides a graphical representation for each of these kinds of relationships, as Figure 5-1 shows. This notation permits you to visualize relationships apart from any specific programming language, and in a way that lets you emphasize the most important parts of a relationship: its name, the things it connects, and its properties.