Object-Oriented Design

Object-Oriented Design:CRC cards and design principles15-413: Introduction to Software EngineeringJonathan AldrichAnnouncements Iteration reports/plans due Wednesday You should already be working on last iteration 2 weeks, 12 hours/personFocus on quality assurance, final value to clients Please fill out faculty course evaluations I will supplement the university form with aanonymous Blackboard survey with more detailedquestionsWe appreciate your input! Next week Mon/Wed: present tool evaluation resultsWed (second half): review for exam Please send me what you’d like to cover!Friday: final exam28 November 20051

OO Design Challenge What classes should be in a system? What should each one do? How should they collaborate? Principles Hide things likely to changeMaximize cohesion Minimize coupling The data and operations of a class should all be relatedOften helpful to look at concepts from problem domainSimplify interactions between classes as much aspossibleProtect user from errorsIterate by considering alternative designs28 November 2005CRC Cards A lightweight collaborative design method Class, Responsibility, Collaboration Responsibilities: tasks to perform, data to trackCollaborations: other objects this object works withClass orations:28 November 20052

CRC Cards A lightweight collaborative design method Class, Responsibility, Collaboration Responsibilities: tasks to perform, data to trackCollaborations: other objects this object works withClass name:ShapeSuperclass:DrawableSubclasses:Circle, Rectange, Responsibilities:Collaborations:Knows location, sizePoint, BoundingBoxDraw self on canvasCanvasMove self28 November 2005CRC Cards A lightweight collaborative design method Class, Responsibility, Collaboration Responsibilities: tasks to perform, data to trackCollaborations: other objects this object works withClass ities:Collaborations:Knows drawablesDrawable, ListDraw selfDrawableDrawing operations28 November 20053

Serialization Framework Design Allow developers to use the frameworkto aid them in serializing data into a fileor databaseClass ities:Collaborations:Knows drawablesDrawable, ListDraw selfDrawableDrawing operations28 November 2005Refining CRC into a design How to go from CRC cards to OOinterfaces (e.g. UML)? Walk through scenarios Operations performed Communication between classes UML: Use case Sequence diagram Infer interface from operations in scenarios Iterate according to design criteria Design for changeCohesionCouplingRobustness to errors28 November 20054

Framework Design Issues Interfaces vs. classes Use interfaces when there are multipleimplementations, or when implementorsmay want to inherit code from elsewhere Use classes when there is useful code toinherit If both apply, provide an interface and anabstract class that implements it Correct extension Use final for all methods that are notspecifically intended for extension28 November 2005Framework Design Issues Correct framework use If possible, ensure that all constructorsresult in fully initialized object Don’t require calls to init or set methods later Check invariants on input and throwmeaningful error Better to increase usability even at cost ofperformance Use assert to make this efficient Use the type system Ensure type dependencies are captured ininterface types Parameterize where necessary28 November 20055

Framework Design Issues Prohibit illegal calls Hide methods clients shouldn’t be able to touch Hide internal objects Make the classes private, or just don’t return the instancefrom public methodsOrdering constraints Internal to frameworkOnly safe to call under certain conditions that othermethods checkNote: Java, C# make this hard because packages are flatIf method A returns an object needed for method B, thencan’t accidentally call B firstProvide read-only access to data structures thatclients shouldn’t modify directly28 November 20056

Object-Oriented Design: CRC cards and design principles 15-413: Introduction to Software Engineering Jonathan Aldrich 28 November 2005 Announcements Iteration reports/plans due Wednesday You should already be working on last iteration 2 weeks, 12 hours/person Focus on quality assurance, final value to clients