Constraint Rules
Much of what you are doing in drawing a class diagram is indicating constraints.
Figure 4-2 indicates that an Order can be placed only by a single Customer. The diagram also implies that each Line Item is thought of separately: You say 40 brown widgets, 40 blue widgets, and 40 red widgets, not 40 red, blue, and brown widgets. Further, the diagram says that Corporate Customers have credit limits but Personal Customers do not.
The basic constructs of association, attribute, and generalization do much to specify important constraints, but they cannot indicate every constraint. These constraints still need to be captured; the class diagram is a good place to do that.
The UML allows you to use anything to describe constraints. The ...
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