SoftwareEngineering第8版《软件工程》课后习题答案

1

SoftwareEngineering

8thedition

Solutionstoselectedexercises

Thesesolutionsaremadeavailablefarinstructionalpurposesonly.Theymayonlybedistributedto

studentsanditisaconditionofdistribuiionthattheyareonlydistributedbyaccreditedinstructors

using'SoftwareEngineering,8thedition'asatextbook..Thesolutionsmaybemadeavailableto

studentsonapassword-protectedintranetbutmustnotbemadeavailableonapublicly-accessible

WWWserver.

NOTFORPUBLICDISTRIBUTION

2

SolutionstotheexercisesareorganisedbychapterandIhaveprovidedsolutionsfor6or7exercises

foreachchapterinthebook.Insomecases,wherethematerialislikelytobeunfamiliarorwhereI

havefoundstudentstohaveparticulardifficulties,alargernumberofsolutionsaregiven.Overall,I

haveprovidedsolutionsforabout60%oftheexercises.Forexercisesconcernedwithethicalissues,

thereareofcourse,nodefinitivesolutions.Fortheseexercises,Ihaveincludedissuesthatmightbe

addressed.

However,thesolutionsherearesimplyindicationsofwhatmightbeexpectedfromstudents

attemptingtheexercises.Manyoftheexerciseshavebeendeliberatelydesignedsothattheymaybe

adaptedtolocalsituations;thereforetheyarenotspecifiedinarigidway.Instructors,therefore,may

usethesesolutionsasaguidebutmanyotherpossible,equallyvalid,solutionsmayalsobe

generated.

Therearestillasmallnumberofchapterswheretherearefewerthan6solutionstoexercises.These

additionalsolutionswillbeavailableinthenextreleaseofthisdocumentinOctober2006.

NOTFORPUBLICDISTRIBUTION

©IanSommerville2006

3

Chapter1Introduction

SolutionsprovidedforExercises1.2,1.3,1.4,1.6,1.7and1.8.

1.2Theessentialdifferenceisthatingenericsoftwareproductdevelopment,thespecificationis

ownedbytheproductdeveloper.Forcustomproductdevelopment,thespecificationisowned

bythecustomer.Ofcourse,theremaybedifferencesindevelopmentprocessesbutthisisnot

necessarilythecase.

1.3Forimportantattributesaremaintainability,dependability,performanceandusability.Other

attributesthatmaybesignificantcouldbereusability(canitbereusedinotherapplications),

distributability(canitbedistributedoveranetworkofprocessors),portability(canitoperate

onmultipleplatforms)andinter-operability(canitworkwithawiderangeofothersoftware

systems).Decompositionsofthe4keyattributese.g.dependabilitydecomposestosecurity,

safety,availability,etc.arealsopossibleanswers.

1.4Asoftwareprocessiswhatactuallygoesonwhensoftwareisdeveloped.Asoftwareprocess

modelisanabstractionandsimplificationofaprocess.Processmodelscanbeusedtohelp

understandrealprocessesandtoidentifywhichaspectsoftheseprocessescouldbesupported

byCASEtools.

1.6MethodsupportprovidedbyCASEtools:

Editorsforspecificgraphicalnotationsused

Checkingofthe'rules1andguidelinesofthemethod

Advicetotoolusersonwhattodonext

Maintenanceofadatadictionary-allnamesusedinthesystem

Automaticgenerationofskeletoncodefromthesystemmodels

Generationofreportsonthedesign

1.7Problemsandchallengesforsoftwareengineering

Developingsystemsformulticulturaluse

Developingsystemsthatcanbeadaptedquicklytonewbusinessneeds

Designingsystemsforoutsourceddevelopment

Developingsystemsthatareresistanttoattack

Developingsystemsthatcanbeadaptedandconfiguredbyend-users

Findingwaysoftesting,validatingandmaintainingend-userdevelopedsystems

Thereareobviouslylotsofotherproblemsthatcouldbementionedhere.

1.9Advantagesofcertification

•Certificationisasignaltoemployersofsomeminimumlevelofcompetence.

•Certificationimprovesthepublicimageoftheprofession.

