ISBN : 978-81-963532-2-3 (E-Book) Fig.13.5:Exampleusecaseinclusion Extends The main idea behind the extends relationship among the use cases is that it allows you to show optional system behavior. An optional system behavior is extended only under certain conditions. This relationship among use cases is also predefined as a stereotype as shown in fig. 13.6. The extends relationship is similar to generalization. But unlike generalization, the extending use case can add additional behavior only at an extension point only when certain conditions are satisfied. The extension points are points within the use case where variation to the mainline (normal) action sequence may occur. The extends relationship is normally used to capture alternate paths or scenarios. Fig.13.6:Exampleusecaseextension Organizationofusecases When the use cases are factored, they are organized hierarchically. The high-level use cases arerefinedintoasetofsmallerandmorerefinedusecasesasshowninfig.13.7.Top-levelusecases are superordinate to the refined use cases. The refined use cases are sub-ordinate to thetoplevelusecases.Notethatonlythecomplexusecasesshouldbedecomposedandorganizedin a hierarchy. It is not necessary to decompose simple use cases. The functionality of the superordinateusecasesistraceabletotheirsub-ordinateusecases.Thus,thefunctionalityprovidedby the superordinate use cases is composite of the functionality of the sub-ordinate use cases. Inthe highest level of the use case model, only the fundamental use cases are shown. The focus ison the application context. Therefore, this level is also referred to as the context diagram. In thecontext diagram, the system limits are emphasized.In the top-level diagram, only those usecaseswithwhichexternalusersofthesystem.Thesubsystem-levelusecasesspecify theservices offered by the subsystems. Any number of levels involving the subsystems may beutilized. In the lowest level of the use case hierarchy, the class-level use cases specify thefunctionalfragments or operations offered 67 Software Engineering Keerthana P, Manasa KN, Ganga D Bengal
73 Publizr Home