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Introduction to Software Technology 3. Software Design Klaus Ostermann Einführung in die Softwaretechnik 1
IntroductiontoSoftwareTechnology3.SoftwareDesign
KlausOstermann
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GoalofSoftwareDesign
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} Foreachdesiredprogrambehaviorthereareinfinitelymanyprogramsthathavethisbehavior} Whatarethedifferencesbetweenthevariants?} Whichvariantshouldwechoose?
} Sinceweusuallyhavetosynthesizeratherthanchoosethesolution…} Howcanwedesignavariantthathasthedesiredproperties?
Example
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} Sortingwithconfigurableorder,variantA
void sort(int[] list, String order) { … boolean mustswap; if (order.equals(“up”)) { mustswap = list[i] < list[j]; } else if (order.equals(“down”)) { mustswap = list[i] > list[j]; } … }
Example
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} Sortingwithconfigurableorder,variantBvoid sort(int[] list, Comparator cmp) { … boolean mustswap; mustswap = cmp.compare(list[i], list[j]); … } interface Comparator { boolean compare(int i, int j); } class UpComparator implements Comparator { boolean compare(int I, int j) { return i<j; }} class DownComparator implements Comparator { boolean compare(int I, int j) { return i>j; }}
(bytheway,thisdesigniscalled“strategypattern”)
QualityofaSoftwareDesign
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} Howcanwemeasuretheinternalqualityofasoftwaredesign?} Extensibility,Maintainability,Understandability,Readability,…} Robustnesstochange} LowCoupling&HighCohesion} Reusability} Allthesequalitiesaretypicallysummarizedbythetermmodularity
} …asopposedtoexternalquality} Correctness:Validimplementationofrequirements} EaseofUse} Resourceconsumption} Legalissues,politicalissues,…
Modularity
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Modularity
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} Asoftwareconstructionmethodismodularifithelpsdesignerstoproducesoftwaresystemsmadeofautonomouselementsconnectedbyacoherent,simplestructure
} Inthefollowingwe’llelaborateonthat:} Fivecriteria} FiveRules} FivePrinciples
FiveCriteria:ModularDecomposability
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AsoftwareconstructionmethodsatisfiesModularDecomposabilityifithelpsinthetaskofdecomposingasoftwareproblemintoasmallnumberoflesscomplexsubproblems,connectedbyasimplestructure,and
independentenoughtoallowfurtherworktoproceedseparatelyoneachofthem.
FiveCriteria:ModularDecomposability
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} ModularDecomposabilityimplies:DivisionofLaborpossible!
} Example:Top-DownDesign} Counter-Example:Productionofaglobalinitializationmodule
FiveCriteria:ModularComposability
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AmethodsatisfiesModularComposabilityifitfavorstheproductsofsoftwareelementswhichmaythenbefreelycombinedwitheachothertoproducenewsystems,
possiblyinanenvironmentquitedifferentfromtheoneinwhichtheywereinitiallydeveloped.
FiveCriteria:ModularComposability
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} Isdualtomodulardecomposability} Isdirectlyconnectedwithreusability
} Old“dream”ofprogramming:programmingasconstructionboxactivity
} Example1:Librarieshavebeenreusedsuccessfullyincountlessdomains
} Example2:UnixShellCommands} Counter-Example:Preprocessors
FiveCriteria:ModularUnderstandability
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AmethodfavorsModularUnderstandabilityifithelpsproducesoftwareinwhichahumanreadercanunderstandeachmodulewithouthavingtoknowtheothers,or,atworst,byhavingtoexamineonlyafewoftheothers.
FiveCriteria:ModularUnderstandability
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} Importantformaintenance} Appliestoallsoftwareartifacts,notjustcode} Counter-example:Sequentialdependenciesbetweenmodules
FiveCriteria:ModularContinuity
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AmethodsatisfiesModularContinuityif,inthesoftwarearchitecturesthatityields,asmallchangeintheproblemspecificationwilltriggerachangeofjustonemodule,ora
smallnumberofmodules.
FiveCriteria:ModularContinuity
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} Example1:Symbolicconstants(asopposedtomagicnumbers)
} Example2:Hidingdatarepresentationbehindaninterface
} Counter-Example:Programdesignsdependingonfragiledetailsofhardwareorcompiler
FiveCriteria:ModularProtection
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AmethodsatisfiedModularProtectionifityieldsarchitecturesinwhichtheeffectofanabnormal
conditionoccurringatruntimeinamodulewillremainconfinedtothatmodule,oratworstwillonlypropagate
toafewneighboringmodules.
