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IntroductiontoSoftwareTechnologySoftwareQuality
KlausOstermann
SomeslidesonrefactoringadaptedfromCS246courseatUWaterloo
EinführungindieSoftwaretechnik1
SoftwareQuality
EinführungindieSoftwaretechnik2
} Howcanwemaintainorimprovethequalityofsoftware?
} Whatissoftwarequality,anyway?} Correctimplementationofrequirementsspecification} Designquality,modularity
} Extensibility,Maintainability,Understandability,Readability,Reusability…
} Robustnesstochange} LowCoupling,HighCohesion
} Reliability,FaultTolerance,Testability,Performance,…
SoftwareQuality
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} Howcanwemeasuresoftwarequality?
} Whataboutsoftwaremetrics?Whatisasoftwaremetric?
“Youcan’tcontrolwhatyoucan'tmeasure”TomDeMarco,1986
Measurementistheempirical,objectiveassignmentofnumbers,accordingtoarulederivedfromamodelor
theory,toattributesofobjectsoreventswiththeintentofdescribingthem.
Kaner&Cem,“SoftwareEngineerMetrics:Whatdotheymeasureandhowdoweknow?”
ExamplesofSoftwareMetrics
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} LinesofCode(LoC)} Bugsperlineofcode} Commentdensity} Cyclomaticcomplexity
} measuresthenumberoflinearlyindependentpathsthroughaprogram'ssourcecode
} Halsteadcomplexitymeasures} Derivesoftwarecomplexityfromnumbersof(distinct)operandsand
operators} Programexecutiontime} TestCoverage} Numberofclassesandinterfaces} Abstractness=ratioofabstractclassestototalnumberofclasses} …
Metricsarerarelyused
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} Fewcompaniesestablishmeasurementprograms,evenfewersucceedwiththem
} ThosethatusemetricsoftendosoonlytoconformtocriteriaestablishedincertainqualitystandardssuchasCMM} seeN.E.Fenton,"SoftwareMetrics:Successes,Failures&New
Directions”,1999
} Onecouldinterpretthisasevidenceoftheimmaturityandunprofessionalismofthefield} Aren’ttheengineerssosuccessfulbecausetheycanmeasure
quality?} Butthisisagainthemisleading“softwareasengineeringproduct”
analogythatwasalreadyrefutedinthefirstlectureofthiscourse
Metricsarerarelyuseduseful!
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} Formallydefinedmetricsareobjective,butwhatdothesemeasurementsmean?} Whatcanweconcludeaboutqualityifthecyclomaticcomplexityofourcodeis12?
} Answer:Nothing.
} Problem:Oftenunclearwhetherthemetriccorrelatestoanyusefulqualityfactor} Similartotheattemptofmeasuringtheintelligenceofapersonintermsoftheweightorcircumferenceofthebrain
} Ifafuturepotentialemployertellsyouabouttheirextensivesoftwaremetricssuite,run!J
TomDeMarco23yearslater…
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“Thebook’smostquotedlineisitsfirstsentence:“Youcan’tcontrolwhatyoucan’tmeasure.”Thislinecontainsarealtruth,butI’vebecomeincreasinglyuncomfortablewithmyuseofit.Implicitinthequoteisthatcontrolisanimportantaspect,maybethemostimportant,ofanysoftwareproject.Butitisn’t.Manyprojectshaveproceededwithoutmuchcontrolbutmanagedtoproducewonderfulproducts.”TomDeMarco,“SoftwareEngineering:AnIdeaWhoseTimeHasComeandGone”,2009.
SoftwareQuality
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} Ifmetricsdon’twork,howdoweassessthequalityofsoftware?
} Answer:Byacase-by-caseanalysisofeachindividualsoftwareproject} Reasonaboutdesignquality,extensibility,…} Reasonbycomparingtodesignsthathaveprovenuseful
} Designpatternsetc.
} Reasonbylookingfor“codesmells”andanti-patterns} Byextensivetestsuites} Byusinganalysistools:Staticanalysis,dynamicanalysis,formalverification
CodeSmells
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} Codesmell:Anysymptominthecodeofaprogramthatpossiblyindicatesadeeperproblem} TermpopularizedbyKentBeckinhis“Refactoring”book
} Commoncodesmells} Duplicatedcode} Longmethod,Largeclass} Featureenvy,inappropriateintimacy} Contrivedcomplexity
Anti-Pattern
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} Ananti-patternisapatternthatmaybecommonlyusedbutisineffectiveand/orcounterproductiveinpractice.
