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Chapter1

JavaFundamentals

KeySkills&Concepts

KnowthehistoryandphilosophyofJava

UnderstandJavascontributiontotheInternet

Understandtheimportanceofbytecode

KnowtheJavabuzzwords

Understandthefoundationalprinciplesofobjectorientedprogramming

Create,compile,andrunasimpleJavaprogram

Usevariables

Usetheifandforcontrolstatements

Createblocksofcode

Understandhowstatementsarepositioned,indented,andterminated

KnowtheJavakeywords

UnderstandtherulesforJavaidentifiers

heriseoftheInternetandtheWorldWideWebfundamentallyreshapedcomputing.PriortotheWeb,thecyberlandscapewas

dominatedbystandalonePCs.Today,nearlyallcomputersareconnectedtotheInternet.TheInternet,itself,wastransformedoriginallyofferingaconvenientwaytosharefilesandinformation.Todayitisavast,distributedcomputinguniverse.Withthesechangescameanewwaytoprogram:Java.

JavaisthepreeminentlanguageoftheInternet,butitismorethanthat.Javarevolutionizedprogramming,changingthewaythatwethinkaboutboththeformandthefunctionofaprogram.TobeaprofessionalprogrammertodayimpliestheabilitytoprograminJavaitisthatimportant.Inthecourseofthisbook,youwilllearntheskillsneededtomasterit.

ThepurposeofthischapteristointroduceyoutoJava,includingitshistory,itsdesignphilosophy,andseveralofitsmostimportantfeatures.Byfar,thehardestthingaboutlearningaprogramminglanguageisthefactthatnoelementexistsinisolation.Instead,thecomponentsofthelanguageworkinconjunctionwitheachother.ThisinterrelatednessisespeciallypronouncedinJava.Infact,itisdifficulttodiscussoneaspectofJavawithoutinvolvingothers.Tohelpovercomethisproblem,thischapterprovidesabriefoverviewofseveralJavafeatures,includingthegeneralformofaJavaprogram,somebasiccontrolstructures,andoperators.Itdoesnotgointotoomanydetailsbut,rather,concentratesonthegeneralconceptscommontoanyJavaprogram.

TheOriginsofJava

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Computerlanguageinnovationisdrivenforwardbytwofactors:improvementsintheartofprogrammingandchangesinthecomputingenvironment.Javaisnoexception.BuildingupontherichlegacyinheritedfromCandC++,Javaaddsrefinementsandfeaturesthatreflectthecurrentstateoftheartinprogramming.Respondingtotheriseoftheonlineenvironment,Javaoffersfeaturesthatstreamlineprogrammingforahighlydistributedarchitecture.

JavawasconceivedbyJamesGosling,PatrickNaughton,ChrisWarth,EdFrank,andMikeSheridanatSunMicrosystemsin1991.ThislanguagewasinitiallycalledOakbutwasrenamedJavain1995.Somewhatsurprisingly,theoriginalimpetusforJavawasnottheInternet!Instead,theprimarymotivationwastheneedforaplatformindependentlanguagethatcouldbeusedtocreatesoftwaretobeembeddedinvariousconsumerelectronicdevices,suchastoasters,microwaveovens,andremotecontrols.Asyoucanprobablyguess,manydifferenttypesofCPUsareusedascontrollers.Thetroublewasthat(atthattime)mostcomputerlanguagesweredesignedtobecompiledforaspecifictarget.Forexample,considerC++.

AlthoughitwaspossibletocompileaC++programforjustaboutanytypeofCPU,todosorequiredafullC++compilertargetedforthatCPU.Theproblem,however,isthatcompilersareexpensiveandtimeconsumingtocreate.Inanattempttofindabettersolution,Goslingandothersworkedonaportable,crossplatformlanguagethatcouldproducecodethatwouldrunonavarietyofCPUsunderdifferingenvironments.ThiseffortultimatelyledtothecreationofJava.

AboutthetimethatthedetailsofJavawerebeingworkedout,asecond,andultimatelymoreimportant,factoremergedthatwouldplayacrucialroleinthefutureofJava.Thissecondforcewas,ofcourse,theWorldWideWeb.HadtheWebnottakenshapeataboutthesametimethatJavawasbeingimplemented,Javamighthaveremainedausefulbutobscurelanguageforprogrammingconsumerelectronics.However,withtheemergenceoftheWeb,Javawaspropelledtotheforefrontofcomputerlanguagedesign,becausetheWeb,too,demandedportableprograms.

Mostprogrammerslearnearlyintheircareersthatportableprogramsareaselusiveastheyaredesirable.Whilethequestforawaytocreateefficient,portable(platformindependent)programsisnearlyasoldasthedisciplineofprogrammingitself,ithadtakenabackseattoother,morepressingproblems.However,withtheadventoftheInternetandtheWeb,theoldproblemofportabilityreturnedwithavengeance.Afterall,theInternetconsistsofadiverse,distributeduniversepopulatedwithmanytypesofcomputers,operatingsystems,andCPUs.

Whatwasonceanirritatingbutalowpriorityproblemhadbecomeahighprofilenecessity.By1993,itbecameobvioustomembersoftheJavadesignteamthattheproblemsofportabilityfrequentlyencounteredwhencreatingcodeforembeddedcontrollersarealsofoundwhenattemptingtocreatecodefortheInternet.ThisrealizationcausedthefocusofJavatoswitchfromconsumerelectronicstoInternetprogramming.So,whileitwasthedesireforanarchitectureneutralprogramminglanguagethatprovidedtheinitialspark,itwastheInternetthatultimatelyledtoJavaslargescalesuccess.

HowJavaRelatestoCandC++

JavaisdirectlyrelatedtobothCandC++.JavainheritsitssyntaxfromC.ItsobjectmodelisadaptedfromC++.JavasrelationshipwithCandC++isimportantforseveralreasons.First,manyprogrammersarefamiliarwiththeC/C++syntax.ThismakesiteasyforaC/C++programmertolearnJavaand,conversely,foraJavaprogrammertolearnC/C++.

Second,Javasdesignersdidnotreinventthewheel.Instead,theyfurtherrefinedanalreadyhighlysuccessfulprogrammingparadigm.ThemodernageofprogrammingbeganwithC.ItmovedtoC++,andnowtoJava.Byinheritingandbuildinguponthatrichheritage,Javaprovidesapowerful,logicallyconsistentprogrammingenvironmentthattakesthebestofthepastandaddsnewfeaturesrequiredbytheonlineenvironment.Perhapsmostimportant,becauseoftheirsimilarities,C,C++,andJavadefineacommon,conceptualframeworkfortheprofessionalprogrammer.Programmersdonotfacemajorriftswhenswitchingfromonelanguagetoanother.