FiveCriteria:ModularProtection
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} Motivation:Bigsoftwarewillalwayscontainbugsetc.,failuresunavoidable
} Example:DefensiveProgramming} Counter-Example:Anerroneousnullpointerinonemoduleleadstoanerrorinadifferentmodule
FiveRules
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} FiveRuleswillfollowwhichwemustobservetoensurehigh-qualitydesign
FiveRules:DirectMapping
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Themodularstructuredevisedintheprocessofbuildingasoftwaresystemshouldremaincompatiblewithanymodularstructuredevisedintheprocessof
modelingtheproblemdomain.
FiveRules:DirectMapping
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} Followsfromcontinuityanddecomposability} A.k.a.“lowrepresentationalgap”[C.Larman]
FiveRules:FewInterfaces
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Iftwomodulescommunicate,theyshouldexchangeaslittleinformationaspossible
FiveRules:FewInterfaces
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} Wanttopologywithfewconnections} Followsfromcontinuityandprotection;otherwisechanges/errorswouldpropagatemore
FiveRules:SmallInterfaces
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Iftwomodulescommunicate,theyshouldexchangeaslittleinformationaspossible
FiveRules:SmallInterfaces
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} Followsfromcontinuityandprotection,requiredforcomposability
} Counter-Example:BigInterfacesJ
FiveRules:ExplicitInterfaces
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WhenevertwomodulesAandBcommunicate,thismustbeobviousfromtheinterfaceofAorBorboth.
FiveRules:ExplicitInterfaces
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} Counter-Example1:GlobalVariables} Counter-Example2:Aliasing–mutationofsharedheapstructures
Intermezzo:LawofDemeter(LoD)
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} LoD:Eachmoduleshouldhaveonlylimitedknowledgeaboutotherunits:onlyunits"closely"relatedtothecurrentunit
} Inparticular:Don’ttalktostrangers!} Forinstance,noa.getB().getC().foo()} Motivatedbycontinuity
FiveRules:InformationHiding
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Thedesignerofeverymodulemustselectasubsetofthemodule’spropertiesastheofficialinformationaboutthemodule,tobemadeavailabletoauthorsofclientmodules.
FiveRules:InformationHiding
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FiveRules:InformationHiding
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} Reynolds’parableaboutcomplexnumbers…} Impliedbycontinuity} Theiceberganalogyisslightlymisleading,sinceaninterfacealsoabstractsovertheimplementation
FivePrinciples
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} Fromtheprecedingrules,andindirectlyfromthecriteria,fiveprinciplesofsoftwareconstructionfollow:} TheLinguisticModularUnitsprinciple.} TheSelf-Documentationprinciple.} TheUniformAccessprinciple.} TheOpen-Closedprinciple.} TheSingleChoiceprinciple.
FivePrinciples:LinguisticModularUnits
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Modulesmustcorrespondtosyntacticunitsinthelanguageused.
FivePrinciples:LinguisticModularUnits
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} Excludesmethodsthatsuggestacertainmoduleconceptandalanguagethatdoesnotofferthecorrespondingmodularconstruct
} Impliedbycontinuityanddirectmapping} Bothrequiredirectcorrespondencebetweenspecification,design,andimplementationmodules
} Impliedbydecomposabilityandcomposability} Theimplementationofeverytaskmustresultinawell-delimitedsyntacticunit
FivePrinciples:Self-DocumentationPrinciple
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Thedesignerofamoduleshouldstrivetomakeallinformationaboutthe
modulepartofthemoduleitself.
FivePrinciples:Self-DocumentationPrinciple
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} Precludeskeepinginformationaboutthemoduleinaseparatedocument
} Justification:} Modularunderstandabilityprinciple} Continuity,hardtokeepseparatedocuments“insync”} Ingeneral:Changeability
} Traditional“heavy-weight”SEprocesseshaveadifferentpointofviewonthis
} Seealsomaterialofpreviouslectureaboutliterateprogrammingandgooddocumentationingeneral
FivePrinciples:UniformAccess
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Allservicesofferedbyamoduleshouldbeavailablethroughauniform
notation,whichdoesnotbetraywhethertheyareimplementedthrough
storageorthroughcomputation
FivePrinciples:UniformAccess
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} Justification:Continuitycriterion,specialcaseofinformationhiding
} Example:Thebalanceofanaccountmaybestoredasdata,oritmaybecomputedfromthelistoftransactions} Thisisatime/spacetradeoff} Differenceshouldnotbevisibleforaclient
} Somelanguagessupportthisprincipledirectly} Ruby,Eiffel,Python(*),Smalltalk(*)} Adesignconventioninotherlanguages
} “getter/setter”methodsinJava
FivePrinciples:Open-ClosedPrinciple
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Modulesshouldbebothopenandclosed.
