www.developer.com/design/article.php/2109801
By Mandar Chitnis, Pravin Tiwari, & Lakshmi Ananthamurthy March 14, 2003 Relationships in Use CasesUse cases share different kinds of relationships. A relationship between two use cases is basically a dependency between the two use cases. Defining a relationship between two use cases is the decision of the modeler of the use case diagram. This reuse of an existing use case using different types of relationships reduces the overall effort required in defining use cases in a system. A similar reuse established using relationships, will be apparent in the other UML diagrams as well. Use case relationships can be one of the following:
On the face of it, both generalizations and extends appear to be more or less similar. But there is a subtle difference between a generalization relationship and an extend relationship. When you establish a generalization relationship between use cases, this implies that the parent use case can be replaced by the child use case without breaking the business flow. On the other hand, an extend relationship between use cases implies that the child use case enhances the functionality of the parent use case into a specialized functionality. The parent use case in an extend relationship cannot be replaced by the child use case. Let us see if we understand things better with an example. From the diagram of a generalization relationship (refer to Figure 3.6), you can see that "Store patient records (paper file)" (parent) use case is depicted as a generalized version of the "Store patient records (computerized file)" (child) use case. Defining a generalization relationship between the two implies that you can replace any occurrence of the "Store patient records (paper file)" use case in the business flow of your system with the "Store patient records (computerized file)" use case without impacting any business flow. This would mean that in future you might choose to store patient records in a computerized file instead of as paper documents without impacting other business actions. Now, if we had defined this as an extend relationship between the two use cases, this would imply that the "Store patient records (computerized file)" use case is a specialized version of the "Store patient records (paper file)" use case. Hence, you would not be able to seamlessly replace the occurrence of the "Store patient records (paper file)" use case with the "Store patient records (computerized file)" use case. |
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