OneofthecentraldesignphilosophiesofbothCandC++isthattheprogrammerisincharge!Javaalsoinheritsthisphilosophy.ExceptforthoseconstraintsimposedbytheInternetenvironment,Javagivesyou,theprogrammer,fullcontrol.Ifyouprogramwell,yourprogramsreflectit.Ifyouprogrampoorly,yourprogramsreflectthat,too.Putdifferently,Javaisnotalanguagewithtrainingwheels.Itisalanguageforprofessionalprogrammers.

JavahasoneotherattributeincommonwithCandC++:itwasdesigned,tested,andrefinedbyreal,workingprogrammers.Itisalanguagegroundedintheneedsandexperiencesofthepeoplewhodevisedit.Thereisnobetterwaytoproduceatopflightprofessionalprogramming



language.

BecauseofthesimilaritiesbetweenJavaandC++,especiallytheirsupportforobjectorientedprogramming,itistemptingtothinkofJavaassimplytheInternetversionofC++.However,todosowouldbeamistake.Javahassignificantpracticalandphilosophicaldifferences.AlthoughJavawasinfluencedbyC++,itisnotanenhancedversionofC++.Forexample,itisneitherupwardlynordownwardlycompatiblewithC++.Ofcourse,thesimilaritieswithC++aresignificant,andifyouareaC++programmer,youwillfeelrightathomewithJava.Anotherpoint:JavawasnotdesignedtoreplaceC++.Javawasdesignedtosolveacertainsetofproblems.C++wasdesignedtosolveadifferentsetofproblems.Theywillcoexistformanyyearstocome.

HowJavaRelatestoC#

AfewyearsafterthecreationofJava,MicrosoftdevelopedtheC#language.ThisisimportantbecauseC#iscloselyrelatedtoJava.Infact,manyofC#sfeaturesdirectlyparallelJava.BothJavaandC#sharethesamegeneralC++stylesyntax,supportdistributedprogramming,andutilizethesameobjectmodel.Thereare,ofcourse,differencesbetweenJavaandC#,buttheoveralllookandfeeloftheselanguagesisverysimilar.ThismeansthatifyoualreadyknowC#,thenlearningJavawillbeespeciallyeasy.Conversely,ifC#isinyourfuture,thenyourknowledgeofJavawillcomeinhandy.

GiventhesimilaritybetweenJavaandC#,onemightnaturallyask,WillC#replaceJava?TheanswerisNo.JavaandC#areoptimizedfortwodifferenttypesofcomputingenvironments.JustasC++andJavawillcoexistforalongtimetocome,sowillC#andJava.

JavasContributiontotheInternet

TheInternethelpedcatapultJavatotheforefrontofprogramming,andJava,inturn,hadaprofoundeffectontheInternet.Inadditiontosimplifyingwebprogrammingingeneral,Javainnovatedanewtypeofnetworkedprogramcalledtheappletthatchangedthewaytheonlineworldthoughtaboutcontent.JavaalsoaddressedsomeofthethorniestissuesassociatedwiththeInternet:portabilityandsecurity.Letslookmorecloselyateachofthese.

JavaApplets

AnappletisaspecialkindofJavaprogramthatisdesignedtobetransmittedovertheInternetandautomaticallyexecutedbyaJavacompatiblewebbrowser.Furthermore,anappletisdownloadedondemand,withoutfurtherinteractionwiththeuser.Iftheuserclicksalinkthatcontainsanapplet,theappletwillbeautomaticallydownloadedandruninthebrowser.Appletsareintendedtobesmallprograms.Theyaretypicallyusedtodisplaydataprovidedbytheserver,handleuserinput,orprovidesimplefunctions,suchasaloancalculator,thatexecutelocally,ratherthanontheserver.Inessence,theappletallowssomefunctionalitytobemovedfromtheservertotheclient.

ThecreationoftheappletchangedInternetprogrammingbecauseitexpandedtheuniverseofobjectsthatcanmoveaboutfreelyincyberspace.Ingeneral,therearetwoverybroadcategoriesofobjectsthataretransmittedbetweentheserverandtheclient:passiveinformationanddynamic,activeprograms.Forexample,whenyoureadyouremail,youareviewingpassivedata.Evenwhenyoudownloadaprogram,theprogramscodeisstillonlypassivedatauntilyouexecuteit.Bycontrast,theappletisadynamic,selfexecutingprogram.Suchaprogramisanactiveagentontheclientcomputer,yetitisinitiatedbytheserver.

Asdesirableasdynamic,networkedprogramsare,theyalsopresentseriousproblemsintheareasofsecurityandportability.Obviously,aprogramthatdownloadsandexecutesautomaticallyontheclientcomputermustbepreventedfromdoingharm.Itmustalsobeabletoruninavarietyofdifferentenvironmentsandunderdifferentoperatingsystems.Asyouwillsee,Javasolvedtheseproblemsinaneffectiveandelegantway.Letslookabitmorecloselyateach.

Security

Asyouarelikelyaware,everytimethatyoudownloadanormalprogram,youaretakingariskbecausethecodeyouaredownloadingmightcontainavirus,Trojanhorse,orotherharmfulcode.Atthecoreoftheproblemisthefactthatmaliciouscodecancauseitsdamagebecauseithasgainedunauthorizedaccesstosystemresources.Forexample,avirusprogrammightgatherprivateinformation,suchascreditcardnumbers,bankaccountbalances,andpasswords,bysearchingthecontentsofyourcomputerslocalfilesystem.InorderforJavatoenableappletstobesafelydownloadedandexecutedontheclientcomputer,itwasnecessarytopreventanappletfromlaunchingsuchanattack.

JavaachievedthisprotectionbyconfininganapplettotheJavaexecutionenvironmentandnotallowingitaccesstootherpartsofthecomputer.(Youwillseehowthisisaccomplishedshortly.)Theabilitytodownloadappletswithconfidencethatnoharmwillbedoneandthatno



securitywillbebreachedisconsideredbymanytobethesinglemostinnovativeaspectofJava.

Portability

PortabilityisamajoraspectoftheInternetbecausetherearemanydifferenttypesofcomputersandoperatingsystemsconnectedtoit.IfaJavaprogramweretoberunonvirtuallyanycomputerconnectedtotheInternet,thereneededtobesomewaytoenablethatprogramtoexecuteondifferentsystems.Forexample,inthecaseofanapplet,thesameappletmustbeabletobedownloadedandexecutedbythewidevarietyofdifferentCPUs,operatingsystems,andbrowsersconnectedtotheInternet.Itisnotpracticaltohavedifferentversionsoftheappletfordifferentcomputers.Thesamecodemustworkinallcomputers.Therefore,somemeansofgeneratingportableexecutablecodewasneeded.Asyouwillsoonsee,thesamemechanismthathelpsensuresecurityalsohelpscreateportability.

