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PrinciplesofSoftwareConstruction:Objects,Design,andConcurrency

TestingandObjectMethodsinJava

JoshBloch CharlieGarrod

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Administrivia

• Homework1dueToday 11:59p.m.– Everyonemustreadandsignourcollaborationpolicy– TAswillbeavailabletohelpyou– Youhavelatedays,buyyoumightwanttosaveforlater

• Secondhomeworkwillbepostedshortly

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KeyconceptsfromTuesday

• Interfaces-baseddesignsareflexible• Informationhidingiscrucialtogooddesign• Exceptionsarewaybetterthanerrorcodes• Usetry-with-resources;notmanualcleanup

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Outline

I. Specifyingprogrambehavior– contractsII. Testingcorrectness– JunitandfriendsIII. OverridingObjectmethods

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Whatisacontract?

• Agreementbetweenanobjectanditsuser• Includes–Methodsignature(typespecifications)– Functionalityandcorrectnessexpectations– Performanceexpectations

• Whatthemethoddoes,nothow itdoesit– Interface(API),notimplementation

• “Focusonconceptsratherthanoperations”

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Methodcontractdetails

• Definesmethod’sandcaller’sresponsibilities• Analogy:legalcontract– Ifyoupaymethisamountonthisschedule…– Iwillbuildaroomwiththefollowingdetailedspec– Somecontractshaveremediesfornonperformance

• Methodcontractstructure– Preconditions:whatmethodrequiresforcorrectoperation– Postconditions:whatmethodestablishesoncompletion– Exceptionalbehavior:whatitdoesifpreconditionviolated

• Definescorrectnessofimplementation6

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FormalcontractspecificationJavaModellingLanguage(JML)

/*@ requires len >= 0 && array != null && array.length == len;@@ ensures \result ==@ (\sum int j; 0 <= j && j < len; array[j]);@*/

int total(int array[], int len);

• Theoreticalapproach– Advantages

• Runtimechecksgeneratedautomatically• Basisforformalverification• Automaticanalysistools

– Disadvantages• Requiresalotofwork• Impracticalinthelarge• Someaspectsofbehaviornotamenabletoformalspecification

postcondition

precondition

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Textualspecification- Javadoc

• Practicalapproach• Document– Everyparameter– Returnvalue– Everyexception(checkedandunchecked)– Whatthemethoddoes,including

• Purpose• Sideeffects• Anythreadsafetyissues• Anyperformanceissues

• Donot documentimplementationdetails

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SpecificationsintherealworldJavadoc

/*** Returns the element at the specified position of this list.* * <p>This method is <i>not</i> guaranteed to run in constant time.* In some implementations, it may run in time proportional to the* element position.* * @param index position of element to return; must be non-negative and * less than the size of this list.* @return the element at the specified position of this list* @throws IndexOutOfBoundsException if the index is out of range* ({@code index < 0 || index >= this.size()})*/E get(int index);

(Nosideeffects)

postcondition

precondition

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Outline

I. Specifyingprogrambehavior– contractsII. Testingcorrectness– JunitandfriendsIII. OverridingObjectmethods

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Semanticcorrectnessadherencetocontracts

• Compilerensurestypesarecorrect(type-checking)– Preventsmanyruntimeerrors,suchas“MethodNotFound”and“Cannotaddboolean toint”

• Staticanalysistools(e.g.,SpotBugs)recognizemanycommonproblems(bugpatterns)– Overridingequals withoutoverridinghashCode

• Buthowdoyouensuresemanticcorrectness?

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Formalverification

• Usemathematicalmethodstoprovecorrectnesswithrespecttotheformalspecification

• Formallyprovethatallpossibleexecutionsofanimplementationfulfillthespecification

• Manualeffort;partialautomation;notautomaticallydecidable

"Testingshowsthepresence,nottheabsenceofbugs.”

Edsger W.Dijkstra,1969

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Testing

• Executingtheprogramwithselectedinputsinacontrolledenvironment

• Goals– Revealbugs,sotheycanbefixed(maingoal)– Assessquality– Clarifythespecification,documentation

“Bewareofbugsintheabovecode;Ihaveonlyproveditcorrect,nottriedit.”

DonaldKnuth,1977

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Who’sright,Dijkstra orKnuth?

• They’rebothright!• Pleasesee“Extra,Extra- ReadAllAboutIt:NearlyAllBinarySearchesandMergesorts areBroken”– Official“GoogleResearch”blog– http://googleresearch.blogspot.com/2006/06/extra-extra-read-all-about-it-nearly.html

• Thereisnosilverbullet– Usealltoolsatyourdisposal

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Manualtesting?

