Lecture3–Programming2ObjectOrientedProgrammingInWeek2wecoveredMethodsarguments,parametersreturnvoidactivationstack/objectheapaccessingmembersviadotoperator(.)ThisWeekConstructsif,switchfor,whileClassdevelopmentConstructorsGet/SetmethodsAccessspecificationNextcoupleofweeks...InheritanceDynamicbindingAbstractclasses/methodsPolymorphismLectureTest!!!NewfunctionalityOurprogramcurrentlyprocessesaSquare,thenaRectangleandthenaCircle.Let’smodifyitsothattheusercanindicatewhichshapeheorshewantstoworkwith.Thenewprogramwillasktheusertoenter1-foraSquare2-foraRectangle3-foraCircleIfStatementLikePython,JavahasanifstatementTheifstatementallowsachoicebetweendifferentexecutionpathsItchecksabooleanconditionalexpression(true/false)IftheexpressionistrueTheifpathisfollowedIftheexpressionisfalseTheelsepath(ifitexists)isfollowedUnlikePythonthereisnoelifkeywordWecanuseelseifhoweverThemodifiedmainmethodSystem.out.print("Enter1forSquare,2forRectangle,3for Circle:");intchoice=scan.nextInt();if(choice==1) processSquare(scan);elseif(choice==2) processRectangle(scan);elseif(choice==3) processCircle(scan);else System.out.println("Wrongnumber");NotesWhenweruntheaboveprogramonlyoneshapewillbeprocessed–not3Theshapeprocessedwillbedeterminedbytheuser–byentering1,2or3Iftheuserenterssomethingotherthan1,2or3theprogramwillgiveanerrormessageMorethanonestatementIftheprogramisrequiredtoexecutemorethanonestatementonanyparticularpath,theyneedtobeenclosedinbraces{}if(choice==1){ System.out.println(“Youhavechosen1”); processSquare(); }IfthebracesareomittedonlythefirststatementwillbelongtotheifprocessSquarewillbecallednomatterwhatvaluechoicehasAnothermethod-ifTestAgaintoavoidclutteringupthemainmethodwe’llputthenewcodeintoamethodcalledifTestThismethodwillbevoid(itwon’treturnavalue)andwilltakeoneparameter(ofclassScanner)ThecodeforthismethodisshownonthenextslideThecodeforthemodifiedmainmethodwillbeshownontheslideafterthatifTestcodevoidifTest(Scannerscan){ System.out.print("Enter1forSquare,2for Rectangle,3 forCircle:"); intchoice=scan.nextInt(); if(choice==1) processSquare(scan); elseif(choice==2) processRectangle(scan); elseif(choice==3) processCircle(scan); else System.out.println("Wrongnumberentered");}LoopsLet’sconsiderasituationwherewewanttoallowtheusertoprocess5shapesandwewanthimorhertocontrolwhichshapestheyareLikePython,Javaprovidestwodifferentloopingstructures(thereareothersbutwewon’tcoverthem)forloop -definiteloopwhileloop -indefiniteloopWhileloopSimilartothewhileloopinPythonStartswiththekeywordwhileGovernedbyaconditionalexpression(true/false)IftheexpressionevaluatestotrueTheloopiteratesTheexpressionisevaluatedagainTheloopwillcontinuetoiterateuntiltheexpressionevaluatestofalseBecarefulofinfiniteloops–usuallyabadthingCodedemonstratingwhileloopvoidwhileTest(Scannerscan){ inti=0; while(i<5){ ifTest(scan); i++; }}NotethattheloopconsistsofthreemajorelementsAstatementthatinitialisesthecountervariable(i)Anexpressionthattestswhetheriislessthan5Astatementtoadd1toi–otherwiseinfiniteloop(why?)AforloopbringstheseelementstogetherintooneplaceForloopfor(inti=0;i<5;i++){ ifTest(scan);}TheforloopismadeupofthefollowingpartsThekeywordforAgroupofthreeexpressionsenclosedinbracketsThefirstexpressionistheinitialiserItiscalledonlyonce–whentheforloopisenteredWecandeclareanewvariableatthesametimeFurthermore,wecangiveitavalueSowecreateanintvariablecalledi,andinitialiseitto0ThesecondexpressionisaconditionalexpressionTheforloopwilliterateonlyifthisexpressionistrueThethirdexpressionupdatestheforloopcounter(i)1isaddedtoiaftereachiterationMethodtotesttheforloopvoidforTest(Scannerscan){ for(inti=0;i<5;i++){ ifTest(scan); }}NotethatthefirstexpressiondeclaresiandintialisesitatthesametimeSwitchstatementNotcoveredinProgramming1(sopayattention)AdecisionconstructliketheifstatementItevaluatesanint(orbyte,char,short,String)expressionItthenexaminesalistofcasesThecasethatmatches(hasthesamevalue)astheexpressionischosenandthecodeassociatedwithitisexecuted.ThecodeonthenextslideshowsthecodeinifTestbeingrewrittenasaswitchstatementMethodtotestswitchstatementvoidswitchTest(Scannerscan){ System.out.print(“1forSquare,2forRectangle,3forCircle:"); intchoice=scan.nextInt(); switch(choice){ case1: processSquare(scan); break; case2: processRectangle(scan); break; case3: processCircle(scan); break; default: System.out.println("Wrongnumberentered"); }}NotesoncodefrompreviousslideNotetheuseofthekeywordsswitchandcaseTheexpressionfollowingswitchisenclosedinbracesItshouldevaluatetoanintvalue.AdoubleorfloatexpressionwillcauseanerrorThekeywordbreakisusedthreetimes.Itcausesexecutiontoleave(breakoutof)theswitchstatementIfthebreakisnottherethenexecutionwillfallthroughtothenextcaseForexample,ifthebreakwasomittedafterprocessSquareandtheuserentered1,thenprocessSquarewouldbecalledandthenprocessRectangle–whichisprobablynotwhatyouwantThefinalalternativeisdefault.Itischosenwheneverchoicedoesnotequal1,2or3ModifiedcodeformethodforTestvoidforTest(Scannerscan){ for(inti=0;i<5;i++){ //ifTest(scan); switchTest(scan); }}WehavecommentedoutifTestandinvokedswitchTest.Theresultshouldbeexactlythesame.SummaryWe’vediscussedtheuseofvariouscontrolstructuresif,switch,while,forWewillnowmoveontoanimportanttopicThecreationofindividualclassesWewilldothisbyidentifyingappropriateobjectsintheShapesclassandwritingclassestomodelthemShapesclassConsidertheShapesclassWecreatedseveralmethodsthatdealtwithvariousshapesCircles,Squares,RectanglesThisclasscanbeinstantiatedanditsmethodsinvokedButwemightfindmoreuseforclassesthatdealwithcircles,squareandrectanglesindividuallyTheseareeasiertoworkwithandtoreuseASquareclassWemightdecidethatwewanttomakeanobjecttomodelaSquareRememberanobjectmodelsbothdatafunctionalityLet’sfirstconsiderthedataassociatedwithaSquareWeneedonepieceofdataThelengthofthesideofthesquareInstanceVariablesThedataisincludedintheclassasinstancevariables(fields).Thesearesimilarinmanywaystothevariablesthatwe’vealreadyencounteredWe’llmakethesideofasquare(we’llcallit‘side’)adouble(althoughwecouldmakeitafloattooifwewanted)OurSquareclasssofarpublicclassSquare{ doubleside;}AboveisthecodeforaclasscalledSquare.Thekeywordpublicmeansthatit’savailableforotherclassestouseThekeywordclassdesignatesthatit’saclassTheidentifierSquareischosenbytheprogrammerandshouldbemeaningfulAlloftheinstancevariablesandmethodsthatbelongtotheclassareenclosedinsidebraces{}TheclasssofarconsistsonlyofasingleinstancevariableandthusisnotveryusefulasyetMethodsNowweneedtothinkaboutthefunctionalityoftheclassWhatsortofthingsarewegoingtowanttodowiththedata?InourSquareclasstherearetwoobviouscandidatesformethodsdoublecalculateAreaOfSquare(double)doublecalculatePerimeterOfSquare(double)Eachofthemtakeasingleparameteroftypedoubleandreturnavalueoftypedouble.OurSquareclasssofarpublicclassSquare{ doubleside; doublecalculateAreaOfSquare(doubleside){ returnside*side; } doublecalculatePerimeterOfSquare(doubleside){ return4*side; }}TheclassSquareWeaddedtwomethodstoourSquareclasscalculateAreaOfSquarecalculatePerimeterOfSquareWe’llmakeacoupleofchangestothese.Firstly,we’llmakethenamesabitshorter.TheybelongtotheSquareclassnowsowedon’tneedtoreflectthatinthename.We’llmaketheirnamescalcAreacalcPerimeterSecondly,wehavesaidthatthepurposeofthemethodsistomanipulatethedataintheinstancevariables.Therefore,thereisnoneedtopass‘side’asanargument.OurSquareclasssofarpublicclassSquare{ doubleside; doublecalcArea(){ returnside*side; } doublecalcPerimeter(){ return4*side; }}ClassdiagramsClassesareoftensummarisedinclassdiagramsForourpurposesaclassdiagramisaboxwiththreesectionsThetopsectionhasthenameoftheclassThemiddlesectioncontainstheinstancevariablesThebottomsectioncontainsthemethodsClassdiagramsusuallycontainmanysuchboxesandrepresenttherelationshipsbetweenthem.However,wewillstartwithasingleclass.ClassDiagramSquaredoubleside;doublecalcArea()doublecalcPerimeter()CreatingSquareobjectsNowthatwehavedefinedaSquareclasshowdoweuseitinourprogram.Theoriginalcodewas

