Summary

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The use case diagram presents an outside view of the system. The functionality of the use case is captured in the flow of events. Scenarios are used to describe how use cases are realized as interactions among societies of objects. A scenario is an instance of a use case ” it is one path through the flow of events for the use case. Thus, each use case is a web of scenarios. Scenarios are developed to help identify the objects, the classes, and the object interactions needed to carry out a piece of the functionality specified by the use case.

The flow of events for a use case typically is captured in text, whereas scenarios are captured in interaction diagrams. There are two types of interaction diagrams ”sequence diagrams and collaboration diagrams. Each diagram is a graphical view of the scenario.

A sequence diagram shows object interactions arranged in time sequence. A collaboration diagram is an alternate way to show a scenario. This type of diagram shows object interactions organized around the objects and their links to each other.

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