• LiveSystem?• ExtraTestingSystem?• Checkoutput/assertions?• Effort,Costs?• Reproducible?

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Automatetesting

• Executeaprogramwithspecificinputs,checkoutputforexpectedvalues

• Setuptestinginfrastructure• Executetestsregularly– Afterevery change

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Unittests

• Unittestsforsmallunits:methods,classes,subsystems– Smallesttestablepartofasystem– Testpartsbeforeassemblingthem– Intendedtocatchlocalbugs

• Typicallywrittenbydevelopers• Manysmall,fast-running,independenttests• Fewdependenciesonothersystempartsorenvironment• Insufficient,butagoodstartingpoint

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JUnit

• Popularunit-testingframeworkforJava• Easytouse• Toolsupportavailable• Canbeusedasdesignmechanism

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KentBeckonautomatedtesting

“Functionalitythatcan’tbedemonstratedbyautomatedtestsimplydon'texist.”

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Selectingtestcases:commonstrategies

• Readspecification• Writetestsfor

– Representativecase– Invalidcases– Boundaryconditions

• Writestresstests– Automaticallygeneratehugenumbersoftestcases

• Thinklikeanattacker– Thetester’sgoalistofindbugs!

• Howmanytestshouldyouwrite?– Aimtocoverthespecification– Workwithintime/moneyconstraints

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JUnit conventions

• TestCasecollectsmultipletests(inoneclass)• TestSuitecollectstestcases(typicallypackage)• Testsshouldrunfast• Testsshouldbeindependent

• Testsaremethodswithoutparameterandreturnvalue• AssertErrorsignalsfailedtest(uncheckedexception)

• TestRunnerknowshowtorunJUnittests– (usesreflectiontofindallmethodswith@Testannotat.)

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Testorganization

• Conventions(notrequirements)• HaveatestclassFooTest foreachpublicclassFoo

• Haveasourcedirectoryandatestdirectory– StoreFooTest andFoointhesamepackage

– Testscanaccessmemberswithdefault(package)visibility

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Testablecode

• Thinkabouttestingwhenwritingcode• Unittestingencouragesyoutowritetestablecode• Modularityandtestabilitygohandinhand• Sametestcanbeusedonmultipleimplementationsofaninterface!

• Test-DrivenDevelopment– Adesignanddevelopmentmethodinwhichyouwritetestsbeforeyouwritethecode

– WritingtestscanexposeAPIweaknesses!

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Runtestsfrequently

• Youshouldonlycommitcodethatispassingalltests• Runtestsbeforeeverycommit• Iftestsuitebecomestoolarge&slowforrapidfeedback– Runlocalpackage-leveltests(“smoketests”)frequently– Runalltestsnightly– Mediumsizedprojectseasilyhavethousandsoftestcases

• Continuousintegrationservershelptoscaletesting

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Continuousintegration- TravisCI

Automatically builds, tests, and displays the result

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Continuousintegration- TravisCI

You can see the results of builds over time

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Outlook:statementcoverage

• Tryingtotestallpartsoftheimplementation• Executeeverystatement,ideally

• Does100%coverageguaranteecorrectness?

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Outline

I. Specifyingprogrambehavior– contractsII. Testingcorrectness– JunitandfriendsIII. OverridingObjectmethods

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Methodscommontoallobjects

• Howdocollectionsknowhowtotestobjectsforequality?• Howdotheyknowhowtohashandprintthem?• TherelevantmethodsareallpresentonObject– equals - returnstrueifthetwoobjectsare“equal”– hashCode - returnsanint thatmustbeequalforequalobjects,andislikelytodifferonunequalobjects

– toString - returnsaprintablestringrepresentation

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Object implementations

• Provideidentitysemantics– equals(Object o) - returnstrue ifo referstothisobject

– hashCode() - returnsanear-randomint thatneverchangesovertheobjectlifetime

– toString() - returnsanastylookingstringconsistingofthetypeandhashcode• Forexample:java.lang.Object@659e0bfd

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OverridingObject implementations

• (nearly)AlwaysoverridetoString– println invokesitautomatically–Whysettleforugly?

• Noneedtooverrideequals andhashCode ifyouwantidentitysemantics–Whenindoubt,don'toverridethem– It'seasytogetitwrong

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OverridingtoString

OverridingtoString iseasyandbeneficialfinal class PhoneNumber {

private final short areaCode;private final short prefix;private final short lineNumber;...