•Certificationgenerallymeansestablishingandcheckingeducationalstandardsandistherefore

amechanismforensuringcoursequality.

•Certificationimpliesresponsibilityintheeventofdisputes.

Certifyingbodyislikelytobeacceptedatanationalandinternationallevelas'speakingfbr

theprofession\

,Certificationmayincreasethestatusofsoftwareengineersandattractparticularlyablepeople

intotheprofession.

Disadvantagesofcertification

NOTFORPUBLICDISTRIBUTION

4

•Certificationtendstoleadtoprotectionismwherecertifiedmemberstendnottoprotectothers

fromcriticism.

•Certificationdoesnotguaranteecompetencemerelythataminimumstandardwasreachedat

thetimeofcertification.

,Certificationisexpensiveandwillincreasecoststoindividualsandorganisations.

•Certificationtendstostultifychange.Thisisaparticularprobleminanareawheretechnology

developmentsareveryrapid.

Thesearepossiblediscussionpoints-anydiscussiononthiswilltendtobewiderangingandtouch

onotherissuessuchasthenatureofprofessionalism,etc.

NOTFORPUBLICDISTRIBUTION

©IanSommerville2006

5

Chapter2Computer-basedsystemengineering

SolutionsprovidedfarExercises2.1,2,2,2.3,2.4,2.6,2.7,and2.8.

2.1Othersystemsinthesystem'senvironmentcanhaveunanticipatedeffectsbecausetheyhave

relationshipswiththesystemoverandabovewhateverformalrelationships(e.g.data

exchange)aredefinedinthesystemspecification.Forexample,thesystemmaysharean

electricalpowersupplyandairconditioningunit,theymaybelocatedinthesameroom(soif

thereisafireinonesystemthentheotherwillbeaffected)etc.

2.2Thisisaninherentlywickedproblembecauseoftheuncertaintiesassociatedwiththe

problem.Itisimpossibletoanticipateexactlywhenandwhereadisasterwilloccur,the

numbersofpeopleinvolved,theeffectsontheenvironment,thetechnologyavailabletothe

emergencyservices,etc.Planningcanonlybeinverygeneraltermsanddetailedsoftware

specificationstocopewithspecificsituationsarealmostimpossibletowrite.

2.3Whenacarisdecommissioned,notallofitspartsarewornout.Softwaresystemscanbe

installedinthecartomonitorthedifferentpartsandtocomputethelifetimewhichtheyare

likelytohaveleft.Whenthecaristobedecommissioned,thepartswhichcanpotentiallybe

reusedcantheneasilybediscovered.

2.4Anoverallarchitecturaldescriptionshouldbeproducedtoidentifysub-systemsmakingupthe

system.Oncethesehavebeenidentified,theymaybespecifiedinparallelwithothersystems

andtheinterfacesbetweensub-systemsdefined.

2.6Thekeyfeaturesofthesolutionare:

•Databasewithdifferenttypesofdata

•Videocontrolsystem

•Operatorconsolesystem

•Riverdatacollection

•Weathersystemlinks

•Communicationcontrolsystem

SeeFigure2.1.

2.7Possibleissuescoveredinthesolutionmightbe:

•Museumsareconservativeplacesandsomestaffmayresenttheintroductionofnew

technology.

•Existingmuseumstaffmaybeaskedtodealwithproblemsoftheequipmentnotworking

andmaynotwishtoappearunabletodealwiththis.

•Otherareasofthemuseummayopposethesystembecausetheyseeitasdiverting

resourcesfromtheirwork.

•Differentmuseumsmayhavedifferentpreferredsuppliersfortheequipmentsothatall

equipmentusedisnotidenticalthuscausingsupportproblems.

•Thenewdisplaystakeupalotofspaceandthisdisplacesotherdisplays.Themaintainers

ofthesedisplaysmayopposetheintroductionofthesystem.

•Somemuseumsmayhavenomechanismforprovidingtechnicalsupportforthesystem.

NOTFORPUBLICDISTRIBUTION

6

Metoffice.

