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PrinciplesofSoftwareConstruction: Objects,Design,andConcurrencyPart1:DesigningclassesDesignforreuse:delegationandinheritanceCharlieGarrodChrisTimperley

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Administrivia

•  Homework1gradedsoon•  Readingduetoday:EffectiveJava,Items17and50

–  OptionalreadingdueThursday–  RequiredreadingduenextTuesday

•  Homework2dueThursday11:59p.m.

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DesigngoalsforyourHomework1solution?

Functionalcorrectness Adherenceofimplementationtothespecifications

Robustness Abilitytohandleanomalousevents

Flexibility Abilitytoaccommodatechangesinspecifications

Reusability Abilitytobereusedinanotherapplication

Efficiency Satisfactionofspeedandstoragerequirements

Scalability Abilitytoserveasthebasisofalargerversionoftheapplication

Security Levelofconsiderationofapplicationsecurity

Source: Braude, Bernstein, Software Engineering. Wiley 2011

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OneHomework1solution…

classDocument{privatefinalStringurl;publicDocument(Stringurl){this.url=url;}publicdoublesimilarityTo(Documentd){…ourText=download(url);…theirText=download(d.url);…ourFreq=computeFrequencies(ourText);…theirFreq=computeFrequencies(theirText);returncosine(ourFreq,theirFreq);}…}

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ComparetoanotherHomework1solution…

classDocument{privatefinalStringurl;publicDocument(Stringurl){this.url=url;}publicdoublesimilarityTo(Documentd){…ourText=download(url);…theirText=download(d.url);…ourFreq=computeFreq(ourText);…theirFreq=computeFreq(theirText);returncosine(ourFreq,theirFreq);}…}

classDocument{privatefinal…frequencies;publicDocument(Stringurl){…ourText=download(url);frequencies=computeFrequencies(ourText);}publicdoublesimilarityTo(Documentd){returncosine(frequencies,d.frequencies);}…}

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UsingtheDocumentclass

Foreachurl:ConstructanewDocumentForeachpairofDocumentsd1,d2:Computed1.similarityTo(d2)…

•  Whatistherunningtimeofthis,fornurls?

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LatencyNumbersEveryProgrammerShouldKnowJeffDean,SeniorFellow,Google

PRIMITIVELATENCY:nsusmsL1cachereference0.5Branchmispredict5L2cachereference7Mutexlock/unlock25Mainmemoryreference100Compress1KbyteswithZippy3,0003Send1Kbytesover1Gbpsnetwork10,00010Read4KrandomlyfromSSD*150,000150Read1MBsequentiallyfrommemory250,000250Roundtripwithinsamedatacenter500,000500Read1MBsequentiallyfromSSD*1,000,0001,0001Diskseek10,000,00010,00010Read1MBsequentiallyfromdisk20,000,00020,00020SendpacketCA->Netherlands->CA150,000,000150,000150

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Thepoint

•  Constantsmatter•  Designgoalssometimesclearlysuggestonealternative

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KeyconceptsfromlastThursday

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KeyconceptsfromlastThursday

•  Testing–  Continuousintegration,practicaladvice–  Coveragemetrics,statementcoverage

•  ExceptionsinJava•  Behavioralsubtyping

–  LiskovSubstitutionPrinciple–  Thejava.lang.Objectcontracts

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Behavioralsubtyping

•  e.g.,Compiler-enforcedrulesinJava:–  Subtypescanadd,butnotremovemethods–  Concreteclassmustimplementallundefinedmethods–  Overridingmethodmustreturnsametypeorsubtype–  Overridingmethodmustacceptthesameparametertypes–  Overridingmethodmaynotthrowadditionalexceptions

•  Alsoappliestospecifiedbehavior.Subtypesmusthave:–  Sameorstrongerinvariants–  Sameorstrongerpostconditionsforallmethods–  Sameorweakerpreconditionsforallmethods

Let q(x) be a property provable about objects x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.

Barbara Liskov

This is called the Liskov Substitution Principle.