} Adescriptionofanti-patternsisuseful} Onecanrecognizetheforcesthatleadtotheirrepetitionandlearnhowothershaverefactoredthemselvesoutofthesebrokenpatterns.
} Examples:} Actionatadistance:Unexpectedinteractionbetweenotherwiseseparatedpartsofasystem
} Sequentialcoupling:Aclassthatrequiresitsmethodstobecalledinaparticularorder
} Circulardependency:Unnecessarydirectorindirectmutualdependenciesbetweensoftwaremodules
Anti-Pattern
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} MoreExamples:} AbstractionInversion:Re-implementlow-levelfunctionsusinghigh-levelfunctions
} Interfacebloat:Makinganinterfacesopowerfulthatitistoohardtoimplement
} Busyspinorbusywaiting:ConsumingCPUwhilewaitingforsomethingtohappen
} Seehttp://c2.com/cgi/wiki?AntiPatternsCatalogforanextensiveoverviewovercommonantipatterns
Whattodoaboutanti-patternsandcodesmells?
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} Refactoringsformalizetheideatosystematicallyremoveanti-patternsandcodesmells} Oftenformalizedtoadegreethatitcanbeautomatedintheformof
anIDEtool} Standardreference:à} Refactoringscanoftenbeunderstoodtoimprovethemodularityofthecode
} Refactoringsdonotchangethebehaviorofcode,e.g.,addafeature
} Let’slookatsomesmellsandassociatedrefactoringsinmoredetail!
Badsmellsandassociatedrefactorings
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} Duplicatedcode–“The#1badsmell”} Wehavealreadydiscussedhowtoabstractoverdifferentformsofduplicatedcodeinthelectureonreuse
} Sameexpressionintwomethodsinthesameclass?} Makeitaprivateauxiliaryroutineandparameterizeit
(Extract method refactoring)
} Samecodeintworelatedclasses?} Pushcommonalitiesintoclosestmutualancestorandparameterize} UsetemplatemethodDPforvariationinsubtasks
(Form template method refactoring)
Badsmellsandassociatedrefactorings
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} Duplicatedcode} Samecodeintwounrelatedclasses?
} Oughttheyberelated?¨ Introduceabstractparent(Extract class, Pull up method)
} Doesthecodereallybelongtojustoneclass?¨ Maketheotherclassintoaclient(Extract method)
} Canyouseparateoutthecommonalitiesintoasubpartorotherfunctionobject?¨ Makethemethodintoasubobjectofbothclasses.¨ StrategyDPallowsforpolymorphicvariationofmethods-as-objects
(Replace method with method object) = apply strategy pattern
ExtractClassRefactoring
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You have one class doing work that should be done by two. Create a new class and move the relevant fields and methods from the
old class into the new class.
Pull-UpMethodRefactoring
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You have methods with identical results on subclasses. Move them to the superclass
ExtractMethod Refactoring
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void printOwing() { printBanner(); //print details System.out.println ("name: " + _name); System.out.println ("amount " + getOutstanding()); }
void printOwing() { printBanner(); printDetails(getOutstanding()); } void printDetails (double outstanding) { System.out.println ("name: " + _name); System.out.println ("amount " + outstanding); }
You have a code fragment that can be grouped together. Turn the fragment into a method whose name explains the purpose of the method.
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Badsmellsincode
} Long method } Oftenasignof:
} Tryingtodotoomanythings} Poorlythoughtoutabstractionsandboundaries
} Besttothinkcarefullyaboutthemajortasksandhowtheyinter-relate.Beaggressive!} Breakupintosmallerprivatemethodswithintheclass
(Extract method) } Delegatesubtaskstosubobjectsthat“knowbest”(i.e.,templatemethodDP)(Extract class/method, Replace data value with object)
ReplaceDataValuewithObjectRefactoring
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You have a data item that needs additional data or behavior. Turn the data item into an object.
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Badsmellsincode
} Long method } Fowler’sheuristic:
} Whenyouseeacomment,makeamethod.} Often,acommentindicates:
¨ Thenextmajorstep¨ Somethingnon-obviouswhosedetailsdetractfromtheclarityoftheroutineasawhole.
} Ineithercase,thisisagoodspotto“breakitup”.