Vidto

cuinc'ay

WeatherinfoOtherservicesi

system

SensordataComms

Cameracontrol

collectioncontroller

system

T

Figure2.1Blockdiagramofthefloodcontrolsystem

2.8Legacysystemsmaybecriticalforthesuccessfuloperationofabusinessfortwobasicreasons

•Theymaybeanintrinsicpartofoneormoreprocesseswhicharefundamentaltothe

operationofabusiness.Forexample,auniversityhasastudentadmissionsprocessand

systemswhichsupportthisarecritical.Theymustbemaintained.

,Theymayincorporateorganisationalandbusinessknowledgewhichissimplynot

documentedelsewhere.Forexample,exceptionsonstudentadmissionsmaysimplyhavebeen

codeddirectlyintothesystemwithnopaperrecordofthese.Withoutthissystem,the

organisationlosesvaluableknowledge.

NOTFORPUBLICDISTRIBUTION

©IanSommerville2006

7

Chapter3Criticalsystems

SolutionsprovidedforExercises3.2,3.5,3.6,3.7,3.8,3.10and3.11.

3.2Sixreasonswhydependabilityisimportantare:

a)Usersmaynotusethesystemiftheydon'ttrustit.

b)Systemfailuremayleadtoalossofbusiness.

c)Anundependablesystemmayloseordamagevaluabledata.

d)Anundependablesystemmaydamageitsexternalenvironment.

d)Thereputationofthecompanywhoproducedthesystemmaybedamagedhenceaffecting

othersystems.

e)Thesystemmaybeinbreachoflawsonconsumerprotectionandthefitnessofgoodsfor

purpose.

3.5Internetserver:Availabilityasfailureofavailabilityaffectsalargenumberofpeople,

reputationofthesupplierandhenceitscurrentandfutureincome.

Acomputer-control1edscalpel:Safetyassafety-relatedfailurescancauseharmtothepatient.

Adirectionalcontrolsystem:Reliabilityasmissionfailurecouldresultfromfailureofthe

systemtoperforintospecification.

Anpersonalfinancemanagementsystem:Securitybecauseofpotentiallossestousers.

3.6Possibledomesticappliancesthatmayincludesafety-criticalsoftwareinclude:

Microwaveoven

Powertoolssuchasadrillorelectricsaw

Lawnmower

Centralheatingfurnace

Garbagedisposalunit

Foodprocessororblender

3.7Ensuringsystemreliabilitydoesnotnecessarilyleadtosystemsafetyasreliabilityis

concernedwithmeetingthesystemspecification(thesystem'shall')whereassafetyis

concernedwithexcludingthepossibilityofdangerousbehavior(thesystem'shallnot').Ifthe

specificationdoesnotexplicitlyexcludedangerousbehaviorthenasystemcanbereliablebut

unsafe.

3.8Possiblehazardisdeliveryoftoomuchradiationtoapatient.Thiscanarisebecauseofa

systemfailurewhereadosegreaterthanthespecifieddoseisdeliveredoranoperatorfailure

wherethedosetobedeliverediswronglyinput.

Possiblesoftwarefeaturestoguardagainstsystemfailurearethedeliveryofradiationin

incrementswithaoperatordisplayshowingthedosedeliveredandtherequirementthatthe

operatorconfirmthedeliveryofthenextincrement.Toreducetheprobabilityofoperator

error,therecouldbeafeaturethatrequiresconfirmationofthedosetobedeliveredandthat

comparesthistopreviousdosesdeliveredtothatpatient.Alternatively,twodifferent

operatorscouldberequiredtoindependentlyinputthedosebeforethemachinecouldoperate.

3.10Anattackisanexploitationofasystemvulnerability.Athreatisacircumstancethathas

thepotentialtocauselossorharm.Anattackcanleadtoathreatiftheexploitationofthe

vulnerabilityleadstoathreat.However,someattackscanbesuccessfulbutdonotleadto

threatsasothersystemfeaturesprotectthesystem.

NOTFORPUBLICDISTRIBUTION

8

3.11Theethicsofdeliveryofafaultysystemarecomplex.Weknowthatthishappensallthetime,

especiallywithsoftware.Issuesthatmightbediscussedincludetheprobabilityofthefault

occurringandtheconsequencesofthefault-ifthefaulthaspotentiallyserious

consequencesthenthedecisionmaybedifferentthanifitisaminor,easilyrecoverablefault.

