Josh Bloch Charlie Garrodcharlie/courses/15-214/2016-fall/slides/... · 2016-09-13 · 1. (Compile...

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SchoolofComputerScience

PrinciplesofSo3wareConstruc9on: Objects,Design,andConcurrencyDesigningclassesDelega9onandinheritanceandbehavioralsubtyping(ohmy)JoshBloch CharlieGarrod

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Administrivia

•  Homework2dueThursday

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KeyconceptsfromThursday…

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Whatisacontract?

•  Agreementbetweenanobjectanditsuser•  Includes

–  Methodsignature(typespecifica9ons)–  Func9onalityandcorrectnessexpecta9ons–  Performanceexpecta9ons

•  Whatthemethoddoes,nothowitdoesit–  Interface(API),notimplementa9on

•  “Focusonconceptsratherthanopera9ons”

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The==operatorvs.equalsmethod

•  Forprimi9vesyoumustuse==•  Forobjectreferencetypes

–  The==operatorprovidesiden*tyseman*cs•  ExactlyasimplementedbyObject.equals•  EvenifObject.equalshasbeenoverridden•  Thisisseldomwhatyouwant!

–  youshould(almost)alwaysuse.equals–  Using==onanobjectreferenceisabadsmellincodeif(input=="yes")//Abug!!!

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TheequalscontractinEnglish

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

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

relaConoverallobjects

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Learninggoalsfortoday

•  Beabletoexplaininheritanceanddelega9on•  Applyinheritanceanddelega9onappropriatelyforreuse

–  Understandtheirtradeoffs•  Behavioralsubtypingandimplica9onsforspecifica9onandtes9ng

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Today:Class-levelreusewithdelega9onandinheritance

•  Delega9on•  Inheritance

–  Java-specificdetailsforinheritance•  Behavioralsubtyping:Liskov'sSubs9tu9onPrinciple•  Thursday:Designpabernsforimprovedclass-levelreuse•  Laterinthecourse:

–  System-levelreusewithlibrariesandframeworks

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Thepromiseofreuse:

# Products

Cost With reuse

Without reuse

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COMPOSITIONANDDELEGATION

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Recallourearliersor9ngexample:

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

interfaceComparator{booleancompare(inti,intj);}finalComparatorASCENDING=(i,j)->i<j;finalComparatorDESCENDING=(i,j)->i>j;staticvoidsort(int[]list,Comparatorcmp){…booleanmustSwap=cmp.compare(list[i],list[j]);…}

Version A:

Version B':

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Delega9on

•  Delega*onissimplywhenoneobjectreliesonanotherobjectforsomesubsetofitsfunc9onality–  e.g.here,theSorterisdelega9ngfunc9onalitytosomeComparator

•  Judiciousdelega9onenablescodereuse

interfaceComparator{booleancompare(inti,intj);}finalComparatorASCENDING=(i,j)->i<j;finalComparatorDESCENDING=(i,j)->i>j;staticvoidsort(int[]list,Comparatorcmp){…booleanmustSwap=cmp.compare(list[i],list[j]);…}

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Delega9on

•  Delega*onissimplywhenoneobjectreliesonanotherobjectforsomesubsetofitsfunc9onality–  e.g.here,theSorterisdelega9ngfunc9onalitytosomeComparator

•  Judiciousdelega9onenablescodereuse–  Sortercanbereusedwitharbitrarysortorders–  Comparatorscanbereusedwitharbitraryclientcodethatneedsto

compareintegers

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Usingdelega9ontoextendfunc9onality

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

•  Supposewewantalistthatlogsitsopera9onstotheconsole…

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Usingdelega9ontoextendfunc9onality

•  Onesolu9on: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)func9onalitytothatList

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Delega9onanddesign

•  Smallinterfaceswithclearcontracts•  Classestoencapsulatealgorithms,behaviors

–  E.g.,theComparator

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IMPLEMENTATIONINHERITANCEANDABSTRACTCLASSES

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Varia9onintherealworld: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(){…}