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Badsmellsincode} Large class
} i.e.,toomanydifferentsubpartsandmethods} Twostepsolution:
1. Gatherupthelittlepiecesintoaggregatesubparts.(Extract class, replace data value with object)
2. Delegatemethodstothenewsubparts.(Extract method)
} Likely,you’llnoticesomeunnecessarysubpartsthathavebeenhidingintheforest!
} Resisttheurgetomicromanagethesubparts!
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Badsmellsincode
} Large class } Counterexample:
} Libraryclassesoftenhavelarge,fatinterfaces(manymethods,manyparameters,lotsofoverloading)¨ Ifthemanymethodsexistforthepurposeofflexibility,that’s
OKinalibraryclass.
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Badsmellsincode
} Long parameter list } Longparameterlistsmakemethodsdifficultfor
clientstounderstand} Thisisoftenasymptomof
} Tryingtodotoomuch} …toofarfromhome} …withtoomanydisparatesubparts
"ifyourprocedurehasmorethanabouthalfadozenparameters,youprobablyforgotafew.“–AlanPerlis
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Badsmellsincode
} Long parameter list } Intheolddays,structuredprogrammingtaught
theuseofparameterizationasacureforglobalvariables.} Withmodules/OOP,objectshavemini-islandsof
statethatcanbereasonablytreatedas“global”tothemethods(yetarestillhiddenfromtherestoftheprogram).
} i.e.,Youdon’tneedtopassasubpartofyourselfasaparametertooneofyourownmethods.
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Badsmellsincode} Long parameter list
} Solution:} Tryingtodotoomuch?
¨ Breakupintosub-tasks(Extract method)
} …toofarfromhome?¨ Localizepassingofparameters;don’tjustpassdownseverallayers
ofcalls(Preserve whole object, introduce parameter object)
} …withtoomanydisparatesubparts?¨ Gatherupparametersintoaggregatesubparts¨ Yourmethodinterfaceswillbemucheasiertounderstand!
(Preserve whole object, introduce parameter object)
PreserveWholeObjectRefactoring
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int low = daysTempRange().getLow(); int high = daysTempRange().getHigh(); withinPlan = plan.withinRange(low, high);
withinPlan = plan.withinRange(daysTempRange());
You are getting several values from an object and passing these values as parameters in a method call.
Send the whole object instead.
IntroduceParameterObjectRefactoring
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You have a group of parameters that naturally go together. Replace them with an object.
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Badsmellsincode
} Divergent change } Occurswhenoneclassiscommonlychangedindifferentwaysfor
differentreasons} Likely,thisclassistryingtodotoomuchandcontainstoomany
unrelatedsubparts} Overtime,someclassesdevelopa“Godcomplex”
} Theyacquiresdetails/ownershipofsubpartsthatrightlybelongelsewhere
} Thisisasignofpoorcohesion} Unrelatedelementsinthesamecontainer
} Solution:} Breakitup,reshuffle,reconsiderrelationshipsandresponsibilities
(Extract class)
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Badsmellsincode} Shotgun surgery
} …theoppositeofdivergentchange} Eachtimeyouwanttomakeasingle,seeminglycoherent
change,youhavetochangelotsofclassesinlittleways
} Alsoaclassicsignofpoorcohesion} Relatedelementsarenotinthesamecontainer!
} Solution:} Looktodosomegathering,eitherinaneworexistingclass.
(Move method/field)
MoveMethodRefactoring
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A method is, or will be, using or used by more features of another class than the class on which it is defined.
Create a new method with a similar body in the class it uses most. Either turn the old method into a simple delegation, or remove it altogether
MoveFieldRefactoring
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A field is, or will be, used by another class more than the class on which it is defined. Create a new field in the target class, and change all its users.
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Badsmellsincode} Feature envy
} Amethodseemsmoreinterestedinanotherclassthantheoneit’sdefinedine.g.,amethodA.m()callslotsofget/setmethodsofclassB
} Solution:} Movem()(orpartofit)intoB!
(Move method/field, extract method) } Exceptions:
} Visitor/iterator/strategyDPwherethewholepointistodecouplethedatafromthealgorithm¨ FeatureenvyismoreofanissuewhenbothAandBhaveinterestingdata
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Badsmellsincode} Data clumps
} Youseeasetofvariablesthatseemto“hangout”togethere.g.,passedasparameters,changed/accessedatthesametime
} Usually,thismeansthatthere’sacoherentsubobjectjustwaitingtoberecognizedandencapsulated
void Scene::setTitle (string titleText, int titleX, int titleY, Colour titleColour){…}
void Scene::getTitle (string& titleText,
int& titleX, int& titleY, Colour& titleColour){…}
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Badsmellsincode} Data clumps
} Intheexample,aTitleclassiswaitingtobeborn} Ifaclientknowshowtochangeatitle’sx,y,text,andcolour,
thenitknowsenoughtobeableto“rollitsown”Titleobjects.} However,thisdoesmeanthattheclientnowhastotalktoanother
class.