Otherissuesarethepricechargedforthesystem(ifitslow,thenwhatlevelofqualityisit

reasonableforthecustomertoexpect).Therecoverymechanismsbuiltintothesystemandthe

compensationmechanismsthatareinplaceifconsequentialdamageoccurs.Makingthe

customerawareofthefaultisthehonestdecisiontomakebutmaybeunwisefromabusiness

perspective.

Claimsaboutthereliabilityofthesoftwareshouldnotbemadeinsuchcircumstancesasthe

softwareproviderdoesnotknowhowthesoftwarewillbeusedandsocannotestimatethe

probabilityofoccurrenceofthefault.

NOTFORPUBLICDISTRIBUTION

©IanSommerville2006

9

Chapter4Softwareprocesses

SolutionsprovidedforExercises4.1,4.3,47,4.9,4.10and4.12.

4.1(a)Anti-lockbrakingsystemSafety-criticalsystemsomethodbasedonformal

transformationswithproofsofequivalencebetweeneachstage.

(b)VirtualrealitysystemSystemwhoserequirementscannotbepredictedinadvanceso

exploratoryprogrammingmodelisappropriate.

(c)UniversityaccountingsystemSystemwhoserequirementsshouldbestablebecauseof

existingsystemthereforewaterfallmodelisappropriate.

(d)InteractivetimetableSystemwithacomplexuserinterfacebutwhichmustbestableand

reliable.Shouldbebasedonthrow-awayprototypingtofindrequirementstheneither

incrementaldevelopmentorwaterfallmodel.

4.3Thewaterfallmodelisaccommodatedwherethereisalowspecificationriskandnoneedfbr

prototypingetc.fbrriskresolution.Theactivitiesinthe2ndquadrantofthespiralmodelare

skipped.Theprototypingmodelisaccommodatedwhenthespecificationphaseislimitedand

theprototyping(riskresolution)phasepredominates.Theactivitiesinthe3rdquadrantofthe

spiralmodelareskippedorreducedinscope.

4.4Solutiontobeadded.

4.7Componentsofadesignmethodare:

Adefinedsetofsystemmodels

Rulesthatapplytothesemodels

Guidelinesfordesign*goodpractice'

Amodelofthedesignprocess

Formatsforreportsonthedesign

4.9Systemsmustchangebecauseastheyareinstalledinanenvironmenttheenvironmentadapts

tothemandthisadaptationnaturallygeneratesnew/differentsystemrequirements.

Furthermore,thesystem'senvironmentisdynamicandconstantlygeneratesnewrequirements

asaconsequenceofchangestothebusiness,businessgoalsandbusinesspolicies.Unlessthe

systemisadaptedtoreflecttheserequirements,itsfacilitieswillbecomeout-of-stepwiththe

facilitiesneededtosupportthebusinessand,hence,itwillbecomelessuseful.

4.10Aclassificationschemecanbehelpfulforsystemprocurementbecauseithelpsidentifygaps

intheCASEtoolcoverageinanorganisation.Procurementmaybeaimedatfillingthese

gaps.Alternatively,aclassificationschememaybeusedtofindtoolswhichsupportarange

ofactivities-thesemayrepresentthemostcosteffectivepurchasesiffundsarelimited.

4.12ThereareobviouslydifferentviewshereandalotdependsonthedevelopmentofCASE

technologyinthefuture.AmajordifferencebetweentheintroductionofCASEtechnology

and,forexample,theintroductionofCADtechnologywhichmadedraftsmenredundant,is

thattheroutineelementsinthedesignanddevelopmentofsoftwarearerelativelyminorparts

ofthewholedevelopmentprocess.Therefore,savingsarenotthatlarge.However,ifAI

technologydevelopssothattrulyintelligenttoolscanbedevelopedthan,obviously,this

situationwillchange.

NOTFORPUBLICDISTRIBUTION

10

Chapter5Projectmanagement

SolutionsprovidedforExercises5.2,5.3,5.6,5.9,5.10and5.11.