JavasMagic:TheBytecode

ThekeythatallowsJavatosolveboththesecurityandtheportabilityproblemsjustdescribedisthattheoutputofaJavacompilerisnotexecutablecode.Rather,itisbytecode.BytecodeisahighlyoptimizedsetofinstructionsdesignedtobeexecutedbytheJavaruntimesystem,whichiscalledtheJavaVirtualMachine(JVM).Inessence,theoriginalJVMwasdesignedasaninterpreterforbytecode.Thismaycomeasabitofasurprisebecausemanymodernlanguagesaredesignedtobecompiledintoexecutablecodeduetoperformanceconcerns.However,thefactthataJavaprogramisexecutedbytheJVMhelpssolvethemajorproblemsassociatedwithwebbasedprograms.Hereiswhy.

TranslatingaJavaprogramintobytecodemakesitmucheasiertorunaprograminawidevarietyofenvironmentsbecauseonlytheJVMneedstobeimplementedforeachplatform.Oncetheruntimepackageexistsforagivensystem,anyJavaprogramcanrunonit.Remember,althoughthedetailsoftheJVMwilldifferfromplatformtoplatform,allunderstandthesameJavabytecode.IfaJavaprogramwerecompiledtonativecode,thendifferentversionsofthesameprogramwouldhavetoexistforeachtypeofCPUconnectedtotheInternet.Thisis,ofcourse,notafeasiblesolution.Thus,theexecutionofbytecodebytheJVMistheeasiestwaytocreatetrulyportableprograms.

ThefactthataJavaprogramisexecutedbytheJVMalsohelpstomakeitsecure.BecausetheJVMisincontrol,itcancontaintheprogramandpreventitfromgeneratingsideeffectsoutsideofthesystem.SafetyisalsoenhancedbycertainrestrictionsthatexistintheJavalanguage.

Whenaprogramisinterpreted,itgenerallyrunsslowerthanthesameprogramwouldrunifcompiledtoexecutablecode.However,withJava,thedifferentialbetweenthetwoisnotsogreat.Becausebytecodehasbeenhighlyoptimized,theuseofbytecodeenablestheJVMtoexecuteprogramsmuchfasterthanyoumightexpect.

AlthoughJavawasdesignedasaninterpretedlanguage,thereisnothingaboutJavathatpreventsontheflycompilationofbytecodeintonativecodeinordertoboostperformance.Forthisreason,theHotSpottechnologywasintroducednotlongafterJavasinitialrelease.HotSpotprovidesajustintime(JIT)compilerforbytecode.WhenaJITcompilerispartoftheJVM,selectedportionsofbytecodearecompiledintoexecutablecodeinrealtimeonapiecebypiece,demandbasis.ItisimportanttounderstandthatitisnotpracticaltocompileanentireJavaprogramintoexecutablecodeallatoncebecauseJavaperformsvariousruntimechecksthatcanbedoneonlyatruntime.Instead,aJITcompilercompilescodeasitisneeded,duringexecution.Furthermore,notallsequencesofbytecodearecompiledonlythosethatwillbenefitfromcompilation.Theremainingcodeissimplyinterpreted.However,thejustintimeapproachstillyieldsasignificantperformanceboost.Evenwhendynamiccompilationisappliedtobytecode,theportabilityandsafetyfeaturesstillapplybecausetheJVMisstillinchargeoftheexecutionenvironment.

AsktheExpert

Q:IhaveheardaboutaspecialtypeofJavaprogramcalledaservlet.Whatisit?

A:Aservletisasmallprogramthatexecutesonaserver.Justasappletsdynamicallyextendthefunctionalityofawebbrowser,servletsdynamicallyextendthefunctionalityofawebserver.Itishelpfultounderstandthatasusefulasappletscanbe,theyarejustonehalfoftheclient/serverequation.NotlongaftertheinitialreleaseofJava,itbecameobviousthatJavawouldalsobeusefulontheserverside.Theresultwastheservlet.Thus,withtheadventoftheservlet,Javaspannedbothsidesoftheclient/serverconnection.Althoughthecreationofservletsisbeyondthescopeofthisbeginnersguide,theyaresomethingthatyouwillwanttostudyfurtherasyouadvanceinJavaprogramming.(CoverageofservletscanbefoundinmybookJava:TheCompleteReference,publishedby



OraclePress/McGrawHillEducation.)

TheJavaBuzzwords

NooverviewofJavaiscompletewithoutalookattheJavabuzzwords.AlthoughthefundamentalforcesthatnecessitatedtheinventionofJavaareportabilityandsecurity,otherfactorsplayedanimportantroleinmoldingthefinalformofthelanguage.ThekeyconsiderationsweresummedupbytheJavadesignteaminthefollowinglistofbuzzwords.

Simple Javahasaconcise,cohesivesetoffeaturesthatmakesiteasytolearnanduse.

Secure JavaprovidesasecuremeansofcreatingInternetapplications.

Portable JavaprogramscanexecuteinanyenvironmentforwhichthereisaJavaruntimesystem.

Objectoriented Javaembodiesthemodern,objectorientedprogrammingphilosophy.

Robust Javaencourageserrorfreeprogrammingbybeingstrictlytypedandperformingruntimechecks.

Multithreaded Javaprovidesintegratedsupportformultithreadedprogramming.

Architectureneutral

Javaisnottiedtoaspecificmachineoroperatingsystemarchitecture.

Interpreted JavasupportscrossplatformcodethroughtheuseofJavabytecode.

Highperformance

TheJavabytecodeishighlyoptimizedforspeedofexecution.

Distributed JavawasdesignedwiththedistributedenvironmentoftheInternetinmind.

Dynamic Javaprogramscarrywiththemsubstantialamountsofruntimetypeinformationthatisusedtoverifyandresolveaccessestoobjectsatruntime.

AsktheExpert

Q:Toaddresstheissuesofportabilityandsecurity,whywasitnecessarytocreateanewcomputerlanguagesuchasJavacouldntalanguagelikeC++beadapted?Inotherwords,couldntaC++compilerthatoutputsbytecodebecreated?

A:WhileitwouldbepossibleforaC++compilertogeneratesomethingsimilartobytecoderatherthanexecutablecode,C++hasfeaturesthatdiscourageitsuseforthecreationofInternetprogramsthemostimportantfeaturebeingC++ssupportforpointers.Apointeristheaddressofsomeobjectstoredinmemory.Usingapointer,itwouldbepossibletoaccessresourcesoutsidetheprogramitself,resultinginasecuritybreach.Javadoesnotsupportpointers,thuseliminatingthisproblem.

