Lecture 6 Design Evaluation and Basic Patterns

Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development (3rd Edition)

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Copyright W. Howden 1 Lecture 6: Design Evaluation and Intro to OO Design Patterns
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Copyright W. Howden 2 Principles Domain Simulation Information Hiding Cohesion and Coupling Design Patterns
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Copyright W. Howden 3 Domain Simulation Structure of system reflects structure of application domain Concepts and relationships derived from domain analysis Concept classes -> system classes Concept associations -> static class relationships
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Copyright W. Howden 4 Domain Simulation Benefits Domain changes less likely than functionality changes Functionality changes less likely to upset basic design structure
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Copyright W. Howden 5 Information Hiding Design decisions “hidden in modules” – Changes to implementation details can be localized Access to information via public methods/routines rather than direct access An original motivation for OO approach
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Copyright W. Howden 6 Information Hiding Examples DS Subsystems – GUI, Domain Logic, DataBase – DL and DB are components with subsystem proxy interfaces All requests for DL and DB services are made to those proxies Proxies hide how these responsibilities are designed and implemented
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Copyright W. Howden 7 Cohesion and Coupling Design expressed in terms of inter-related components • Cohesion – Things in components belong together – Cohesion should be strong • Coupling – Connections between components – Coupling should be weak
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Copyright W. Howden 8 Extreme Cases Single component – Bad cohesion – Good coupling Components contain a “single thing” – Good cohesion – Bad coupling Design is a balance between cohesion and coupling
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Copyright W. Howden 9 General Levels of Cohesion • Coincidental • Logical • Temporal • Procedural Data Flow • Communicational • Functional
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Copyright W. Howden 10 Coincidental Accidental, no functional rationale Division of code into modules determined by size consideration Isolation of commonly used but not functionally related pieces of code
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Copyright W. Howden 11 Logical Group things together that are the same kind of thing E.g. Module containing all the error handling routines E.g. Initialization module that opens files, initializes variables, etc.
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Copyright W. Howden 12 Temporal Group things together than can be done at the same time E.g. Initialization code E.g. Termination/clean up
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Copyright W. Howden
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