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PrinciplesofSoftwareConstruction: Objects,Design,andConcurrencySoftwareengineeringinpracticeTeams,branch-baseddevelopment,andworkflowsJoshBloch CharlieGarrod

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Administrivia

•  Homework5teamsign-updeadlineThursday11:59p.m.–  Teamsizes,presentationslots…

•  2ndmidtermexam"inclass"onThursday–  Pleasehavemobilephoneorsomeotherwaytoscandocuments–  Reviewsessiontoday6-8pmEDT:https://cmu.zoom.us/j/343150293

•  RequiredreadingduenextTuesday:–  JavaConcurrencyinPractice,Sections11.3and11.4

•  Homework5frameworksdiscussion•  Onlineformat...

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KeyconceptsfromlastThursday

•  APIdesignprinciples,part2

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Keydesignprinciple:Informationhiding

•  "Whenindoubt,leaveitout."

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Minimizemutability

•  Classesshouldbeimmutableunlessthere'sagoodreasontodootherwise–  Advantages:simple,thread-safe,reusable

•  Seejava.lang.String–  Disadvantage:separateobjectforeachvalue

•  Mutableobjectsrequirecarefulmanagementofvisibilityandsideeffects–  e.g.Component.getSize()returnsamutableDimension

•  Documentmutability–  Carefullydescribestatespace

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Failfast

•  Reporterrorsassoonastheyaredetectable–  Checkpreconditionsatthebeginningofeachmethod–  Avoiddynamictypecasts,run-timetype-checking

//APropertiesinstancemapsStringstoStringspublicclassPropertiesextendsHashTable{publicObjectput(Objectkey,Objectvalue);//ThrowsClassCastExceptionifthisinstance//containsanykeysorvaluesthatarenotStringspublicvoidsave(OutputStreamout,Stringcomments);}

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Subtletiesofinformationhiding

•  Preventsubtleleaksofimplementationdetails–  Documentation–  Lackofdocumentation–  Implementation-specificreturntypes–  Implementation-specificexceptions–  Outputformats–  implementsSerializable

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Don'tletyouroutputbecomeyourdefactoAPI

•  Documentthefactthatoutputformatsmayevolveinthefuture•  Provideprogrammaticaccesstoalldataavailableinstringform

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Don'tletyouroutputbecomeyourdefactoAPI

•  Documentthefactthatoutputformatsmayevolveinthefuture•  Provideprogrammaticaccesstoalldataavailableinstringform

publicclassThrowable{publicvoidprintStackTrace(PrintStreams);publicStackTraceElement[]getStackTrace();//since1.4}publicfinalclassStackTraceElement{publicStringgetFileName();publicintgetLineNumber();publicStringgetClassName();publicStringgetMethodName();publicbooleanisNativeMethod();}

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Today:Towardsoftwareengineeringinpractice

•  Twopuzzlers•  Softwareengineeringforteams

–  Challengesofworkingasateam–  Toolsandprocessesforteams

•  Branch-baseddevelopment,etal.

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1.“TimeforaChange” (2002)

Ifyoupay$2.00foragasketthatcosts$1.10,howmuchchangedoyouget? publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

From An Evening Of Puzzlers by Josh Bloch

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Whatdoesitprint?

(a) 0.9 (b) 0.90 (c) It varies (d) None of the above

publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

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(a)0.9(b)0.90(c)Itvaries(d)Noneoftheabove:0.8999999999999999 Decimalvaluescan'tberepresentedexactlyby float or double

Whatdoesitprint?

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Anotherlook

publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(2.00-1.10);}}

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Howdoyoufixit?

//Youcouldfixitthisway...importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(newBigDecimal("2.00").subtract(newBigDecimal("1.10")));}}

//...oryoucouldfixitthiswaypublicclassChange{publicstaticvoidmain(Stringargs[]){System.out.println(200-110);}}

Prints0.90

Prints90

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Themoral

•  Avoid float and doublewhereexactanswersarerequired–  Forexample,whendealingwithmoney

•  UseBigDecimal,int,orlonginstead

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2.“AChangeisGonnaCome”

Ifyoupay$2.00foragasketthatcosts$1.10,howmuchchangedoyouget? importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

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Whatdoesitprint?