ObjectOrientedProgramming

AtthecenterofJavaisobjectorientedprogramming(OOP).TheobjectorientedmethodologyisinseparablefromJava,andallJavaprogramsare,toatleastsomeextent,objectoriented.BecauseofOOPsimportancetoJava,itisusefultounderstandinageneralwayOOPsbasicprinciplesbeforeyouwriteevenasimpleJavaprogram.Laterinthisbook,youwillseehowtoputtheseconceptsintopractice.

OOPisapowerfulwaytoapproachthejobofprogramming.Programmingmethodologieshavechangeddramaticallysincetheinventionofthecomputer,primarilytoaccommodatetheincreasingcomplexityofprograms.Forexample,whencomputerswerefirstinvented,programmingwasdonebytogglinginthebinarymachineinstructionsusingthecomputersfrontpanel.Aslongasprogramswerejustafewhundredinstructionslong,thisapproachworked.Asprogramsgrew,assemblylanguagewasinventedsothataprogrammercoulddealwithlarger,increasinglycomplexprograms,usingsymbolicrepresentationsofthemachineinstructions.Asprogramscontinuedtogrow,highlevellanguageswereintroducedthatgavetheprogrammermoretoolswithwhichtohandlecomplexity.Thefirstwidespreadlanguagewas,ofcourse,FORTRAN.AlthoughFORTRANwasaveryimpressivefirststep,itishardlyalanguagethatencouragesclear,easytounderstandprograms.

The1960sgavebirthtostructuredprogramming.ThisisthemethodencouragedbylanguagessuchasCandPascal.Theuseofstructured



languagesmadeitpossibletowritemoderatelycomplexprogramsfairlyeasily.Structuredlanguagesarecharacterizedbytheirsupportforstandalonesubroutines,localvariables,richcontrolconstructs,andtheirlackofrelianceupontheGOTO.Althoughstructuredlanguagesareapowerfultool,eventheyreachtheirlimitwhenaprojectbecomestoolarge.

Considerthis:Ateachmilestoneinthedevelopmentofprogramming,techniquesandtoolswerecreatedtoallowtheprogrammertodealwithincreasinglygreatercomplexity.Eachstepoftheway,thenewapproachtookthebestelementsofthepreviousmethodsandmovedforward.PriortotheinventionofOOP,manyprojectswerenearing(orexceeding)thepointwherethestructuredapproachnolongerworks.Objectorientedmethodswerecreatedtohelpprogrammersbreakthroughthesebarriers.

Objectorientedprogrammingtookthebestideasofstructuredprogrammingandcombinedthemwithseveralnewconcepts.Theresultwasadifferentwayoforganizingaprogram.Inthemostgeneralsense,aprogramcanbeorganizedinoneoftwoways:arounditscode(whatishappening)orarounditsdata(whatisbeingaffected).Usingonlystructuredprogrammingtechniques,programsaretypicallyorganizedaroundcode.Thisapproachcanbethoughtofascodeactingondata.

Objectorientedprogramsworktheotherwayaround.Theyareorganizedarounddata,withthekeyprinciplebeingdatacontrollingaccesstocode.Inanobjectorientedlanguage,youdefinethedataandtheroutinesthatarepermittedtoactonthatdata.Thus,adatatypedefinespreciselywhatsortofoperationscanbeappliedtothatdata.

Tosupporttheprinciplesofobjectorientedprogramming,allOOPlanguages,includingJava,havethreetraitsincommon:encapsulation,polymorphism,andinheritance.Letsexamineeach.

Encapsulation

Encapsulationisaprogrammingmechanismthatbindstogethercodeandthedataitmanipulates,andthatkeepsbothsafefromoutsideinterferenceandmisuse.Inanobjectorientedlanguage,codeanddatacanbeboundtogetherinsuchawaythataselfcontainedblackboxiscreated.Withintheboxareallnecessarydataandcode.Whencodeanddataarelinkedtogetherinthisfashion,anobjectiscreated.Inotherwords,anobjectisthedevicethatsupportsencapsulation.

Withinanobject,code,data,orbothmaybeprivatetothatobjectorpublic.Privatecodeordataisknowntoandaccessiblebyonlyanotherpartoftheobject.Thatis,privatecodeordatacannotbeaccessedbyapieceoftheprogramthatexistsoutsidetheobject.Whencodeordataispublic,otherpartsofyourprogramcanaccessiteventhoughitisdefinedwithinanobject.Typically,thepublicpartsofanobjectareusedtoprovideacontrolledinterfacetotheprivateelementsoftheobject.

Javasbasicunitofencapsulationistheclass.Althoughtheclasswillbeexaminedingreatdetaillaterinthisbook,thefollowingbriefdiscussionwillbehelpfulnow.Aclassdefinestheformofanobject.Itspecifiesboththedataandthecodethatwilloperateonthatdata.Javausesaclassspecificationtoconstructobjects.Objectsareinstancesofaclass.Thus,aclassisessentiallyasetofplansthatspecifyhowtobuildanobject.

Thecodeanddatathatconstituteaclassarecalledmembersoftheclass.Specifically,thedatadefinedbytheclassarereferredtoasmembervariablesorinstancevariables.Thecodethatoperatesonthatdataisreferredtoasmembermethodsorjustmethods.MethodisJavastermforasubroutine.IfyouarefamiliarwithC/C++,itmayhelptoknowthatwhataJavaprogrammercallsamethod,aC/C++programmercallsafunction.

Polymorphism

Polymorphism(fromGreek,meaningmanyforms)isthequalitythatallowsoneinterfacetoaccessageneralclassofactions.Thespecificactionisdeterminedbytheexactnatureofthesituation.Asimpleexampleofpolymorphismisfoundinthesteeringwheelofanautomobile.Thesteeringwheel(i.e.,theinterface)isthesamenomatterwhattypeofactualsteeringmechanismisused.Thatis,thesteeringwheelworksthesamewhetheryourcarhasmanualsteering,powersteering,orrackandpinionsteering.Therefore,onceyouknowhowtooperatethesteeringwheel,youcandriveanytypeofcar.

Thesameprinciplecanalsoapplytoprogramming.Forexample,considerastack(whichisafirstin,lastoutlist).Youmighthaveaprogramthatrequiresthreedifferenttypesofstacks.Onestackisusedforintegervalues,oneforfloatingpointvalues,andoneforcharacters.Inthiscase,thealgorithmthatimplementseachstackisthesame,eventhoughthedatabeingstoreddiffers.Inanonobjectorientedlanguage,youwouldberequiredtocreatethreedifferentsetsofstackroutines,witheachsetusingdifferentnames.However,becauseofpolymorphism,inJavayoucancreateonegeneralsetofstackroutinesthatworksforallthreespecificsituations.Thisway,onceyouknowhowtouseonestack,youcanusethemall.



