计算机科学导论课件：Software-Engineering

Chapter7

SoftwareEngineeringObjectivesUnderstandthesoftwarelifecycle.Describethedevelopmentprocessmodels..Understandtheconceptofmodularityinsoftwareengineering.Understandtheimportanceofqualityinsoftwareengineering.Understandtheroleofdocumentationinsoftwareengineering.SystemlifecycleSoftware,likemanyotherproducts,goesthroughacycleofrepeatingphases.SystemdevelopmentphasesThedevelopmentprocessinthesoftwarelifecycleinvolves4phases.AnalysisDefinetheuser(genericorspecific?)Definetheneeds(expectations)DefinetherequirementsDefinethemethods(tomeetrequirements)DesignphaseThedesignphasedefineshowthesystemwillaccomplishwhatwasdefinedintheanalysisphase.ModularityToolsStructurechartUMLImplementationphaseToolsFlowchartPseudocodeCodingDevelopmentprocessmodelsDevelopmentprocessmodelsIntheincrementalmodel,theprocessisdevelopedinaseriesofsteps.Firstcompleteasimplifiedversionofthewholepackage.Graduallyaddmoredetails.ModularityModularitymeansbreakingalargeprojectintosmallerpartsthatcanbeunderstoodandhandledeasily.ToolsStructurechartClassdiagramUnifiedModelingLanguage(UML)Modularity-couplingCouplingisameasureofhowtightlytwomodulesareboundtoeachother.Loosecouplingismoredesirable,because:IndependentfunctionsaremoreresuableLesslikelytocauseproblemsMaintenancemodificationsareeasierModularity-couplingDatacoupling:passesonlytheminimumrequireddatafromthecallingfunctiontothecalledfunction.Mostdesirable.Stampcoupling:whentheparametersarecompositeobjectssuchasarrays.Controlcoupling:passesflagstodirectthecontrolflowofafunction.Globalcoupling:usesglobalvariables.Shouldneverbeused.Contentcoupling:whenonefunctionrefersdirectlytothedataorstatementsinanotherfunction.Modularity-cohesionCohesionisameasureofhowcloselytheprocessesinaprogramarerelated.Functioncohesion:Amodulewithfunctioncohesioncontainonlyoneprocess.Forexample,inprintingareport,thereportfunctionshouldcall3lowerlevelfunctions:onetogetthedata,onetoformatandprintthereportheader,andonetoformatandprintthedata.OnlyonethingInoneplaceModularity-cohesionSequentialcohesion:Amodulewithsequentialcohesioncontainstwoormorerelatedtasksthatarecloselytiedtogether,usuallywiththeoutputofoneflowingasinputtotheother.Example:thecalculationsforasale:ExtenditempricesSumitemsCalculatesalestaxCalculatetotalModularity-cohesionCommunicationalcohesion:Itcombinesprocessesthatworkonthesamedata.Example:afunctionthatreadsaninventoryfile,printsthecurrentstatusoftheparts,andthencheckstoseeifanypartsneedtobeordered.Thefirst3levelsofcohesionareallconsideredgoodstructuredprogrammingprinciples.Beyondthispoint,easeofunderstandingandimplementation,maintanability,andaccuracybegintodroprapidly.Modularity-cohesionThe4thlevel,proceduralcohesion,combinesunrelatedprocessesthatarelinkedbyacontrolflow.The5thlevel,temporalcohesion,combinesunrelatedprocessesthatalwaysoccurtogether.Thelast2levelsareseldomfoundinprogramstoday.Logicalcohesioncombinesprocessesthatarerelatedonlybytheentitythatcontrolsthem.Coincidentalcohesioncombinesprocessesthatareunrelated.QualitydefinedQualitysoftwareisdefinedas:Softwarethatsatisfiestheusers’explicitandimplicitrequirements,iswelldocumented,meetstheoperatingstandardsoftheorganization,andrunsefficientlyonthehardwareforwhichitwasdeveloped.Ifyouwanttoattainit,youhavetobeabletomeasureit.Wheneverpossible,thesemeasurementshouldbequantitative.Forexample:#oferrorsperthousandlinesofcode,themeantimebetweenfailuresforsoftwaresystems.QualityfactorsOperabilityAccuracycanbemeasuredbysuchmetricsasmeantimebetweenfailures,numberofbugsperthousandlinesofcode,andnumberofuserrequestforchange.Efficiencyis,byandlarge,asubjectiveterm.Insomecases,theuserwillspecifyaperformancestandard,suchasareal-timeresponsethatmustbereceivedwithin1second95percentofthetime.Reliabilityisreallythesumoftheotherfactors.OperabilityMaintainabilityTransferabilityTransferabilityreferstotheabilitytomovedataand/orasystemfromoneplatformtoanotherandtoreusecode.Codereusability:isthelikelihoodasegmentofstructuredcodecanbeusedagaintoaddnewfunctionalitieswithslightornomodification.Interoperability:istheabilityofsendingdatatoothersystems.Portability:istheabilitytomovesoftwarefromonehardwareplatformtoanother.ThequalitycircleThequalitycircleThequalitycircleDocumentationUserdocumentation:traditionallycalledamanual,userdocumentationshowshowtousethepackagestepbystep.Agoodusermanualcanbeaverypowerfulmarketingtool.Systemdocumentaiondefinesthepackageitself.SystemdocumentationDocumentationintheanalysisphase.Infocollectedshouldbecarefullydocumented.Analystshoulddefinethesourceofinfo.Requirementsandmethodschosenmustbeclearlystatedwiththerationalebehindthem.SystemdocumentationDocumentationinthedesignphase.Toolsusedinthefinalcopymustbed