FivePrinciples:Open-ClosedPrinciple
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} Amoduleissaidtobeopenifitisstillavailableforextension.} Forexample,itshouldbepossibletoexpanditssetofoperationsoraddfieldstoitsdatastructures.
} Amoduleissaidtobeclosedifitisavailableforusebyothermodules.} Well-definedstableinterface} Canbecompiled,stored,…
} Motivation:Opennessforfutureextensions,closednessforcomposition
FivePrinciples:Open-ClosedPrinciple
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} Example:ClassesinOOlanguagesareopenthroughinheritanceyetcanbeusedthroughconstructorcalls
} Counter-Example:PackagesinJava} Whathappensifmodulesarenotopen:
} “Monkeypatching”inJavascript
(youdon’tneedtounderstandthisexampleindetail)
eval("getBrowser().removeTab ="+ getBrowser().removeTab.toString().replace( 'this.addTab("about:blank");', 'if (SpeedDial.loadInLastTab) {this.addTab(' +'"chrome://speeddial/content/speeddial.xul"' +')} else { this.addTab("about:blank")}' ));
FivePrinciples:SingleChoice
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Wheneverasoftwaresystemmustsupportasetofalternatives,oneandonlyonemoduleinthesystemshouldknowtheirexhaustivelist.
FivePrinciples:SingleChoice
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} SpecialcaseoftheDRYprinciple(Don’trepeatyourself):} Everypieceofknowledgemusthaveasingle,unambiguous,authoritativerepresentationwithinasystem
} Typicalexamplesofviolationsofthisprinciple:} MultipleIf/then/elseorcasestatementswithidenticalconditions
} PatternMatchinginfunctionalprogramming
} Isaconsequenceofopen-closedprinciple} Isaformofinformationhiding
FivePrinciples:SingleChoice
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} AvoidedinOOlanguagesbyusingsubtypingandlatebinding} Cf.sortingexampleinthebeginningofthispresentation} However,OOviolatestheprincipleitselfinthatthelistofmethodsinaninterfaceisclosedandreplicatedinallimplementations
} Simpleprinciplebutquitehardtorealize} Cf.solutionstotheso-called‘expressionproblem’
Discussion
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} Examinethemodularstructuresofanyprogramminglanguagewhichyouknow
} Assesshowtheysupportthecriteriaandprinciplespresentedinthislecture
Reusability
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Reusability
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} Olddreamof“mass-producedsoftwarecomponents”byMcIlroyatNATOconference} Idea:Mimicengineering} However,cf.pitfallsofconfusingproductandplan,seefirstlecture
} Other(equallyproblematic)commonanalogy:SoftwareasLEGO
} Althoughthesevisionsarebothproblematic,reuseisamajorconcerninsoftwaredesign
GoalsofReuse
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} Timeliness:Havinglesssoftwaretodevelopmeansthatwecanbuilditfaster
} Decreasedmaintenanceeffort:Ifsomeoneelseisresponsibleforthesoftwareheorsheisalsoresponsibleforitsevolution} Andhedoessoonceforallclients:Fixonce,profitmanytimes
} Redundancyisasourceofinconsistency.Reusecanavoidredundancy.
} Reliability} Ifsoftwareisreusedmanytimesmoreeffortisputintosoftware
quality} Efficiency
} Bythesameargumentsasreliability} Consistency} Investment
Whatshouldwereuse?
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} Reuseofpersonnel} E.g.avoidlossofknow-howbytransferringsoftwareengineersfromprojecttoproject
} Reuseofdesignsandspecifications} Notionofdesign/specificationassoftwareproductindependentofimplementationisdubious,cf.self-documentationprinciple
} Theimplementationisthe(detailed)design} Withcontinuity,directmappingetc.,thedistinctionbetweenreusingmodulesandreusingdesignstendstofadeaway} High-LevelDesign(e.g.classdiagram)islikeatableofcontentsofthedetaileddesign(implementation)
Whatshouldwereuse?
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} DesignPatterns} =architecturalideasapplicableacrossabroadrangeofapplicationdomains
} Animportantformofarchitecturereusewhichwewillstudyindetaillateron
} SourceCode} Wewanttoreusesomethingthat“runs”,sinceeventuallywewanttorunprograms(“Bubblesdon’tcrash!”)
Reuseofabstractedmodules
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} Ifwewanttoreusesourcecode,inwhatformshouldwereuseit?