5.2Managementactivitiessuchasproposalwriting,projectplanningandpersonnelselection

requireasetofskillsincludingpresentationandcommunicationskills,organisationalskills

andtheabilitytocommunicatewithotherprojectteammembers.Programmingskillsare

distinctfromthese(indeed,itisacommoncriticismofprogrammersthattheylackhuman

communicationskills)soitdoesnotfollowthatgoodprogrammerscanre-orienttheir

abilitiestobegoodmanagers.

5.3Projectplanningcanonlybebasedonavailableinformation.Atthebeginningofaproject,

therearemanyuncertaintiesintheavailableinformationandsomeinformationaboutthe

projectandtheproductmaynotbeavailable.Astheprojectdevelops,moreandmore

informationbecomesavailableanduncertaintiesareresolved.Theprojectplanthereforemust

bereviewedandupdatedregularlytoreflectthischanginginformationenvironment.

5.6TheactivitychartandbarchartareshownasFigures5.1and5.2.

5.9Otherpossiblerisksare:

Technology:Communicationsnetworksaturatesbeforeexpectedtransactionlimitisreached.

People:Levelofskillofavailablepeopleislowerthanexpected.

Organisational:Organisationalchangesmeanthattheprojectscheduleisaccelerated.

Tools:CASEtoolscannothandlethevolumeofdataavailablefbrlargesystems.

Requirements:Newnon-functionalrequirementsareintroducedthatrequirechangestothe

systemarchitecture.

Estimation:Thedifficultofthesoftwareisunderestimated.

©IanSommerville2006

11

Figure5.2Taskbarchart

NOTFORPUBLICDISTRIBUTION

12

5.10Fixedpricecontractsincreasethechancesofproductrisksbecausetheyremoveoptionsfrom

thedevelopmentprocess.Becausethecontractisfixed-price,thecontractorisnaturally

reluctanttoincreasetheeffortortimeexpendedontheprojectasthiswillreducetheirprofits

onthework.Therefore,ifproblemsarisetheywilllookforwaystoreducethescopeofthe

productortoreducethecostsofproductdevelopment(e.g.byreducingtheeffortdevotedto

testing).Bothofthesefactorscanleadtoproductsthatarenotasexpectedbythecustomer.

5.11Issueswhichmightbecoveredincludetheproblemsoffindingabalancebetweenfamilylife

andorganisationaldemands,whetherororganisationsshouldexpectpeopletobehaveas

professionals.Thisperhapsimpliesworkingthenumberofhoursrequiredtocompletesome

jobbutalsoimpliesthatengineersshouldhaveadegreeofautonomyabouthowtheyarrange

theirworkinglives(e.g.theymaychoosetoworkfromhomeortheirownworkinghours).

Factorswhichaffectthisdecisionmightbethefinancialstateofthecompany,thegeneral

companycultureandattitude,theavailabilityofalternativelocalemployment,particular

personalcircumstances(e.g.arepeoplesingleparents,dotheyhavebabieswhichdon'tsleep

well,etc.)

©IanSommerville2006

13

Chapter6Softwarerequirements

SolutionsprovidedforExercises6.1,6.3,6.6,6.7,6.8and6.9.

6.1Functionalrequirementsthatspecifysomeservicesorfunctionalitytobeprovidedbythe

system.

Nonfunctionalrequirementsthatdefineoperationalconstraintsonthebehaviourofthe

system

Designrequirementsthatdefineconstraintsonthesystemdesignandimplementation

Processrequirementsthatdefineconstraintsonthesystemdevelopmentprocess.

6.3Ambiguitiesandomissionsinclude:

•Canacustomerbuyseveralticketsfbrthesamedestinationtogetherormusttheybebought

oneatatime?

,Cancustomerscancelarequestifamistakehasbeenmade?

•Howshouldthesystemrespondifaninvalidcardisinput?

•Whathappensifcustomerstrytoputtheircardinbeforeselectingadestination(asthey

wouldinATMmachines)?

•Musttheuserpressthestartbuttonagainiftheywishtobuyanothertickettoadifferent

destination?

,Shouldthesystemonlysellticketsbetweenthestationwherethemachineissituatedand

directconnectionsorshoulditincludeallpossibledestinations?

6.6NotethatFigure6.1isatop-levelrequirementsdefinitionforthewholesystem.Figures6.2

and6.3aremoredetailedfunctiondefinitions.

