© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 1XXIII. Class DesignXXIII. Class DesignWhat is Class Design?Types of Design ClassesClass Specifications and InterfacesComponents, Sub-Systems and PackagesCohesion and CouplingDesigning AssociationsIntegrity ConstraintsReferential, Dependency and Domain Integrity© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 2Class DesignClass Design Within the context of architectural design, classdesign: Produces full definitions of classes, associations,algorithms and interfaces of operations; Adds classes that will be useful duringimplementation; Defines object interactions and object lifetimes interms of interaction and state diagrams; Optimises data structures and algorithms.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 3Input/Output for Class DesignInput/Output for Class Design The input is assumed to consist of: Use cases that describe functional requirements;also sequence, state/activity diagrams thatdescribe the use cases in more detail; Class diagrams that describe the kinds of thingsthe information system will be managinginformation about. The outputs of class design are: Class packages which describe the overallsoftware architecture of the new system; Supporting sequence, state/activity diagrams thatgive additional details about the design.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 4Types of Design ClassesTypes of Design Classes Most classes defined during requirements analysisrepresent objects about which information will bestored in the system database. Assuming a 4-tier layered architecture, we distinguishfour types of classes:Persistent database classes (D), correspond toapplication classes and describe what will bestored persistently in the system database;Entity classes (E) represent in-memory, run-timedata structures for persistent database classes;Boundary classes (B) specify interface functions;Control classes (C) specify business logicfunctions.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 5Class SpecificationsClass Specifications Attribute signaturename: `:´ type-expr `=´ init-value `{property-string}´ Operation signature Operation name: `(´ param-list ´)´ `:´ return-type-expr Object Visibility + Public -- feature directly accessible by any class; - Private -- feature may only be used by the classthat includes it; # Protected -- feature maybe used by either theclass that includes it or by a subclass of that class;© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 6AnAnExampleExampleDatabaseDatabaseClassClass<<database>>© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 7AnAnExampleExampleEntityEntityClassClass <<entity>> Customername:Stringaddr: StringlistOfAccts:Listcreate()Notes: Customer objects are created by accessing theCustomerDB and BankAccounts part of the database tobuild a single Customer object which collects allaccount information about a customer.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 8AnAnExampleExampleControlControlClassClass <<control>>TransferAmounttransfer(acc1, acc2, amount)AnAnExampleExampleBoundaryBoundaryClassClassLook at examples of view andcontrol classes from the previoussection.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 9Class InterfacesClass Interfaces An interfaceinterface is a group of externally visible (public)operations. An interface is like a class, but contains no internalstructure, has no attributes, no associations and noimplementation of its operations. The realizes realizes relationship indicates that the targetclass supports at least the operations listed in theinterface© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 10ClassClassInterfacesInterfaces© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 11Class DesignClass Design So, a class design consists of a set of packageswhich contain classes and other packages, and whichrepresent components or sub-systems. The grouping of classes into packages may be donefrom several different points of view: By architectural tier -- Boundary, Control, Entity,Database; By functional relationship -- MVC; By authorship -- who designed what; …© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 12Cohesion and CouplingCohesion and CouplingCriteria for good sub-system/package design:Coupling measures the degree ofinterconnectedness between designclasses/components/sub-systems/packages. The degree of coupling is reflected by the number oflinks a class has, and by the degree of interaction theclass has with other classes. Low coupling is preferrable in a design for many goodreasons, e.g., easier to understand and modify thedesign.Cohesion, on the other hand, measures the degreeto which an element (class/component/sub-system/package) contributes to a single purpose. Of course, we want a highly cohesive design.© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 13 Interaction coupling Measures number of message typesand the number of parameterspassed with these message types; Should be kept to a minimum inorder to reduce the possibility ofchanges rippling through interfaces; Inheritance coupling Degree to which a subclass actuallyneeds the features it inherits; A subclass with unnecessaryattributes or operations is morecomplex than it needs to be.Minimizing CouplingMinimizing Coupling© 2004 Jaelson Castro and John MylopoulosCSC340Information Systems Analysis and DesignClass Design -- 14 Operation cohesion Measures degree to which anoperation focuses on a singlefunctional requirement. Good design produces highlycohesive operations, each ofwhich deals with a singlefunctional requirement. Class cohesion Degree to which a class isfocused on a
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