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

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DEV475 Mastering Object-Oriented Analysis and Design with UMLOne Interaction Diagram Is Not Good EnoughOne Interaction Diagram Is Not Good EnoughBasic FlowAlternate Flow 1 Alternate Flow 2 Alternate Flow 3AF3AF1AF2Alternate Flow 4 Alternate Flow 5 Alternate Flow nMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 34Model most of the flow of events to make sure that all requirements on theoperations of the participating classes are identified. Start with describing the basicflow, which is the most common or most important flow of events. Then describevariants such as exceptional flows. You do not have to describe all the flow of events,as long as you employ and exemplify all operations of the participating objects. Verytrivial flows can be omitted, such as those that concern only one object.Examples of exceptional flows include the following:• Error handling. What should the system do if an error is encountered?• Time-out handling. If the user does not reply within a certain period, the use caseshould take some special measures.• Handling of erroneous input to the objects that participate in the use case (forexample, incorrect user input).Examples of optional flows include the following:• The actor decides-from a number of options — what the system is to do next.• The subsequent flow of events depends on the value of stored attributes orrelationships.• The subsequent flow of events depends on the type of data to be processed.You can use either Collaboration or Sequence diagrams.6 - 34
Module 6 - Use-Case AnalysisCollaboration Diagrams vs. Sequence DiagramsCollaboration Diagrams vs. Sequence Diagrams• CollaborationDiagrams• Show relationships inaddition to interactions• Better for visualizingpatterns of collaboration• Better for visualizing allof the effects on a givenobject• Easier to use forbrainstorming sessions• Sequence Diagrams• Show the explicitsequence of messages• Better for visualizingoverall flow• Better for real-timespecifications and forcomplex scenariosMastering Object Oriented Analysis and Design with UMLCopyright © 2003 Rational Software, all rights reserved 35Sequence diagrams and Collaboration diagrams express similar information, but showit in different ways.Collaboration diagrams emphasize the structural collaboration of a society of objectsand provide a clearer picture of the patterns of relationships and control that existamongst the objects participating in a use case. Collaboration diagrams show morestructural information (that is, the relationships among objects). Collaborationdiagrams are better for understanding all the effects on a given object and forprocedural design.Sequence diagrams show the explicit sequence of messages and are better for realtimespecifications and for complex scenarios. A Sequence diagram includeschronological sequences, but does not include object relationships. Sequencenumbers are often omitted in Sequence diagrams, in which the physical location ofthe arrow shows the relative sequence. On Sequence diagrams, the time dimension iseasier to read, operations and parameters are easier to present, and a larger numberof objects are easier to manage than in Collaboration diagrams.Both Sequence and Collaboration diagrams allow you to capture semantics of theuse-case flow of events; they help identify objects, classes, interactions, andresponsibilities; and they help validate the architecture.6 - 35
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