Classes,Objects,andInterfaces

CS5010ProgramDesignParadigms"Bootcamp"Lesson9.1

1©MitchellWand,2012-2015ThisworkislicensedunderaCreative Commons Attribution-NonCommercial 4.0 International License.

ModuleIntroduction

• Inthismodule,wewillseehowclasses,objects,andinterfacesfitintoouraccountofinformationanalysisanddatadesign

• We'llseehowthefunctionalandtheobject-orientedmodelsarerelated

• We'lllearnhowtoapplythedesignrecipeinanobject-orientedsetting.

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Generalization

OverConstants

OverExpressions

OverContexts

OverDataRepresentations

OverMethodImplementations

MixedData

DataRepresentations

Basics

RecursiveData

FunctionalData

Objects&Classes

Stateful Objects

Module09

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DesignStrategies

Combinesimplerfunctions

Useatemplate

DivideintoCases

Callamoregeneralfunction

CommunicateviaState

Recuronsubproblem(s)

Goalsofthislesson

• Learnthebasicsaboutclasses,objects,fields,andmethods.

• LearnhowtheseideasareexpressedintheRacketobjectsystem

• Weassumethatyoualreadyknowalittleaboutobject-orientedprogramming.

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Slogansforthislesson

• Classesarelikedefine-structs,butwithmethods(functions)aswellasfields.

• Everyobjectknowsitsclass.• Invokeamethodofanobjectbysendingitamessage.

• Theinterfaceofanobjectisthesetofmessagestowhichitresponds.

• Interfacesaredatatypes

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ClassesandObjects

• Aclassislikeadefine-struct.• Itspecifiesthenamesofthefieldsofitsobjects.

• Italsocontainssomemethods. Eachmethodhasanameandadefinition.

• TocreateanobjectofclassC,wesay(new C)

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Yousaymorethanthis,butthisisgoodenoughright

now.

Whatisanobject?• Anobjectisanotherwayofrepresentingcompounddata,like

astruct.• Likeastruct,ithasfields.• Ithasonebuilt-infield,calledthis,whichalwaysreferstothis

object• Herearepicturesoftwosimpleobjects:

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x = 10y = 20r = 10this =

h = 30w = 15color = "blue"this =

Weassumethatyou've seensomekindofobject-orientedprogrammingbefore,sowe'rejustreviewingvocabularyhere.

Ifyou'vereallyneverusedOOPbefore,godosomeoutsidereadingbeforecontinuing.

Everyobjectknowsitsclass(1)

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x = 10y = 20r = 10

x = 15y = 35r = 5

(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))...)

Herearetwoobjectsofthesameclass.Intheclassdefinition,theinit-fielddeclarationspecifiesthateachobjectofthisclasshas3fields,namedx,y,andr.Theclassdefinitionalsodefinestwomethods,namedfoo andbar,thatareapplicabletoanyobjectofthisclass.

Theseobjectsalsohaveathisfield,butwedon'tshowit

unlessweneedto.

Howdoyoucomputewithanobject?

• Toinvokeamethodofanobject,wesendtheobjectamessage.

• Forexample,toinvoketheareamethodofanobjectobj1,wewrite

(send obj1 area)• Ifobj1 isanobjectofclassC,thisinvokestheareamethodinclassC.

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Everyobjectknowsitsclass(2)

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x = 10y = 20r = 10

x = 15y = 35r = 5

(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))...)

obj1

obj2 Thevariablesinthemethoddeclarationsrefertothefieldsintheobject.So:

(sendobj1foo)returns30(sendobj2foo)returns50

Everyobjectknowsitsclass(3)

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x = 10y = 20r = 10

x = 15y = 35r = 5

(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))...)

obj1

obj2 Methodscanalsotakearguments,justlikefunctions.So

(sendobj1bar8)returns18(sendobj2bar8)returns13

Everyobjectknowsitsclass(4)

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(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))(define/public (baz n) (+ (send this foo)

n))...)

MethodsarejustRacketfunctions,sotheycandoanythingaRacketfunctioncando,includingsendmessagestoobjects.

(sendobj1baz 20)returns(+3020)=50(sendobj2baz 20)returns(+5020)=70

x = 10y = 20r = 10this =

obj1x = 15y = 35r = 5this =

obj2

Everyobjectknowsitsclass(5)

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(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))(define/public (baz n) (+ (send this foo)

n))...)

obj2obj1

x = 10y = 20r = 10this =

x = 15y = 35r = 5this =

a = 15b = 35c = 5this =

(class* object% () (init-field a b c)(define/public (foo) (+ a b))(define/public (bar n) (* c n))(define/public (baz n)

(+ (send this foo) n))...)

obj3

Here'sanotherobject,obj3,ofadifferentclass.Ifwesendamessagetoobj3,thenobj3'smethodswillbeinvoked.

Everyobjectknowsitsclass(6)

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(class* object% () (init-field x y r)(define/public (foo) (+ x y))(define/public (bar n) (+ r n))(define/public (baz n) (+ (send this foo)

n))...)

obj2obj1

x = 10y = 20r = 10this =

x = 15y = 35r = 5this = So

(sendobj2bar8)=(+58)=13(sendobj3bar8)=(*58)=40

a = 15b = 35c = 5this =

(class* object% () (init-field a b c)(define/public (foo) (+ a b))(define/public (bar n) (* c n))(define/public (baz n)

(+ (send this foo) n))...)

obj3

Theimportantthingaboutanobjectiswhatmethodsitrespondsto

• SoifIwrote(define (foo1 x) (send x bar 8))

• Icouldcallfoo1 onobj1,obj2,orobj3,becauseallofthemrespondtothebarmessagewithanintegerargument.

