DEV475 Mastering Object-Oriented Analysis and Design with UML ...

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DEV475 Mastering Object-Oriented Analysis and Design with UMLAssociation or Aggregation?Association or Aggregation?• If two objects are tightly bound by a whole-partrelationship• The relationship is an aggregation.CarDoor10..2,4• If two objects are usually considered asindependent, although they are often linked• The relationship is an association.CarDoor10..2,4When in doubt use associationMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 46Aggregation should be used only where the "parts" are incomplete outside the contextof the whole. If the classes can have independent identity outside the contextprovided by other classes, or if they are not parts of some greater whole, then theassociation relationship should be used.When in doubt, an association is more appropriate. Aggregations are generallyobvious. A good aggregate should be a natural, coherent part of the model. Themeaning of aggregates should be simple to understand from the context. Choosingaggregation is only done to help clarify; it is not something that is crucial to thesuccess of the modeling effort.The use of aggregation versus association is dependent on the application you aredeveloping. For example, if you are modeling a car dealership, the relationshipbetween a car and the doors might be modeled as aggregation, because the caralways comes with doors, and doors are never sold separately. However, if you aremodeling a car parts store, you might model the relationship as an association, as thecar (the body) might be independent of the doors.6 - 46
Module 6 - Use-Case AnalysisWhat Are Roles?What Are Roles?• The “face” that a class plays in theassociationCourseOfferinginstructorProfessorDepartmentDepartment HeadRole NameCoursepreRequisitesMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 47Each end of an association has a role in relationship to the class on the other end ofthe association. The role specifies the face that a class presents on each side of theassociation. A role must have a name, and the role names on opposite sides of theassociation must be unique. The role name should be a noun indicating theassociated object's role in relation to the associating object.The use of association names and role names is mutually exclusive: one would notuse both an association name and a role name. For each association, decide whichconveys more information.The role name is placed next to the end of the association line of the class itdescribes.In the case of self-associations, role names are essential to distinguish the purpose forthe association.In the above example, the Professor participates in two separate associationrelationships, playing a different role in each.6 - 47
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