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ThisSquareisnotabehavioralsubtypeofRectangle

classRectangle{//@invarianth>0&&w>0;inth,w;Rectangle(inth,intw){ this.h=h;this.w=w;}

//@requiresfactor>0;

voidscale(intfactor){ w=w*factor; h=h*factor;}

//@requiresneww>0;//@ensuresw==neww&&h==old.h;

voidsetWidth(intneww){ w=neww;}

}

classSquareextendsRectangle{//@invarianth>0&&w>0;//@invarianth==w;

Square(intw){ super(w,w);}

//@requiresneww>0;//@ensuresw==neww&&h==neww;@OverridevoidsetWidth(intneww){w=neww;h=neww;}}

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Today

•  Designforreuse:delegationandinheritance–  Java-specificdetailsforinheritance

•  Anexerciseinequality

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Recallourearliersortingexample:

staticvoidsort(int[]list,booleanascending){…booleanmustSwap;if(ascending){mustSwap=list[i]>list[j];}else{mustSwap=list[i]<list[j];}…}

interfaceOrder{booleanlessThan(inti,intj);}finalOrderASCENDING=(i,j)->i<j;finalOrderDESCENDING=(i,j)->i>j;staticvoidsort(int[]list,Ordercmp){…booleanmustSwap=cmp.lessThan(list[i],list[j]);…}

Version A:

Version B':

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Delegation

•  Delegationissimplywhenoneobjectreliesonanotherobjectforsomesubsetofitsfunctionality–  e.g.here,theSorterisdelegatingfunctionalitytosomeOrder

•  Judiciousdelegationenablescodereuse

interfaceOrder{booleanlessThan(inti,intj);}finalOrderASCENDING=(i,j)->i<j;finalOrderDESCENDING=(i,j)->i>j;staticvoidsort(int[]list,Ordercmp){…booleanmustSwap=cmp.lessThan(list[i],list[j]);…}

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Delegation

•  Delegationissimplywhenoneobjectreliesonanotherobjectforsomesubsetofitsfunctionality–  e.g.here,theSorterisdelegatingfunctionalitytosomeOrder

•  Judiciousdelegationenablescodereuse–  Sortercanbereusedwitharbitrarysortorders–  Orderscanbereusedwitharbitraryclientcodethatneedstocompare

integersinterfaceOrder{booleanlessThan(inti,intj);}finalOrderASCENDING=(i,j)->i<j;finalOrderDESCENDING=(i,j)->i>j;staticvoidsort(int[]list,Ordercmp){…booleanmustSwap=cmp.lessThan(list[i],list[j]);…}

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Usingdelegationtoextendfunctionality

•  Considerthejava.util.List(excerpted):publicinterfaceList<E>{publicbooleanadd(Ee);publicEremove(intindex);publicvoidclear();…}

•  Supposewewantalistthatlogsitsoperationstotheconsole…

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Usingdelegationtoextendfunctionality

•  Onesolution:publicclassLoggingList<E>implementsList<E>{privatefinalList<E>list;publicLoggingList<E>(List<E>list){this.list=list;}publicbooleanadd(Ee){System.out.println("Adding"+e);returnlist.add(e);}publicEremove(intindex){System.out.println("Removingat"+index);returnlist.remove(index);}…

TheLoggingListiscomposedofaList,anddelegates(thenon-logging)functionalitytothatList

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Delegationanddesign

•  Smallinterfaceswithclearcontracts•  Classestoencapsulatealgorithms,behaviors

–  E.g.,theOrder

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Today

•  Designforreuse:delegationandinheritance–  Java-specificdetailsforinheritance

•  Anexerciseinequality

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Consider:typesofbankaccounts

publicinterfaceCheckingAccount{publiclonggetBalance();publicvoiddeposit(longamount);publicbooleanwithdraw(longamount);publicbooleantransfer(longamount,Account???target);publiclonggetFee();}

publicinterfaceSavingsAccount{publiclonggetBalance();publicvoiddeposit(longamount);publicbooleanwithdraw(longamount);publicbooleantransfer(longamount,Account???target);publicdoublegetInterestRate();}

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Interfaceinheritanceforanaccounttypehierarchy

publicinterfaceAccount{publiclonggetBalance();publicvoiddeposit(longamount);publicbooleanwithdraw(longamount);publicbooleantransfer(longamount,Accounttarget);publicvoidmonthlyAdjustment();}

publicinterfaceCheckingAccountextendsAccount{publiclonggetFee();}

publicinterfaceSavingsAccountextendsAccount{publicdoublegetInterestRate();}

publicinterfaceInterestCheckingAccountextendsCheckingAccount,SavingsAccount{}

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Thepowerofobject-orientedinterfaces

•  Subtypepolymorphism–  Differentkindsofobjectscanbetreateduniformlybyclientcode–  Eachobjectbehavesaccordingtoitstype

•  e.g.,ifyouaddnewkindofaccount,clientcodedoesnotchange:

Iftodayisthelastdayofthemonth:ForeachacctinallAccounts:acct.monthlyAdjustment();