NOTFORPUBLICDISTRIBUTION

14

1.Fueldeliverysystem

1.1Thesystemshouldprovideanunattendedfueldeliveryservicewhereaspecifiedamountof

fuelisdeliveredtocustomers,Thecostisdeductedfromthecustomer'screditcardaccount.

1.2Thesequenceofactionstodispensefuelshouldbe:

1.Thecustomerselectsthetypeoffueltobedelivered.

2.Thecustomerinputseitheracashlimitoramaximumamountoffueltobedelivered

3.Thecustomervalidatesthetransactionbyprovidingcreditcardaccountdetails.

Rationale,.Theamountoffuelalloweddependsonthecreditlimitbutcustomersmaywishto'fillup'

ratherthanhaveaspecifiedamountoffuel.Byspecifyingamaximum,thesystemcancheckifcredit

isavailable.Notethatthedefinitiondoesnotsetouthowcreditcarddetailsshouldbeprovided.

4.Thepumpisactivatedandfuelisdelivered,undercustomercontrol.

5.Thetransactionisterminatedeitherwhenthepumpnozzleisreturnedtoitsholsterfor15

secondsorwhenthecustomersfuelorcashlimitisreached.

Rationale'.Terminationshouldnotbeimmediatewhenthenozzleisreturnedasthecustomermay

wishtorestartthetransactione.g.tofillafuelcanaswellasthecarfueltank.Ifapumpdisplayis

available,itmaybeappropriatetoissuea'Pleasewaitforyourreceipt'message.

6.Areceiptisprintedforthecustomer.

7.Thefuelstockisupdated.

Specification'.PUMPSYS/FS.Section1

Figure6.1Requirementsforafueldeliverysystem

2.Dispensingcash

2.1Thesystemmustprovideafacilitywhichallowsaspecifiedamounttocashtobeissued

tocustomers.Theamountisrequestedbythecustomerbutthesystemmayreducethisamountif

thecustomer'sdailylimitoroverdraftlimitisreached.

2.1.1Thesequenceofactionstodispensecashshouldbe:

1.Thecustomerinputstheamountofcashrequired

2.Thesystemchecksthisagainstdailycardlimitsandthecustomer'soverdraftlimit.

3.Iftheamountbreacheseitheroftheselimits,thenamessageisissuedwhichtellsthe

customerofthemaximumamountallowedandthetransactioniscancelled.

4.Iftheamountiswithinlimits,therequestedcashshouldbedispensed

5.Thecustomer'saccountbalanceanddailycardlimitshouldbereducedbytheamountof

cashdispensed.

Specification:ATM/Customerfunctionality/FS.Section2.1

Figure6.2ATMsystem-cashdispensing

7.2Spellchecking

7.2.1Thesystemshallprovideauser-activatedfacilitywhichchecksthespellingofwords

inthedocumentagainstspellingsinthesystemdictionaryanduser-supplieddictionaries.

©IanSommerville2006

15

7.2.2Whenawordisfoundinthedocumentwhichisnotinanydictionary,auserqueryshould

beissuedwiththefollowingoptions:

1.Ignorethisinstanceoftheword

2.Ignoreallinstancesoftheword

3.Replacethewordwithasuggestedwordfromthedictionary

4.Replacethewordwithuser-suppliedtext

5.Ignorethisinstanceandaddthewordtoaspecifieddictionary

7.2.3Whenawordisdiscoveredwhichisnotinthedictionary,thesystemshouldpropose10

alternativewordsbasedonamatchbetweenthewordfoundandthoseinthedictionaries.

Specification'.NewWP/Tools/FS.Section7.2

Figure6.3Spellchecking

6.7Therearemanypossibilitieshere.SomesuggestionsareshowninFigure6.4.

Non-functionalDescriptionExamples

requirement

PerformancePerformancerequirementssetThesystemmustprocessatleast150

outlimitstotheperformancetransactionspersecond.

expectedofthesystem.These

maybeexpressedindifferentThemaximumresponsetimeforanyuser

waysdependingonthetypeofrequestshouldbe2seconds.

systeme.g.numberof

transactionsprocessedper

second,responsetimetouser

requests,etc.

Implement