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

(a)0.9(b)0.90(c)0.8999999999999999(d)None of the above

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(a)0.9(b)0.90(c)0.8999999999999999(d)Noneoftheabove:0.899999999999999911182158029987476766109466552734375

WeusedthewrongBigDecimalconstructor

Whatdoesitprint?

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Anotherlook

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal(2.00);BigDecimalcost=newBigDecimal(1.10);System.out.println(payment.subtract(cost));}}

Thespecsays: publicBigDecimal(doubleval)TranslatesadoubleintoaBigDecimalwhichistheexactdecimalrepresentationofthedouble'sbinaryfloating-pointvalue.

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Howdoyoufixit?

importjava.math.BigDecimal;publicclassChange{publicstaticvoidmain(Stringargs[]){BigDecimalpayment=newBigDecimal("2.00");BigDecimalcost=newBigDecimal("1.10");System.out.println(payment.subtract(cost));}}

Prints0.90

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The moral

•  Use newBigDecimal(String), not newBigDecimal(double)

•  BigDecimal.valueOf(double) is better, but not perfect –  Use it for non-constant values.

•  For API designers –  Make it easy to do the commonly correct thing –  Make it hard to misuse –  Make it possible to do exotic things

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Today:Towardsoftwareengineeringinpractice

•  Twopuzzlers•  Softwareengineeringforteams

–  Challengesofworkingasateam–  Toolsandprocessesforteams

•  Branch-baseddevelopment,etal.

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Softwareengineeringisinherentlycollaborative

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Challengesofworkingasateam:

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Challengesofworkingasateam:Aligningexpectations

•  Howdoestheteammakedecisions?•  Howdoyoudividethework?•  Doestheteamsharethesamegoalsandincentives?•  Whathappenswhenworkisn’tcompletedasexpected?•  Whendoteammembersliketowork?•  Whatothercommitmentsdoyourteammembershave?•  Wherewillyougettheworkdone?•  ...

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Decidewhattobuild,thendesigntheAPI

//Acollectionofelements(rootofthecollectionhierarchy)publicinterfaceCollection<E>{//Ensuresthatcollectioncontainsobooleanadd(Eo);//Removesaninstanceofofromcollection,ifpresentbooleanremove(Objecto);//Returnstrueiffcollectioncontainsobooleancontains(Objecto);//Returnsnumberofelementsincollectionintsize();//ReturnstrueifcollectionisemptybooleanisEmpty();...//Remainderomitted}

BasicProcess:(1)  Determineminimal

featureset(2)  DrawUMLonthe

whiteboard.(3)  SketchoutyourAPIon

paper(4)  Writeexamplecode(5)  Review(6)  Repeat

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BreakuptasksintoGitHubIssues

Issuescanrepresentbothtasksandbugsthatneedtobefixed.Issuesshouldbe:●  areasonablechunkofwork●  focusedandcohesive

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BreakuptasksintoGitHubIssues

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Uselabelstoindicatepriorityanddifferentiatebugsfromfeatures

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Considerusingmilestones(e.g.,HW5a,HW5b)

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Howdoesalargesoftwareprojectgettobeoneyearlate?

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Howdoesalargesoftwareprojectgettobeoneyearlate?Onedayatatime.— FredBrooks,TheMythicalMan-Month

https://en.wikipedia.org/wiki/The_Mythical_Man-Month

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UseasimpleKanbanboardtomeasureprogress

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UseasimpleKanbanboardtomeasureprogress

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Single-branchdevelopmentdoesn’tscaletoteams

Master

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Usesimplebranch-baseddevelopment

Createanewbranchforeachfeature.●  allowsparalleldevelopment●  nodealingwithhalf-finishedcode●  nomergeconflicts!

Everycommitto“master”shouldpassyourCIchecks.

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Git,practically

•  Gitstoreseachversionasasnapshot•  Iffileshavenotchanged,onlyalinktothepreviousfileisstored•  EachversionisreferredbytheSHA-1hashofthecontents

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gitcommit

Graphicsbyhttps://learngitbranching.js.org

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gitbranchnewImage

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gitcommit

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gitcheckoutnewImage;gitcommit

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Summary

•  Identifyanddiscussriskswithinyourteam–  Gettoknowyourteammates,andagreeonyourprocess

•  Usestandardtoolstoimproveyourprocess