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publicabstractclassAbstractAccount implementsAccount{protectedlongbalance=0;publiclonggetBalance(){ returnbalance;}abstractpublicvoidmonthlyAdjustment();//othermethods…

}publicclassCheckingAccountImpl extendsAbstractAccount implementsCheckingAccount{publicvoidmonthlyAdjustment(){ balance-=getFee();}publiclonggetFee(){…}

}

Implementationinheritanceforcodereuse

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publicabstractclassAbstractAccount implementsAccount{protectedlongbalance=0;publiclonggetBalance(){ returnbalance;}abstractpublicvoidmonthlyAdjustment();//othermethods…

}publicclassCheckingAccountImpl extendsAbstractAccount implementsCheckingAccount{publicvoidmonthlyAdjustment(){ balance-=getFee();}publiclonggetFee(){…}

}

Implementationinheritanceforcodereuse

protectedelementsarevisibleinsubclasses

anabstractclassismissingtheimplementationofone

ormoremethods

anabstractmethodislefttobe

implementedinasubclass

noneedtodefinegetBalance()–thecodeisinheritedfrom

AbstractAccount

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Inheritance:aglimpseatthehierarchy

•  ExamplesfromJava–  java.lang.Object–  Collectionslibrary

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JavaCollectionsAPI(excerpt)

Collection

List Set AbstractCollection

AbstractList

LinkedList

Vector

HashSet

AbstractSequentialList

AbstractSet

Cloneable

ArrayList

interfaces

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Theabstractjava.util.AbstractList<E>

abstractEget(inti);abstractintsize();booleanset(inti,Ee);//pseudo-abstractbooleanadd(Ee);//pseudo-abstractbooleanremove(Ee);//pseudo-abstractbooleanaddAll(Collection<?extendsE>c);booleanremoveAll(Collection<?>c);booleanretainAll(Collection<?>c);booleancontains(Ee);booleancontainsAll(Collection<?>c);voidclear();booleanisEmpty();Iterator<E>iterator();Object[]toArray()<T>T[]toArray(T[]a);…

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Usingjava.util.AbstractList<E>

publicclassReversedList<E>extendsjava.util.AbstractList<E>implementsjava.util.List<E>{privatefinalList<E>list;publicReversedList(List<E>list){this.list=list;}@Overridepublicintsize(){returnlist.size();}@OverridepublicEget(intindex){returnlist.get(size()-index-1);}}

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Benefitsofinheritance

•  Reuseofcode•  Modelingflexibility

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Inheritanceandsubtyping

•  Inheritanceisforpolymorphismandcodereuse–  Writecodeonceandonlyonce–  Superclassfeaturesimplicitlyavailablein

subclass

•  Subtypingisforpolymorphism–  Accessingobjectsthesameway,butgetting

differentbehavior–  Subtypeissubstitutableforsupertype

classAextendsB

classAimplementsBclassAextendsB

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Typicalrolesforinterfacesandclasses

•  Aninterfacedefinesexpectations/commitmentsforclients•  Aclassfulfillstheexpectationsofaninterface

–  Anabstractclassisaconvenienthybrid–  Asubclassspecializesaclass'simplementation

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Javadetails:extendedreusewithsuper

publicabstractclassAbstractAccountimplementsAccount{protectedlongbalance=0;publicbooleanwithdraw(longamount){//withdrawsmoneyfromaccount(codenotshown)}

}publicclassExpensiveCheckingAccountImpl extendsAbstractAccountimplementsCheckingAccount{publicbooleanwithdraw(longamount){ balance-=HUGE_ATM_FEE; booleansuccess=super.withdraw(amount) if(!success) balance+=HUGE_ATM_FEE; returnsuccess;}

}

Overrideswithdrawbutalsousesthesuperclasswithdrawmethod

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Javadetails:constructorswiththisandsuper

publicclassCheckingAccountImpl extendsAbstractAccountimplementsCheckingAccount{

privatelongfee;

publicCheckingAccountImpl(longinitialBalance,longfee){ super(initialBalance); this.fee=fee;}

publicCheckingAccountImpl(longinitialBalance){ this(initialBalance,500);}/*othermethods…*/} Invokesanother

constructorinthissameclass

Invokesaconstructorofthesuperclass.Mustbethe

firststatementoftheconstructor.