}

Implementa9oninheritanceforcodereuse

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

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

}

Implementa9oninheritanceforcodereuse

protectedelementsarevisibleinsubclasses

anabstractclassismissingtheimplementa9onofone

ormoremethods

anabstractmethodisle3tobe

implementedinasubclass

noneedtodefinegetBalance()–thecodeisinheritedfrom

AbstractAccount

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Inheritance:aglimpseatthehierarchy

•  ExamplesfromJava–  java.lang.Object–  Collec9onslibrary

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JavaCollec9onsAPI(excerpt)

Collection

List Set AbstractCollection

AbstractList

LinkedList

Vector

HashSet

AbstractSequentialList

AbstractSet

Cloneable

ArrayList

interfaces

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Benefitsofinheritance

•  Reuseofcode•  Modelingflexibility•  AJavaaside:

–  Eachclasscandirectlyextendonlyoneparentclass–  Aclasscanimplementmul9pleinterfaces

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Inheritanceandsubtyping

•  Inheritanceisforcodereuse–  Writecodeonceandonlyonce–  Superclassfeaturesimplicitlyavailablein

subclass

•  Subtypingisforpolymorphism–  Accessingobjectsthesameway,butgefng

differentbehavior–  Subtypeissubs9tutableforsupertype

classAextendsB

classAimplementsIclassAextendsB

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Typicalrolesforinterfacesandclasses

•  Aninterfacedefinesexpecta9ons/commitmentsforclients•  Aclassfulfillstheexpecta9onsofaninterface

–  Anabstractclassisaconvenienthybrid–  Asubclassspecializesaclass'simplementa9on

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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:preventsreassignmenttothefielda3erini9aliza9on•  Afinalmethod:preventsoverridingthemethod•  Afinalclass:preventsextendingtheclass

–  e.g.,publicfinalclassCheckingAccountImpl{…

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Note:type-cas9nginJava

•  Some9mesyouwantadifferenttypethanyouhave–  e.g., doublepi=3.14; intindianaPi=(int)pi;

•  Usefulifyouknowyouhaveamorespecificsubtype:–  e.g., Accountacct=…;CheckingAccountcheckingAcct=(CheckingAccount)acct;longfee=checkingAcct.getFee();–  WillgetaClassCastExceptioniftypesareincompa9ble

•  Advice:avoiddowncas9ngtypes–  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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Javadetails:Dynamicmethoddispatch

1.  (Compile9me)Determinewhichclasstolookin2.  (Compile9me)Determinemethodsignaturetobeexecuted

1.  Findallaccessible,applicablemethods2.  Selectmostspecificmatchingmethod

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Javadetails:Dynamicmethoddispatch

1.  (Compile9me)Determinewhichclasstolookin2.  (Compile9me)Determinemethodsignaturetobeexecuted

1.  Findallaccessible,applicablemethods2.  Selectmostspecificmatchingmethod

3.  (Run9me)Determinedynamicclassofthereceiver4.  (Run9me)Fromdynamicclass,locatemethodtoinvoke

1.  Lookformethodwiththesamesignaturefoundinstep22.  Otherwisesearchinsuperclassandetc.

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Designwithinheritance(ornot)

•  Favorcomposi9onoverinheritance–  Inheritanceviolatesinforma9onhiding

•  Designanddocumentforinheritance,orprohibitit–  Documentrequirementsforoverridinganymethod

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BEHAVIORALSUBTYPING

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Behavioralsubtyping

•  e.g.,Compiler-enforcedrulesinJava:–  Subtypescanadd,butnotremovemethods–  Concreteclassmustimplementallundefinedmethods–  Overridingmethodmustreturnsametypeorsubtype–  Overridingmethodmustacceptthesameparametertypes–  Overridingmethodmaynotthrowaddi9onalexcep9ons

