Object-Oriented Software Development with UML - UNU-IIST ...

56 Conceptual Model – AssociationsLibrary1 Contains 1..*0..8BookOn-loan-to1BorrowerAirline1Employs1..*Assigned-toPerson1 1..*1 1..*FlightAssinged-to* 1PlaneSupervisesStore11..*1 1..* Paid-byPOST Sale PaymentCaptures1 1Figure 5.7: Examples of associationsThis can be taken as a criterion for whether two objects should be linked.Multiple associations between two classes Two classes may have multiple associations between them;this is not uncommon. For example, in the domain of an airline example, there are two relationshipsbetween a Flight and an Airport, Flies-to and Flies-from, as shown in Figure 5.9.These two associations are distinctly different relationships between flights and airports. Note that not everyflight is guaranteed to land at any airport! However, there is always one airport from where a flight flies.5.2.1 Strategies for identifying associationsFor the requirement analysis, we have the following principles:An useful association usually implies knowledge of a relationship that needs to be preserved for some=duration (“need-to-know” association); andan important link between two objects should fulfill the role to provide a means for the objects to=collaborate or interact with each other.Report No. 259, July 2002UNU/IIST, P.O. Box 3058, Macau