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Javadetails:final

•  Afinalfield:preventsreassignmenttothefieldafterinitialization

•  Afinalmethod:preventsoverridingthemethod•  Afinalclass:preventsextendingtheclass

–  e.g.,publicfinalclassCheckingAccountImpl{…

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Note:type-castinginJava

•  Sometimesyouwantadifferenttypethanyouhave–  e.g., doublepi=3.14; intindianaPi=(int)pi;

•  Usefulifyouknowyouhaveamorespecificsubtype:–  e.g., Accountacct=…;CheckingAccountcheckingAcct=(CheckingAccount)acct;longfee=checkingAcct.getFee();–  WillgetaClassCastExceptioniftypesareincompatible

•  Advice:avoiddowncastingtypes–  Never(?)downcastwithinsuperclasstoasubclass

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Note:instanceof

•  OperatorthattestswhetheranobjectisofagivenclasspublicvoiddoSomething(Accountacct){longadj=0;if(acctinstanceofCheckingAccount){checkingAcct=(CheckingAccount)acct;adj=checkingAcct.getFee();

}elseif(acctinstanceofSavingsAccount){savingsAcct=(SavingsAccount)acct;adj=savingsAcct.getInterest();

}…}

•  Advice:avoidinstanceofifpossible–  Never(?)useinstanceofinasuperclasstochecktypeagainstsubclass

Warning: This code is bad.

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Delegationvs.inheritancesummary

•  Inheritancecanimprovemodelingflexibility•  Usually,favorcomposition/delegationoverinheritance

–  Inheritanceviolatesinformationhiding–  Delegationsupportsinformationhiding

•  Designanddocumentforinheritance,orprohibitit–  Documentrequirementsforoverridinganymethod

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Today

•  Designforreuse:delegationandinheritance–  Java-specificdetailsforinheritance

•  Anexerciseinequality

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AnObjectmethodexercise

Provideallcodeneededforareasonableequalsmethod:publicfinalclassName{privatefinalStringfirst,last;publicName(Stringfirst,Stringlast){if(first==null||last==null)thrownewNullPointerException();this.first=first;this.last=last;}…}

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Whatdoesthefollowingcodeprint?

publicfinalclassName{privatefinalStringfirst,last;publicName(Stringfirst,Stringlast){if(first==null||last==null)thrownewNullPointerException();this.first=first;this.last=last;publicbooleanequals(Nameo){returnfirst.equals(o.first)&&last.equals(o.last);}publicinthashCode(){return31*first.hashCode()+last.hashCode();}publicstaticvoidmain(String[]args){Set<Name>s=newHashSet<>();s.add(newName("Mickey","Mouse"));System.out.println(s.contains(newName("Mickey","Mouse")));}}

(a)true(b)false(c)Itvaries(d)Noneoftheabove

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Whatdoesitprint?

(a)true(b)false(c)Itvaries(d)Noneoftheabove TheNameclassoverrideshashCodebutnotequals!ThetwoNameinstancesarethusunequal.

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Object.equalshasnotbeenoverridden

publicfinalclassName{privatefinalStringfirst,last;publicName(Stringfirst,Stringlast){if(first==null||last==null)thrownewNullPointerException();this.first=first;this.last=last;publicbooleanequals(Nameo){//Accidentaloverloadingreturnfirst.equals(o.first)&&last.equals(o.last);}publicinthashCode(){return31*first.hashCode()+last.hashCode();}publicstaticvoidmain(String[]args){Set<Name>s=newHashSet<>();s.add(newName("Mickey","Mouse"));System.out.println(s.contains(newName("Mickey","Mouse")));}}

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Javadetails:Dynamicmethoddispatch

1.  (Compiletime)Determinewhichclasstolookin2.  (Compiletime)Determinemethodsignaturetobeexecuted

1.  Findallaccessible,applicablemethods2.  Selectmostspecificmatchingmethod

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Javadetails:Dynamicmethoddispatch

1.  (Compiletime)Determinewhichclasstolookin2.  (Compiletime)Determinemethodsignaturetobeexecuted

1.  Findallaccessible,applicablemethods2.  Selectmostspecificmatchingmethod

3.  (Runtime)Determinedynamicclassofthereceiver4.  (Runtime)Fromdynamicclass,locatemethodtoinvoke

1.  Lookformethodwiththesamesignaturefoundinstep22.  Otherwisesearchinsuperclassandetc.

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Acorrectequalsimplementation

@Overridepublicbooleanequals(Objecto){if(!(oinstanceofName))returnfalse;Namen=(Name)o;returnn.first.equals(first)&&n.last.equals(last);}

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Thelesson

•  AlwaysoverridehashCodeiffyouoverrideequals•  Alwaysuse@Overrideifyouintendtooverrideamethod

–  orletyourIDEgeneratethesemethodsforyou…