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

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Behavioralsubtyping

•  e.g.,Compiler-enforcedrulesinJava:–  Subtypescanadd,butnotremovemethods–  Concreteclassmustimplementallundefinedmethods–  Overridingmethodmustreturnsametypeorsubtype–  Overridingmethodmustacceptthesameparametertypes–  Overridingmethodmaynotthrowaddi9onalexcep9ons

•  Alsoappliestospecifiedbehavior:–  Sameorstrongerinvariants–  Sameorstrongerpostcondi9onsforallmethods–  Sameorweakerprecondi9onsforallmethods

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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abstract class Vehicle { int speed, limit;

//@ invariant speed < limit;

//@ requires speed != 0; //@ ensures speed < \old(speed) void brake();

}

class Car extends Vehicle { int fuel; boolean engineOn; //@ invariant speed < limit; //@ invariant fuel >= 0;

//@ requires fuel > 0 && !engineOn; //@ ensures engineOn; void start() { … }

void accelerate() { … }

//@ requires speed != 0; //@ ensures speed < \old(speed) void brake() { … }

}

Subclass fulfills the same invariants (and additional ones) Overridden method has the same pre and postconditions

Behavioralsubtyping(LiskovSubs9tu9onPrinciple)

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class Car extends Vehicle { int fuel; boolean engineOn; //@ invariant fuel >= 0;

//@ requires fuel > 0 && !engineOn; //@ ensures engineOn; void start() { … }


//@ requires speed != 0; //@ ensures speed < old(speed) void brake() { … }

}

class Hybrid extends Car { int charge; //@ invariant charge >= 0;

//@ requires (charge > 0 || fuel > 0) && !engineOn; //@ ensures engineOn; void start() { … }


//@ requires speed != 0; //@ ensures speed < \old(speed) //@ ensures charge > \old(charge) void brake() { … }

}

Subclass fulfills the same invariants (and additional ones) Overridden method start has weaker precondition Overridden method brake has stronger postcondition


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class Rectangle { int h, w; Rectangle(int h, int w) { this.h=h; this.w=w; }

//methods

}

class Square extends Rectangle { Square(int w) { super(w, w); }

}

Is this Square a behavioral subtype of Rectangle?


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class Rectangle { int h, w; Rectangle(int h, int w) { this.h=h; this.w=w; }

//methods

}

class Square extends Rectangle { Square(int w) { super(w, w); }

}

Is this Square a behavioral subtype of Rectangle? (Yes.)


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class Rectangle { //@ invariant h>0 && w>0; int h, w; Rectangle(int h, int w) { this.h=h; this.w=w; }

//methods

}

class Square extends Rectangle { //@ invariant h>0 && w>0;

//@ invariant h==w; Square(int w) { super(w, w); }

}



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//methods

}

class Square extends Rectangle { //@ invariant h>0 && w>0;

//@ invariant h==w; Square(int w) { super(w, w); }

}



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//@ requires factor > 0;

void scale(int factor) { w=w*factor; h=h*factor; }

}

class Square extends Rectangle { //@ invariant h>0 && w>0; //@ invariant h==w; Square(int w) { super(w, w); }

}



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//@ requires factor > 0;


}


}



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//@ requires factor > 0; void scale(int factor) { w=w*factor; h=h*factor; } //@ requires neww > 0; void setWidth(int neww) { w=neww; }

}


}



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void setWidth(int neww) { w=neww; }

}


}

Yes? (But the Square is not actually a square…)

← Invalidates stronger invariant (w==h) in subclass

classGraphicProgram{voidscaleW(Rectangler,intfactor){r.setWidth(r.getWidth()*factor);}}


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Summary:Designingreusableclasses

•  Reusableimplementa9onswithsimple,clearcontracts•  Inheritanceforreuse,itspipalls,anditsalterna9ves•  Liskov'sSubs9tu9onPrincipleforbehavioralsubtyping

Josh Bloch Charlie Garrodcharlie/courses/15-214/2016-fall/slides/... · 2016-09-13 · 1. (Compile...

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