Moregenerally,theconceptofpolymorphismisoftenexpressedbythephraseoneinterface,multiplemethods.Thismeansthatitispossibletodesignagenericinterfacetoagroupofrelatedactivities.Polymorphismhelpsreducecomplexitybyallowingthesameinterfacetobeusedtospecifyageneralclassofaction.Itisthecompilersjobtoselectthespecificaction(i.e.,method)asitappliestoeachsituation.You,theprogrammer,dontneedtodothisselectionmanually.Youneedonlyrememberandutilizethegeneralinterface.

Inheritance

Inheritanceistheprocessbywhichoneobjectcanacquirethepropertiesofanotherobject.Thisisimportantbecauseitsupportstheconceptofhierarchicalclassification.Ifyouthinkaboutit,mostknowledgeismademanageablebyhierarchical(i.e.,topdown)classifications.Forexample,aRedDeliciousappleispartoftheclassificationapple,whichinturnispartofthefruitclass,whichisunderthelargerclassfood.Thatis,thefoodclasspossessescertainqualities(edible,nutritious,etc.)whichalso,logically,applytoitssubclass,fruit.Inadditiontothesequalities,thefruitclasshasspecificcharacteristics(juicy,sweet,etc.)thatdistinguishitfromotherfood.Theappleclassdefinesthosequalitiesspecifictoanapple(growsontrees,nottropical,etc.).ARedDeliciousapplewould,inturn,inheritallthequalitiesofallprecedingclasses,andwoulddefineonlythosequalitiesthatmakeitunique.

Withouttheuseofhierarchies,eachobjectwouldhavetoexplicitlydefineallofitscharacteristics.Usinginheritance,anobjectneedonlydefinethosequalitiesthatmakeituniquewithinitsclass.Itcaninherititsgeneralattributesfromitsparent.Thus,itistheinheritancemechanismthatmakesitpossibleforoneobjecttobeaspecificinstanceofamoregeneralcase.

ObtainingtheJavaDevelopmentKit

NowthatthetheoreticalunderpinningofJavahasbeenexplained,itistimetostartwritingJavaprograms.Beforeyoucancompileandrunthoseprograms,however,youmusthavetheJavaDevelopmentKit(JDK)installedonyourcomputer.TheJDKisavailablefreeofchargefromOracle.Atthetimeofthiswriting,thecurrentreleaseoftheJDKisJDK8.ThisistheversionusedbyJavaSE8.(SEstandsforStandardEdition.)BecauseJDK8containsmanynewfeaturesthatarenotsupportedbyearlierversionsofJava,itisrecommendedthatyouuseJDK8(orlater)tocompileandruntheprogramsinthisbook.Ifyouuseanearlierversion,thenprogramscontainingnewfeatureswillnotcompile.

TheJDKcanbedownloadedfromwww.oracle.com/technetwork/java/javase/downloads/index.html(http://www.oracle.com/technetwork/java/javase/downloads/index.html)

Justgotothedownloadpageandfollowtheinstructionsforthetypeofcomputerthatyouhave.AfteryouhaveinstalledtheJDK,youwillbeabletocompileandrunprograms.TheJDKsuppliestwoprimaryprograms.Thefirstisjavac,whichistheJavacompiler.Thesecondisjava,whichisthestandardJavainterpreterandisalsoreferredtoastheapplicationlauncher.

Oneotherpoint:TheJDKrunsinthecommandpromptenvironmentandusescommandlinetools.Itisnotawindowedapplication.Itisalsonotanintegrateddevelopmentenvironment(IDE).

NOTE

InadditiontothebasiccommandlinetoolssuppliedwiththeJDK,thereareseveralhighqualityIDEsavailableforJava,suchasNetBeansandEclipse.AnIDEcanbeveryhelpfulwhendevelopinganddeployingcommercialapplications.Asageneralrule,youcanalsouseanIDEtocompileandruntheprogramsinthisbookifyousochoose.However,theinstructionspresentedinthisbookforcompilingandrunningaJavaprogramdescribeonlytheJDKcommandlinetools.Thereasonsforthisareeasytounderstand.First,theJDKisreadilyavailabletoallreaders.Second,theinstructionsforusingtheJDKwillbethesameforallreaders.Furthermore,forthesimpleprogramspresentedinthisbook,usingtheJDKcommandlinetoolsisusuallytheeasiestapproach.IfyouareusinganIDE,youwillneedtofollowitsinstructions.BecauseofdifferencesbetweenIDEs,nogeneralsetofinstructionscanbegiven.

AsktheExpert

Q:Youstatethatobjectorientedprogrammingisaneffectivewaytomanagelargeprograms.However,itseems



thatitmightaddsubstantialoverheadtorelativelysmallones.SinceyousaythatallJavaprogramsare,tosomeextent,objectoriented,doesthisimposeapenaltyforsmallerprograms?

A:No.Asyouwillsee,forsmallprograms,Javasobjectorientedfeaturesarenearlytransparent.AlthoughitistruethatJavafollowsastrictobjectmodel,youhavewidelatitudeastothedegreetowhichyouemployit.Forsmallerprograms,theirobjectorientednessisbarelyperceptible.Asyourprogramsgrow,youwillintegratemoreobjectorientedfeatureseffortlessly.

AFirstSimpleProgram

Letsstartbycompilingandrunningtheshortsampleprogramshownhere:

Youwillfollowthesethreesteps:

1.Entertheprogram.

2.Compiletheprogram.

3.Runtheprogram.

EnteringtheProgram

TheprogramsshowninthisbookareavailablefromMcGrawHillEducationswebsite:www.oraclepressbooks.com(http://www.oraclepressbooks.com).

However,ifyouwanttoentertheprogramsbyhand,youarefreetodoso.Inthiscase,youmustentertheprogramintoyourcomputerusingatexteditor,notawordprocessor.Wordprocessorstypicallystoreformatinformationalongwithtext.ThisformatinformationwillconfusetheJavacompiler.IfyouareusingaWindowsplatform,youcanuseWordPadoranyotherprogrammingeditorthatyoulike.

Formostcomputerlanguages,thenameofthefilethatholdsthesourcecodetoaprogramisarbitrary.However,thisisnotthecasewithJava.ThefirstthingthatyoumustlearnaboutJavaisthatthenameyougivetoasourcefileisveryimportant.Forthisexample,thenameofthesourcefileshouldbeExample.java.Letsseewhy.