} Thiswillgreatlyshortenandsimplifyyourparameterlists(whichaidsunderstanding)andmakesyourclassconceptuallysimplertoo.
} Movingthedatamaycreatefeature envyinitially} Mayhavetoiterateonthedesignuntilitfeelsright.(Preserve whole object, extract class, introduce parameter
object)
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Badsmellsincode} Primitive obsession
} Allsubpartsofanobjectareinstancesofprimitivetypes(int, string, bool, double, etc.)e.g.,dates,currency,SIN,tel.#,ISBN,specialstringvalues
} Often,thesesmallobjectshaveinterestingandnon-trivialconstraintsthatcanbemodellede.g.,fixednumberofdigits/chars,checkdigits,specialvalues
} Solution:} Createsome“smallclasses”thatcanencapsulatecoherent
subsetsoftheprimitivedataandvalidateandenforcetheconstraints.
(Replace data value with object, extract class, introduce parameter object)
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Badsmellsincode} Switch statements
} Wesawthisbefore;here’sFowler’sexample:
Double getSpeed () { switch (_type) {
case EUROPEAN: return getBaseSpeed(); case AFRICAN: return getBaseSpeed() – getLoadFactor() * _numCoconuts; case NORWEGIAN_BLUE: return (_isNailed) ? 0 : getBaseSpeed(_voltage);
} }
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Badsmellsincode} Switch statements
} Thisisanexampleofalackofunderstandingpolymorphismandalackofencapsulation.
} Solution:} RedesignasapolymorphicmethodofPythonBird (Replace conditional with polymorphism, replace type code with
subclasses)
Replace conditional with polymorphism, replace type code with subclasses
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Replace conditional with polymorphism
replace type code with subclasses
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Badsmellsincode} Lazy class
} Classesthatdoesn’tdomuchthat’sdifferentfromotherclasses.} Ifthereareseveralsiblingclassesthatdon’texhibitpolymorphic
behaviouraldifferences,thenconsiderjustcollapsingthembackintotheparentandaddsomeparameters
} Often,lazy classesarelegaciesofambitiousdesignorarefactoringthatguttedtheclassofinterestingbehaviour
(Collapse hierarchy, inline class)
Collapse hierarchy, inline class
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A class isn't doing very much. Move all its features into another class and delete it.
A superclass and subclass are not very different. Merge them together.
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Badsmellsincode} Speculative generality
} “Wemightneedthisoneday…”} Fairenough,butdidyoureallyneeditafterall?} Extraclassesandfeaturesaddtocomplexity.
} “ExtremeProgramming”philosophy:} “Assimpleaspossiblebutnosimpler.”} “Ruleofthree”.
} Keepinmindthatrefactoringisanongoingprocess.} Ifyoureallydoneeditlater,youcanadditbackin.(Collapse hierarchy, inline class, remove parameter)
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Badsmellsincode} Message chains
} Aclientasksoneobjectforanotherobject,whichtheclientthenasksforyetanotherobject,whichtheclientthenasksforyetanotheranotherobject,
} Navigatingthiswaymeanstheclientiscoupledtothestructureofthenavigation.
} Anychangetotheintermediaterelationshipscausestheclienttohavetochange(cf.LawofDemeter)
} Solution:Hide delegate
Hide delegate refactoring
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A client is calling a delegate class of an object. Create methods on the server to hide the delegate.
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Badsmellsincode} Middle man
} “Allhardproblemsinsoftwareengineeringcanbesolvedbyanextralevelofindirection.”} OODPsprettywellallboildowntothis,albeitinquitecleverand
elegantways.
} Ifyounoticethatmanyofaclass’smethodsjustturnaroundandbegservicesofdelegatesubobjects,thebasicabstractionisprobablypoorlythoughtout.
} Anobjectshouldbemorethanthesumofitspartsintermsofbehaviours!
(Remove middle man, replace delegation with inheritance)
RemoveMiddleManRefactoring
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A class is doing too much simple delegation. Get the client to call the delegate directly
ReplaceDelegationwithInheritance
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You're using delegation and are often writing many simple delegations for the entire interface.