• Thecontractforfoo1 shouldspecifythatitsargumentwillacceptabarmessagewithanintegerargument.

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Interfacesaredatatypes

• Thesetofmessagestowhichanobjectresponds(alongwiththeircontracts)iscalleditsinterface.

• Sothecontractforfoo1 (oranyotherfunctionthattakesanobjectasanargument)shouldbeexpressedintermsofinterfaces.

• SointerfacesplaytheroleofdatatypesintheOOPsetting.

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UsingTheRacketClassSystem

• WewillusefullRacket(yay!)• Write

#lang racketatthebeginningofeachfile• AndsettheLanguagelevelto"DetermineLanguagefromSource"

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Interfacedefinition#lang racket

(require "extras.rkt")(require rackunit)

;; examples from Lesson 9.1

(define Interface1<%>(interface ()

foo ; -> Intbar ; Int -> Intbaz ; Int -> Int))

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InRacket,namesofinterfacesendwith<%> (by

convention)

Ignorethat ()fornow

Wewritedowneachmethodname,withitscontractasacomment.We

canwritetheminanyorder

foo isafunctionofnoarguments (legalin#lang racket)

AClassDefinition(1)(define Class1%

(class* object% (Interface1<%>)

(init-field x y r) ;; x,y,r : Int

(super-new) ;; required magic

;; foo : -> Int(define/public (foo) (+ x y))

;; bar : Int -> Int(define/public (bar n) (+ r n))

;; baz : Int -> Int(define/public (baz n) (+ (send this foo) n))

))

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Thismeansthatthisclassissupposed toimplementInterface1<%>. Ifweleaveoffoneofthemethods,we’llgetanerrormessage.

x,y,andr arethefieldnames.We’veputintheircontractsasacomment.Inarealexample,you’dputaninterpretation foreachfield,justasyoudothefields ofastruct.

object%and(super-new)arerequiredmagic.We’lllearnabouttheminalatermodule

AClassDefinition(2)(define Class1%

(class* object% (Interface1<%>)

(init-field x y r) ;; x,y,r : Int

(super-new) ;; required magic

;; foo : -> Int(define/public (foo) (+ x y))

;; bar : Int -> Int(define/public (bar n) (+ r n))

;; baz : Int -> Int(define/public (baz n) (+ (send this foo) n))

))

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Weusedefine/public todefinemethods.Herewe’vewrittenthe

contractforeachmethod; laterwe’llseewhattheDesignRecipedeliverablesformethodsare.

Anotherclassdefinition(define Class2%(class* object% (Interface1<%>)

(init-field a b c) ; a, b, c : Int

(super-new)

;; foo : -> Int(define/public (foo) (+ a b))

;; bar : Int -> Int(define/public (bar n) (* c n))

;; baz : Int -> Int(define/public (baz n)

(+ (send this foo) n))

))

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Here’sthedefinition ofClass2%.Observethatithasdifferent fieldnames,butthesamemethodnames.Themethoddefinitions refertothe

newfieldnames.

Yetanotherclassdefinition(define Class2%(class* object% (Interface1<%>)

(init-field a b c) ; a, b, c : Int

(super-new)

;; foo : -> Int(define/public (foo) (+ a b))

;; bar : Int -> Int(define/public (bar n) (* c n))

;; baz : Int -> Int(define/public (baz n) (+ (send this foo) n))

))

(define Class2a%(class* object% (Interface1<%>)

(init-field a b c) ; a, b, c : Int

; add a new field, initialized to (– a)(field [a1 (- a)])

(super-new)

;; foo : -> Int(define/public (foo) (- b a1))

;; bar : Int -> Int(define/public (bar n) (* c n))

;; baz : Int -> Int(define/public (baz n) (+ (send this foo) n))

))

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ObjectsofClass2%andClass2a%arebuiltthesamewayandgivethesameanswerforeverymethodcall.Anyprocedure thatworkswithonewillworkthesamewaywiththe

other.

Thisisanotherreasonwewritecontractsintermsofinterfaces,not

classes.

Creatingobjectsandtesting(define obj1 (new Class1% [x 10][y 20][r 10]))(define obj2 (new Class1% [y 35][x 15][r 5]))(define obj3 (new Class2% [a 15][b 35][c 5]))

(begin-for-test

(check-equal? (send obj1 foo) 30)(check-equal? (send obj2 foo) 50)

(check-equal? (send obj1 bar 8) 18)(check-equal? (send obj2 bar 8) 13)

(check-equal? (send obj1 baz 20) 50)(check-equal? (send obj2 baz 20) 70)

(check-equal? (send obj2 bar 8) 13)(check-equal? (send obj3 bar 8) 40)

)

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Hereisthesyntaxforcreatingobjects.The

fieldscanbelistedinanyorder.

Andherewesendtheobjectssomemessagesandcheckthattheresultsareaswepredicted

ontheslidesabove.

LessonSummary

• Inthislessonwe’velearned:– Classesarelikedefine-structs,butwithmethods(functions)aswellasfields.

– Everyobjectknowsitsclass.– Invokeamethodofanobjectbysendingitamessage.– Theinterfaceofanobjectisthesetofmessagestowhichitresponds.

– Interfacesaredatatypes.• We’veseenhowtodefineclasses,objects,andinterfacesintheRacketobjectsystem.

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NextSteps

• Studythefile09-1-basics.rktintheExamplesfolder

• Ifyouhavequestionsaboutthislesson,askthemontheDiscussionBoard

• Goontothenextlesson

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