InJava,asourcefileisofficiallycalledacompilationunit.Itisatextfilethatcontains(amongotherthings)oneormoreclassdefinitions.(Fornow,wewillbeusingsourcefilesthatcontainonlyoneclass.)TheJavacompilerrequiresthatasourcefileusethe.javafilenameextension.Asyoucanseebylookingattheprogram,thenameoftheclassdefinedbytheprogramisalsoExample.Thisisnotacoincidence.InJava,allcodemustresideinsideaclass.Byconvention,thenameofthemainclassshouldmatchthenameofthefilethatholdstheprogram.Youshouldalsomakesurethatthecapitalizationofthefilenamematchestheclassname.ThereasonforthisisthatJavaiscasesensitive.Atthispoint,theconventionthatfilenamescorrespondtoclassnamesmayseemarbitrary.However,thisconventionmakesiteasiertomaintainandorganizeyourprograms.

CompilingtheProgram

TocompiletheExampleprogram,executethecompiler,javac,specifyingthenameofthesourcefileonthecommandline,asshownhere:

ThejavaccompilercreatesafilecalledExample.classthatcontainsthebytecodeversionoftheprogram.Remember,bytecodeisnotexecutablecode.BytecodemustbeexecutedbyaJavaVirtualMachine.Thus,theoutputofjavacisnotcodethatcanbedirectlyexecuted.

Toactuallyruntheprogram,youmustusetheJavainterpreter,java.Todoso,passtheclassnameExampleasacommandlineargument,asshownhere:

Whentheprogramisrun,thefollowingoutputisdisplayed:



WhenJavasourcecodeiscompiled,eachindividualclassisputintoitsownoutputfilenamedaftertheclassandusingthe.classextension.ThisiswhyitisagoodideatogiveyourJavasourcefilesthesamenameastheclasstheycontainthenameofthesourcefilewillmatchthenameofthe.classfile.WhenyouexecutetheJavainterpreterasjustshown,youareactuallyspecifyingthenameoftheclassthatyouwanttheinterpretertoexecute.Itwillautomaticallysearchforafilebythatnamethathasthe.classextension.Ifitfindsthefile,itwillexecutethecodecontainedinthespecifiedclass.

NOTE

If,whenyoutrytocompiletheprogram,thecomputercannotfindjavac(andassumingthatyouhaveinstalledtheJDKcorrectly),youmayneedtospecifythepathtothecommandlinetools.InWindows,forexample,thismeansthatyouwillneedtoaddthepathtothecommandlinetoolstothepathsdefinedforthePATHenvironmentalvariable.Forexample,ifJDK8wasinstalledundertheProgramFilesdirectory,thenthepathtothecommandlinetoolswillbesimilartoC:\ProgramFiles\Java\jdk1.8.0\bin.(Ofcourse,youwillneedtofindthepathtoJavaonyourcomputer,whichmaydifferfromtheonejustshown.AlsothespecificversionoftheJDKmaydiffer.)Youwillneedtoconsultthedocumentationforyouroperatingsystemonhowtosetthepath,becausethisprocedurediffersbetweenOSes.

TheFirstSampleProgramLinebyLine

AlthoughExample.javaisquiteshort,itincludesseveralkeyfeaturesthatarecommontoallJavaprograms.Letscloselyexamineeachpartoftheprogram.

Theprogrambeginswiththefollowinglines:

Thisisacomment.Likemostotherprogramminglanguages,Javaletsyouenteraremarkintoaprogramssourcefile.Thecontentsofacommentareignoredbythecompiler.Instead,acommentdescribesorexplainstheoperationoftheprogramtoanyonewhoisreadingitssourcecode.Inthiscase,thecommentdescribestheprogramandremindsyouthatthesourcefileshouldbecalledExample.java.Ofcourse,inrealapplications,commentsgenerallyexplainhowsomepartoftheprogramworksorwhataspecificfeaturedoes.

Javasupportsthreestylesofcomments.Theoneshownatthetopoftheprogramiscalledamultilinecomment.Thistypeofcommentmustbeginwith/*andendwith*/.Anythingbetweenthesetwocommentsymbolsisignoredbythecompiler.Asthenamesuggests,amultilinecommentmaybeseverallineslong.

Thenextlineofcodeintheprogramisshownhere:

Thislineusesthekeywordclasstodeclarethatanewclassisbeingdefined.Asmentioned,theclassisJavasbasicunitofencapsulation.Exampleisthenameoftheclass.Theclassdefinitionbeginswiththeopeningcurlybrace({)andendswiththeclosingcurlybrace(}).Theelementsbetweenthetwobracesaremembersoftheclass.Forthemoment,dontworrytoomuchaboutthedetailsofaclassexcepttonotethatinJava,allprogramactivityoccurswithinone.ThisisonereasonwhyallJavaprogramsare(atleastalittlebit)objectoriented.

Thenextlineintheprogramisthesinglelinecomment,shownhere:

ThisisthesecondtypeofcommentsupportedbyJava.Asinglelinecommentbeginswitha//andendsattheendoftheline.Asageneralrule,programmersusemultilinecommentsforlongerremarksandsinglelinecommentsforbrief,linebylinedescriptions.

Thenextlineofcodeisshownhere:



Thislinebeginsthemain()method.Asmentionedearlier,inJava,asubroutineiscalledamethod.Asthecommentprecedingitsuggests,thisisthelineatwhichtheprogramwillbeginexecuting.AllJavaapplicationsbeginexecutionbycallingmain().Theexactmeaningofeachpartofthislinecannotbegivennow,sinceitinvolvesadetailedunderstandingofseveralotherofJavasfeatures.However,sincemanyoftheexamplesinthisbookwillusethislineofcode,letstakeabrieflookateachpartnow.

Thepublickeywordisanaccessmodifier.Anaccessmodifierdetermineshowotherpartsoftheprogramcanaccessthemembersoftheclass.Whenaclassmemberisprecededbypublic,thenthatmembercanbeaccessedbycodeoutsidetheclassinwhichitisdeclared.(Theoppositeofpublicisprivate,whichpreventsamemberfrombeingusedbycodedefinedoutsideofitsclass.)Inthiscase,main()mustbedeclaredaspublic,sinceitmustbecalledbycodeoutsideofitsclasswhentheprogramisstarted.Thekeywordstaticallowsmain()tobecalledbeforeanobjectoftheclasshasbeencreated.Thisisnecessarybecausemain()iscalledbytheJVMbeforeanyobjectsaremade.Thekeywordvoidsimplytellsthecompilerthatmain()doesnotreturnavalue.Asyouwillsee,methodsmayalsoreturnvalues.Ifallthisseemsabitconfusing,dontworry.Alloftheseconceptswillbediscussedindetailinsubsequentchapters.

