CS427a: Object-Oriented Programming Design Patterns for Flexible and Reusable designReusability, Flexibility, and MaintainabilityThe Waterfall Software ProcessWhy a Pure Waterfall Process is Usually Not Practical The Spiral ProcessAdvantage of OO DesignAspect of ReusabilityMaking a Class Re-usableReducing Dependency Among ClassesAspect of FlexibilitySome Techniques to Achieve FlexibilityRoadmapWhat is a Design PatternUML/OMT NotationCS427a: Object-Oriented Programming Design Patterns for Flexible and Reusable designMichael J. Fischer(from slides by Y. Richard Yang)Lecture 22bNovember 17, 2011Reusability, Flexibility, and Maintainability•One thing constant in software development is CHANGE•For software that is used over a period of years, the cost of keeping it current in the face of changing needs often exceeds the cost of originally developing it.•A key need in software design is the ability for maintenance and modification to keep abreast of changes.The Waterfall Software ProcesstimeRequirementsAnalysisDesignMilestone(s)Phases (activities)ImplementationTestingMaintenanceRelease product XTwo phases may occur at the same time for a short periodWhy a Pure Waterfall Process is Usually Not Practical Don’t know up front everything wanted and needed–Usually hard to visualize every detail in advance To gain confidence in an estimate, we need to design and actually implement parts, especially the riskiest ones, this may probably lead to modify requirements as a resultWe often need to execute intermediate builds–Stakeholders need to gain confidence–Designers and developers need confirmation they're building what’s needed and wantedTeam members can't be idle while the requirements are being completedThe Spiral Processtime 1 RequirementsanalysisDesignCodingTesting1Iteration # 11 2 2 23 3 3 Product released XIntermediate version* completed X*typically a prototypeM I L E S T O N E S 2 3 2 3 1Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.Advantage of OO DesignOO systems exhibit recurring structures that promote–Abstraction–Modularity–Flexibility–Extensibility–EleganceAspect of Reusability• Classes – in source code form–Thus, we write generic code whenever possible• Assemblies of related classes A toolkittoolkit is a library of reusable classes designed to provide useful, general-purpose functionality.•E.g., C++ standard library, BoostAn application framework application framework is a specific set of classes that cooperate closely with each other and together embody a reusable design for a category of problems.•E.g., Java APIs (Applet, Thr ead, etc), gtkmm•Design patternMaking a Class Re-usableDefine a useful abstraction–attain broad applicabilityReduce dependencies on other classes…Reducing Dependency Among ClassesStudentReplace …Coursewith …Student CourseEnrollmentAspect of Flexibility•Making small variation to existing functionality•Adding new kinds of functionality•Changing functionalitySome Techniques to Achieve FlexibilityFlexibility Aspect: ability to … Some techniques… create objects in variable configurations determined at runtime“Creational” design patterns… create variable trees of objects or other structures at runtime “Structural” design patterns… change, recombine, or otherwise capture the mutual behavior of a set of objects“Behavioral” design patterns… create and store a possibly complex object of a class.Component… configure objects of predefined complex classes – or sets of classes – so as to interact in many waysComponentRoadmap•We will focus on flexibility and reusability–It is important to remember that real systems also need to consider efficiency and robustness•We will start with design patterns, and then look into the design of some OO libraries/toolkit/framework•We will learn by examples:–Example is not another way to teach, it is the only way to teach. -- Albert EinsteinWhat is a Design Pattern•Abstracts a recurring design structure•Comprises class and/or objectdependenciesstructuresinteractionsconventions•Distills design experience•Names & specifies the design structure explicitly•Language- & implementation-independent•A “micro-architecture”14UML/OMT
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