Make the delegating class a subclass of the delegate
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Badsmellsincode} Inappropriate intimacy
} Sharingofsecretsbetweenclasses,esp.outsideoftheholyboundsofinheritancee.g.,publicvariables,indiscriminatedefinitionsofget/setmethods,C++
friendship,protecteddatainclasses} Leadstodatacoupling,intimateknowledgeofinternalstructures
andimplementationdecisions.} Makesclientsbrittle,hardtoevolve,easytobreak.
} Solution:} Appropriateuseofget/setmethods} Rethinkbasicabstraction.} Mergeclassesifyoudiscover“truelove”(Move/extract method/field, change bidirectional association to
unidirectional, hide delegate)
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Badsmellsincode
} Alternative classes with different interfaces } Classes/methodsseemtoimplementthesame
orsimilarabstractionyetareotherwiseunrelated.
} Solution:} Movetheclasses“closer”together.
¨ Findacommoninterface,perhapsanABC.¨ Findacommonsubpartandremoveit.
(Extract[super]class,movemethod/field,renamemethod)
ExtractSuperclassRefactoring
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You have two classes with similar features. Create a superclass and move the common features to the superclass.
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Badsmellsincode
} Data class } Classconsistsof(simple)datafieldsandsimpleaccessor/mutator
methodsonly.} Solution:
} Havealookatusagepatternsintheclients} Trytoabstractsomecommonalitiesofusageintomethodsofthedata
classandmovesomefunctionalityover} “Dataclassesarelikechildren.TheyareOKasastartingpoint,but
toparticipateasagrownupobject,theyneedtotakeonsomeresponsibility.”
(Extract/move method)
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Badsmellsincode} Comments
} XPphilosophydiscouragescomments,ingeneral:} Instead,makemethodsshortanduselongidentifiers
} Inthecontextofrefactoring,Fowlerclaimsthatlongcommentsareoftenasignofopaque,complicated,inscrutablecode.} Theyaren’tagainstcommentssomuchasinfavourofself-
evidentcodingpractices.} Ratherthanexplainingopaquecode,restructureit!(Extract method/class, [many others applicable] …)
} Commentsarebestusedtodocumentrationalei.e.,explainwhyyoupickedoneapproachoveranother.
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Summary} Fowleretal.‘sRefactoringisawell-writtenbookthatsummarizesalotof
“bestpractices”ofOOD/OOPwithniceexamples.} ManybooksonOODheuristicsarevagueandlackconcreteadvice.} Mostoftheadviceinthisbookisaimedatlow-levelOOprogramming.
} i.e.,loops,variables,methodcalls,andclassdefinitions.} Nextobviousstepupinabstraction/scaleistoOODPs.
} i.e.,collaboratingclasses
} Thisisanexcellentbookfortheintermediate-levelOOprogrammer.} ExperiencedOOprogrammerswillhavediscoveredalotofthetechniques
alreadyontheirown.
} Manysourcesaboutrefactoringontheweb:} http://refactoring.com/
Testing
SomeslidesbyT.BallandJ.Aldrich
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Whytest?
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Testing:Challenges
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} Testingiscostly} Testeffectivenessandsoftwarequalityhardtomeasure} Incomplete,informalandchangingspecifications} Downstreamcostofbugsisenormous} Lackofspecandimplementationtestingtools} Integrationtestingacrossproductgroups} Patchingnightmare} Versionsexploding
Example:TestingMSWord
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} inputs} keyboard} mouse/pen} .doc,.htm,.xml,…
} outputs(WYSIWYG)} Printers} displays} doc,.htm,.xml,…
} variables} fonts} templates} languages} dictionaries} styles
} Interoperability} Access} Excel} COM} VB} sharepoint
} Otherfeatures} 34toolbars} 100sofcommands} ?dialogs
} Constraints} hugeuserbase
FromMicrosoftOfficeEULA…
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FromGPL
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Thegoalsoftesting
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} Not-quite-rightanswers} Makesureitdoesn’tcrash} Regressiontesting–nonewbugs} Makesureyoumeetthespec} Makesureyoudon’thaveharmfulsideeffects
} Actualgoals} Revealfaults} Establishconfidence} Clarifyorrepresentthespecification
THElimitationoftesting
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Testingcanonlyshowthepresenceoferrors,nottheirabsence
-E.W.Dijkstra
…tobecontinuedinthenextlecture!
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