Asstated,main()isthemethodcalledwhenaJavaapplicationbegins.Anyinformationthatyouneedtopasstoamethodisreceivedbyvariablesspecifiedwithinthesetofparenthesesthatfollowthenameofthemethod.Thesevariablesarecalledparameters.Ifnoparametersarerequiredforagivenmethod,youstillneedtoincludetheemptyparentheses.Inmain()thereisonlyoneparameter,Stringargs[],whichdeclaresaparameternamedargs.ThisisanarrayofobjectsoftypeString.(Arraysarecollectionsofsimilarobjects.)ObjectsoftypeStringstoresequencesofcharacters.Inthiscase,argsreceivesanycommandlineargumentspresentwhentheprogramisexecuted.Thisprogramdoesnotmakeuseofthisinformation,butotherprogramsshownlaterinthisbookwill.

Thelastcharacteronthelineisthe{.Thissignalsthestartofmain()sbody.Allofthecodeincludedinamethodwilloccurbetweenthemethodsopeningcurlybraceanditsclosingcurlybrace.

Thenextlineofcodeisshownhere.Noticethatitoccursinsidemain().

Thislineoutputsthestring"JavadrivestheWeb."followedbyanewlineonthescreen.Outputisactuallyaccomplishedbythebuiltinprintln()method.Inthiscase,println()displaysthestringthatispassedtoit.Asyouwillsee,println()canbeusedtodisplayothertypesofinformation,too.ThelinebeginswithSystem.out.Whiletoocomplicatedtoexplainindetailatthistime,briefly,Systemisapredefinedclassthatprovidesaccesstothesystem,andoutistheoutputstreamthatisconnectedtotheconsole.Thus,System.outisanobjectthatencapsulatesconsoleoutput.ThefactthatJavausesanobjecttodefineconsoleoutputisfurtherevidenceofitsobjectorientednature.

Asyouhaveprobablyguessed,consoleoutput(andinput)isnotusedfrequentlyinrealworldJavaapplications.Sincemostmoderncomputingenvironmentsarewindowedandgraphicalinnature,consoleI/Oisusedmostlyforsimpleutilityprograms,fordemonstrationprograms,andforserversidecode.Laterinthisbook,youwilllearnotherwaystogenerateoutputusingJava,butfornow,wewillcontinuetousetheconsoleI/Omethods.

Noticethattheprintln()statementendswithasemicolon.AllstatementsinJavaendwithasemicolon.Thereasonthattheotherlinesintheprogramdonotendinasemicolonisthattheyarenot,technically,statements.

Thefirst}intheprogramendsmain(),andthelast}endstheExampleclassdefinition.

Onelastpoint:Javaiscasesensitive.Forgettingthiscancauseyouseriousproblems.Forexample,ifyouaccidentallytypeMaininsteadofmain,orPrintLninsteadofprintln,theprecedingprogramwillbeincorrect.Furthermore,althoughtheJavacompilerwillcompileclassesthatdonotcontainamain()method,ithasnowaytoexecutethem.So,ifyouhadmistypedmain,thecompilerwouldstillcompileyourprogram.However,theJavainterpreterwouldreportanerrorbecauseitwouldbeunabletofindthemain()method.

HandlingSyntaxErrors

Ifyouhavenotyetdoneso,enter,compile,andruntheprecedingprogram.Asyoumayknowfromyourpreviousprogrammingexperience,itisquiteeasytoaccidentallytypesomethingincorrectlywhenenteringcodeintoyourcomputer.Fortunately,ifyouentersomethingincorrectlyintoyourprogram,thecompilerwillreportasyntaxerrormessagewhenittriestocompileit.TheJavacompiler



attemptstomakesenseoutofyoursourcecodenomatterwhatyouhavewritten.Forthisreason,theerrorthatisreportedmaynotalwaysreflecttheactualcauseoftheproblem.Intheprecedingprogram,forexample,anaccidentalomissionoftheopeningcurlybraceafterthemain()methodcausesthecompilertoreportthefollowingtwoerrors:

Clearly,thefirsterrormessageiscompletelywrongbecausewhatismissingisnotasemicolon,butacurlybrace.

Thepointofthisdiscussionisthatwhenyourprogramcontainsasyntaxerror,youshouldntnecessarilytakethecompilersmessagesatfacevalue.Themessagesmaybemisleading.Youmayneedtosecondguessanerrormessageinordertofindtherealproblem.Also,lookatthelastfewlinesofcodeinyourprogramthatprecedethelinebeingflagged.Sometimesanerrorwillnotbereporteduntilseverallinesafterthepointatwhichtheerroractuallyoccurred.

ASecondSimpleProgram

Perhapsnootherconstructisasimportanttoaprogramminglanguageastheassignmentofavaluetoavariable.Avariableisanamedmemorylocationthatcanbeassignedavalue.Further,thevalueofavariablecanbechangedduringtheexecutionofaprogram.Thatis,thecontentofavariableischangeable,notfixed.Thefollowingprogramcreatestwovariablescalledvar1andvar2:

Whenyourunthisprogram,youwillseethefollowingoutput:

Thisprogramintroducesseveralnewconcepts.First,thestatement

declaresavariablecalledvar1oftypeinteger.InJava,allvariablesmustbedeclaredbeforetheyareused.Further,thetypeofvaluesthatthevariablecanholdmustalsobespecified.Thisiscalledthetypeofthevariable.Inthiscase,var1canholdintegervalues.Thesearewholenumbervalues.InJava,todeclareavariabletobeoftypeinteger,precedeitsnamewiththekeywordint.Thus,theprecedingstatementdeclaresavariablecalledvar1oftypeint.

Thenextlinedeclaresasecondvariablecalledvar2:

Noticethatthislineusesthesameformatasthefirstlineexceptthatthenameofthevariableisdifferent.

Ingeneral,todeclareavariableyouwilluseastatementlikethis:

typevarname

Here,typespecifiesthetypeofvariablebeingdeclared,andvarnameisthenameofthevariable.Inadditiontoint,Javasupportsseveralotherdatatypes.

Thefollowinglineofcodeassignsvar1thevalue1024:

InJava,theassignmentoperatoristhesingleequalsign.Itcopiesthevalueonitsrightsideintothevariableonitsleft.

Thenextlineofcodeoutputsthevalueofvar1precededbythestring"var1contains":



Inthisstatement,theplussigncausesthevalueofvar1tobedisplayedafterthestringthatprecedesit.Thisapproachcanbegeneralized.Usingthe+operator,youcanchaintogetherasmanyitemsasyouwantwithinasingleprintln()statement.