System.out.println(“Enterthesideofasquare:”);doubleside=scan.nextDouble();doubleareaOfSquare=s.calculateAreaOfSquare(side); System.out.println(“Areais“+areaOfSquare);WewillrewritethiscodeinthenextcoupleofslidestouseournewSquareclassCreatingaSquareobjectWe’vepreviouslyusedthenewoperatortocreatevariousobjects.WewillnowuseittocreateanewSquareobject Squaresq=newSquare()Wecannowusesqtointeractwiththenewlycreatedobject.GivingtheinstancevariableavalueWewillnowgivetheinstancevariablesideavalue.Thevaluewillbewhateverisenteredbytheuser.Wecandothisthroughtheuseofthedotoperator(.) sq.side=side;Wecanthenusethemethodtocalculatethearea area=sq.calculateArea();DatahidingRememberwhenwemadeuseoftheAPIspecificationsinordertousetheJFrameclass?Rememberthatallthatwaslistedweremethods.Wedidnotseeanyinstancevariableslistedforthisclass.ThiswillbetrueofthevastmajorityofclassesintheAPI.Thisisadeliberatedecisiononthepartoftheclassdeveloperstohidethedataandmakeaccesstoitavailableonlythroughitsmethods.AccessSpecificationJavaprovidesthreeaccessspecifierswhichdeterminehowaparticularmembercanbeaccessed.Thesearepublic -availabletoanyotherclassprivate -onlyavailabletomethodsinitsown classprotected -availabletosubclasses(moreonthis whenwecoverinheritance)Ifyoudon’tgiveamemberanaccessspecifierthenthedefault(sometimescalledpackage)isused.Thememberwillbeavailabletoallclassesinthesamepackagebutnoothers.Whatshouldbeprivate?We’vealreadyalludedtothisItisusualtomakeallinstancevariablesprivateThemethodsthatyouwanttomakeavailabletootherclassesshouldbepublicTheremaybesomemethodswhoseonlyjobistobecalledbyothermethodsintheclass.Theseshouldbeprivateaswell.OurSquareclasssofarpublicclassSquare{ privatedoubleside; publicdoublecalcArea(){ returnside*side; } publicdoublecalcPerimeter(){ return4*side; }}calcAreaandcalcPerimetercanbeusedbyotherclasses.Theinstancevariable,side,canonlybeaccessedbymethodsbelongingtoSquareWhymakethedataprivate?Makingthedataprivategivestheclassdevelopersomecontroloverhowtheobjectsareused.Let’ssaythatweasclassdevelopersdecidethatthesideofaSquareshouldneverbenegative.Itshouldonlyeverbe0orpositive.Byallowing sq.side=lengthWecannotpreventsidefromreceivinganegativenumber.AccessingprivateinstancevariableNowthatwehavemadesideprivatethestatementaboveisnolongerlegalandwillresultinanerror.ButnowwehavenowayatallofgivingsideavalueandsoourclassnolongerhasanyuseTherearetwoverycommonsortsofmethodthatallowustoovercomethisproblem.Thesearecalledgetmethods(orgetters)setmethods(orsetters)GetmethodThecommonformofnamingagetmethodisbyusingtheprefix‘get’andthenthenameoftheinstancevariablestartingwithanuppercaseletterThusagetmethodforsidewouldbecalledgetSideAgetmethodhasnoparameterandhasareturntypewhichisthesameasitsinstancevariableInthiscasedoubleIt’susedtoreturnthecurrentvalueoftheinstancevariabledoublegetSide(){ returnside;}SetmethodThecommonformofnamingasetmethodisbyusingtheprefix‘set’andthenthenameoftheinstancevariablestartingwithanuppercaseletterThusasetmethodforsidewouldbecalledsetSideAsetmethodhasnoreturntype(void)andhasasingleparameterwhichisthesametypeasitsinstancevariableInthiscasedoubleIt’susedtosetthecurrentvalueoftheinstancevariablevoidsetSide(doublelen){ side=len;}ModifyingthesetmethodNowthatwecanonlysetthevalueofsidethroughitssetmethod,wecanwritecodetoensurethatonlypositivenumbers(andzero)canbeused.Ifthevalueoftheparameterisnegativethesetterwillsetthevalueofsideto0.Otherwiseitwillsetittothevalueoftheparameter voidsetSide(doublelen){ if(len<0) side=0;