} Reusingitintheformoftheactualsourcetextisproblematic} Itremovesinformationhiding:Usersmayrelyonimplementationdetailsonlyvisiblebystudyingthesourcetext
} Developersofsoftwaredistributedinsourcetextmaybetemptedtoviolatemodularityrules
} Whatwewanttoreuseareabstractedmodules} Softwaredescribedthroughawell-definedinterfacethathidesimplementationdetails
} Thesoftwaremaystillbedistributedinsourcetext;thedifferenceistheprimarysourceofinformationaboutit
ReuseandPatterns
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} Areusablepieceofcodeabstractsovercertainpatterns(patternsinthegeneralsense,notdesignpatterns)
} Conversely,aprogramcontainsopportunitiesforreuseifitcontainspatterns} Canbeformalizedas:TheKolmogorovcomplexityoftheprogramissmallerthantheprogram
} Let’slookatsometypicalpatternsofredundancy(patternsofpatternsJ)andwaystoabstractoverthem
Reuse:Constants
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} Sometimesthesamenumber/string/…showsupmanytimesinaprogram(“magicnumber”)} Asourceofinconsistency,ifthenumber/stringmaychange
} E.g.,thevalueofPiisunlikelytochangeJ
} Nodocumentationoftheintentionofthenumber/string/…} Reusebydefiningaconstant
} Canreusetheconstantinallplaceswhereitisneeded} Canchangeinoneplace} Cangivemeaningfulnametoconstant
Reuse:CommonSubexpressions
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} Codemaycontainthesamesubexpressionsinmanyplaces,e.g.,5*89+3orx+y/2 } Mustkeeptrackofscopingrulesifvariablesareinvolved
} Canabstractoverpatternbyusingalocalvariable} Ifthesubexpressionperformsside-effects(I/O,mutation,…)canabstractoverpatternusingprocedure/method
} Thismayormaynotyieldmoreefficientcode} Dependingonhowsmartthecompileris
} Moreimportantly,itwilltypicallyimprovethecode} Givestheideabehindthecommonsubexpressionaname} Lessredundancy,…
Reuse:Almostcommonsubexpressions
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} Codecontainssimilar,butnotidenticalsubexpressions} Differonlyinthevalueof“first-class”expressions
} Canabstractoverpatternusingproceduralabstraction(or,methods,functions,lambdas,…)
} E.g.average(x,y)=x+y/2insteadofcomputingaveragesinplace
} Givesabstractionaname,avoidsredundancy,…
Reuse:Almostcommonsubexpressions
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} Whatiftheexpressionsdifferinthesubroutinestheycall?} Nothingchangesinalanguagewithfirst-classsubroutines;canabstractoverthesecallsandturnthemintoparameters} E.g.functionallanguages
} InOOlanguages,thisproblemhasgivenrisetodesignpatternssuchas“strategy”and“templatemethod”whichwewilldiscussindetaillateron
Reuse:Almostcommonsubexpressions
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} Whatiftheexpressionsdifferinthetypestheyuse?} E.g.quicksortalgorithmonintegersvs.quicksortonstrings
} Canusegenerictypes!} List<T>sort<T>(List<T>in,Comparator<T>cmp){…}
Reuse:Similarclassdefinitions
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} Canfactoroutcommonalitiesincommonsuperclass} Latebindingandpossibilitytoextendfields/methodsallowonetospecializeinpowerfulways} However,subtypinglimitsthewayshowasubclasscanbedifferentfromasuperclass} “LiskovSubstitutionPrinciple”àSEDesignLecture
} However,differentpointsofviewonwhentouseinheritance} “Scandinavian”styleofinheritance:Mechanismforconceptualspecialization,drivenbymodeling
} “American”styleofinheritance:Reuseandpatchcode
Somepatternsarehardtoabstractover,though
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ReuseandEfficiency
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} Abstractdesignscanbelessefficient(intermsofspace/timebehavior)thantheirredundantexpansions} “Alldesignisjustaddingmorelevelsofindirection”} Aconflictbetweenreuseandefficiency?
} Mostofthetimenotarealconcern} Compilertechniquesremovemuchabstractionoverhead
} Inlining,devirtualization,tail-calloptimization,partialevaluation,staging,…
} Costofmethod/functioncallingrarelythemainbottleneck} Onlyafractionofthecodeisperformance-criticalanyway
} Canbeaconcernifveryinefficientabstractiontechniquesareused} E.g.,reflection
Literature
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} BertrandMeyer,Object-OrientedSoftwareConstruction,PrenticeHall,1997[Chapter3,4]
