Aggregation Relationships

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An aggregation relationship is a specialized form of association in which a whole is related to its part(s). Aggregation is known as a "part-of" or containment relationship. The UML notation for an aggregation relationship is an association with a diamond next to the class denoting the aggregate (whole), as shown in Figure 6-3.

Figure 6-3. UML Notation for an Aggregation Relationship

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The following tests may be used to determine if an association should be an aggregation:

  • Is the phrase "part of" used to describe the relationship?

  • Are some operations on the whole automatically applied to its parts ? For example, delete a course then delete all of its course offerings.

  • Is there an intrinsic asymmetry to the relationship where one class is subordinate to the other?

For example, a Course (Math 101) may be offered at different times during a semester. Each offering is represented as a Course Offering (e.g., Math 101, Section 1, and Math 101, Section 2). The relationship between a Course and a CourseOffering is modeled as an aggregationa Course "has" CourseOfferings.

CREATING AN AGGREGATION RELATIONSHIP IN RATIONAL ROSE

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  1. Select the Aggregation icon from the toolbar. The Aggregation icon may be added to the toolbar by right-clicking on the toolbar and selecting the Customize menu command.

  2. Click on the class playing the role of the "whole" in a class diagram and drag the aggregation line to the class playing the role of the "part."

An aggregation relationship is shown in Figure 6-4.

Figure 6-4. Aggregation Relationship

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Visual Modeling with Rational Rose 2002 and UML
Visual Modeling with Rational Rose 2002 and UML (3rd Edition)
ISBN: 0201729326
EAN: 2147483647
Year: 2002
Pages: 134

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