@Override public String toString() {return String.format("(%03d) %03d-%04d",

areaCode, prefix, lineNumber);}

}

PhoneNumber jenny = ...;System.out.println(jenny);Prints:(707) 867-5309

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Theequals contract

Theequalsmethodimplementsanequivalencerelation.Itis:– Reflexive:Foranynon-nullreferencevaluex,x.equals(x)must

returntrue.– Symmetric:Foranynon-nullreferencevaluesxandy,x.equals(y)

mustreturntrueifandonlyify.equals(x)returnstrue.– Transitive:Foranynon-nullreferencevaluesx,y,z,ifx.equals(y)

returnstrueandy.equals(z)returnstrue,thenx.equals(z)mustreturntrue.

– Consistent:Foranynon-nullreferencevaluesxandy,multipleinvocationsofx.equals(y)consistentlyreturntrueorconsistentlyreturnfalse,providednoinformationusedinequalscomparisonsontheobjectsismodified.

– Foranynon-nullreferencevaluex,x.equals(null)mustreturnfalse.

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Theequals contractinEnglish

• Reflexive – everyobjectisequaltoitself• Symmetric – if a.equals(b) thenb.equals(a)• Transitive – ifa.equals(b) andb.equals(c),thena.equals(c)

• Consistent– equalobjectsstayequalunlessmutated• “Non-null”– a.equals(null) returnsfalse• Takentogethertheseensurethatequalsisaglobalequivalencerelationoverallobjects

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equals OverrideExamplepublic final class PhoneNumber {

private final short areaCode;private final short prefix;private final short lineNumber;

@Override public boolean equals(Object o) {if (!(o instanceof PhoneNumber)) // Does null check

return false;PhoneNumber pn = (PhoneNumber) o;return pn.lineNumber == lineNumber

&& pn.prefix == prefix&& pn.areaCode == areaCode;

}

...}

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ThehashCode contract

Wheneveritisinvokedonthesameobjectmorethanonceduringanexecutionofanapplication,thehashCode methodmustconsistentlyreturnthesameinteger,providednoinformationusedinequalscomparisonsontheobjectismodified.Thisintegerneednotremainconsistentfromoneexecutionofanapplicationtoanotherexecutionofthesameapplication.– Iftwoobjectsareequalaccordingtotheequals(Object)method,thencallingthe

hashCode methodoneachofthetwoobjectsmustproducethesameintegerresult.

– Itisnotrequiredthatiftwoobjectsareunequalaccordingtotheequals(Object)method,thencallingthehashCode methodoneachofthetwoobjectsmustproducedistinctintegerresults.However,theprogrammershouldbeawarethatproducingdistinctintegerresultsforunequalobjectsmayimprovetheperformanceofhashtables.

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ThehashCode contractinEnglish

• Equalobjectsmusthaveequalhashcodes– Ifyouoverrideequals youmustoverridehashCode

• Unequalobjectsshould havedifferenthashcodes– Takeallvaluefieldsintoaccountwhenconstructingit

• Hashcodemustnotchangeunlessobjectmutated

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hashCode overrideexample

public final class PhoneNumber {private final short areaCode;private final short prefix;private final short lineNumber;

@Override public int hashCode() {int result = 17; // Nonzero is goodresult = 31 * result + areaCode; // Constant must be oddresult = 31 * result + prefix; // " " " " result = 31 * result + lineNumber; // " " " " return result;

}

...}

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AlternativehashCode overrideLessefficient,butotherwiseequallygood!

public final class PhoneNumber {private final short areaCode;private final short prefix;private final short lineNumber;

@Override public int hashCode() {return Objects.hash(areaCode, prefix, lineNumber);

}

...}

Aoneliner.NoexcuseforfailingtooverridehashCode!

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Formorethanyouwanttoknowaboutoverridingobjectmethods,seeEffectiveJavaChapter2

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The== operatorvs.equalsmethodreview

• Forprimitivesyoumust use==• Forobjectreferencetypes– The== operatorprovidesidentitysemantics• ExactlyasimplementedbyObject.equals• EvenifObject.equals hasbeenoverridden• Thisisseldomwhatyouwant!

– Youshould(almost)alwaysuse.equals– Using == onanobjectreferenceisabadsmellincode

if (input == "yes") // A bug!!!

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Summary

• Contractsspecifymethodbehavior– Documentthecontractofeverymethod

• Testearly,testoften!• AlwaysoverridetoString• Overrideequalswhenyouneedvaluesemantics• OverridehashCodewhenyouroverrideequals