Thenextlineofcodeassignsvar2thevalueofvar1dividedby2:

Thislinedividesthevalueinvar1by2andthenstoresthatresultinvar2.Thus,afterthelineexecutes,var2willcontainthevalue512.Thevalueofvar1willbeunchanged.Likemostothercomputerlanguages,Javasupportsafullrangeofarithmeticoperators,includingthoseshownhere:

+ Addition

Subtraction

* Multiplication

/ Division

Herearethenexttwolinesintheprogram:

Twonewthingsareoccurringhere.First,thebuiltinmethodprint()isusedtodisplaythestring"var2containsvar1/2:".Thisstringisnotfollowedbyanewline.Thismeansthatwhenthenextoutputisgenerated,itwillstartonthesameline.Theprint()methodisjustlikeprintln(),exceptthatitdoesnotoutputanewlineaftereachcall.Second,inthecalltoprintln(),noticethatvar2isusedbyitself.Bothprint()andprintln()canbeusedtooutputvaluesofanyofJavasbuiltintypes.

Onemorepointaboutdeclaringvariablesbeforewemoveon:Itispossibletodeclaretwoormorevariablesusingthesamedeclarationstatement.Justseparatetheirnamesbycommas.Forexample,var1andvar2couldhavebeendeclaredlikethis:

AnotherDataType

Intheprecedingprogram,avariableoftypeintwasused.However,avariableoftypeintcanholdonlywholenumbers.Thus,itcannotbeusedwhenafractionalcomponentisrequired.Forexample,anintvariablecanholdthevalue18,butnotthevalue18.3.Fortunately,intisonlyoneofseveraldatatypesdefinedbyJava.Toallownumberswithfractionalcomponents,Javadefinestwofloatingpointtypes:floatanddouble,whichrepresentsingleanddoubleprecisionvalues,respectively.Ofthetwo,doubleisthemostcommonlyused.

Todeclareavariableoftypedouble,useastatementsimilartothatshownhere:

Here,xisthenameofthevariable,whichisoftypedouble.Becausexhasafloatingpointtype,itcanholdvaluessuchas122.23,0.034,or19.0.

Tobetterunderstandthedifferencebetweenintanddouble,trythefollowingprogram:

Theoutputfromthisprogramisshownhere:



Asyoucansee,whenvarisdividedby4,awholenumberdivisionisperformed,andtheoutcomeis2thefractionalcomponentislost.However,whenxisdividedby4,thefractionalcomponentispreserved,andtheproperanswerisdisplayed.

Thereisoneothernewthingtonoticeintheprogram.Toprintablankline,simplycallprintln()withoutanyarguments.

AsktheExpert

Q:WhydoesJavahavedifferentdatatypesforintegersandfloatingpointvalues?Thatis,whyarentallnumericvaluesjustthesametype?

A:Javasuppliesdifferentdatatypessothatyoucanwriteefficientprograms.Forexample,integerarithmeticisfasterthanfloatingpointcalculations.Thus,ifyoudontneedfractionalvalues,thenyoudontneedtoincurtheoverheadassociatedwithtypesfloatordouble.Second,theamountofmemoryrequiredforonetypeofdatamightbelessthanthatrequiredforanother.Bysupplyingdifferenttypes,Javaenablesyoutomakebestuseofsystemresources.Finally,somealgorithmsrequire(oratleastbenefitfrom)theuseofaspecifictypeofdata.Ingeneral,Javasuppliesanumberofbuiltintypestogiveyouthegreatestflexibility.

TryThis11 ConvertingGallonstoLiters

AlthoughtheprecedingsampleprogramsillustrateseveralimportantfeaturesoftheJavalanguage,theyarenotveryuseful.EventhoughyoudonotknowmuchaboutJavaatthispoint,youcanstillputwhatyouhavelearnedtoworktocreateapracticalprogram.Inthisproject,wewillcreateaprogramthatconvertsgallonstoliters.Theprogramwillworkbydeclaringtwodoublevariables.Onewillholdthenumberofthegallons,andthesecondwillholdthenumberoflitersaftertheconversion.Thereare3.7854litersinagallon.Thus,toconvertgallonstoliters,thegallonvalueismultipliedby3.7854.Theprogramdisplaysboththenumberofgallonsandtheequivalentnumberofliters.

1.CreateanewfilecalledGalToLit.java.

2.Enterthefollowingprogramintothefile:

3.Compiletheprogramusingthefollowingcommandline:

4.Runtheprogramusingthiscommand:

Youwillseethisoutput:

5.Asitstands,thisprogramconverts10gallonstoliters.However,bychangingthevalueassignedtogallons,youcanhavetheprogramconvertadifferentnumberofgallonsintoitsequivalentnumberofliters.

TwoControlStatements

Insideamethod,executionproceedsfromonestatementtothenext,toptobottom.However,itispossibletoalterthisflowthroughtheuseofthevariousprogramcontrolstatementssupportedbyJava.Althoughwewilllookcloselyatcontrolstatementslater,twoarebrieflyintroducedherebecausewewillbeusingthemtowritesampleprograms.

TheifStatement



YoucanselectivelyexecutepartofaprogramthroughtheuseofJavasconditionalstatement:theif.TheJavaifstatementworksmuchliketheIFstatementinanyotherlanguage.Itssimplestformisshownhere:

if(condition)statement

Here,conditionisaBooleanexpression.Ifconditionistrue,thenthestatementisexecuted.Ifconditionisfalse,thenthestatementisbypassed.Hereisanexample:

Inthiscase,since10islessthan11,theconditionalexpressionistrue,andprintln()willexecute.However,considerthefollowing:

Inthiscase,10isnotlessthan9.Thus,thecalltoprintln()willnottakeplace.

Javadefinesafullcomplementofrelationaloperatorsthatmaybeusedinaconditionalexpression.Theyareshownhere:

Operator Meaning

< Lessthan

Greaterthan

>= Greaterthanorequal

== Equalto

!= Notequal

Noticethatthetestforequalityisthedoubleequalsign.

Hereisaprogramthatillustratestheifstatement:

Theoutputgeneratedbythisprogramisshownhere:

Noticeoneotherthinginthisprogram.Theline

declaresthreevariables,a,b,andc,byuseofacommaseparatedlist.Asmentionedearlier,whenyouneedtwoormorevariablesofthesametype,theycanbedeclaredinonestatement.Justseparatethevariablenamesbycommas.

TheforLoop

Youcanrepeatedlyexecuteasequenceofcodebycreatingaloop.Java



suppliesapowerfulassortmentofloopconstructs.Theonewewilllookathereistheforloop.Thesimplestformoftheforloopisshownhere:

for(initializationconditioniteration)statement

Initsmostcommonform,theinitializationportionoftheloopsetsaloopcontrolvariabletoaninitialvalue.TheconditionisaBooleanexpressionthatteststheloopcontrolvariable.Iftheoutcomeofthattestistrue,theforloopcontinuestoiterate.Ifitisfalse,theloopterminates.Theiterationexpressiondetermineshowtheloopcontrolvariableischangedeachtimetheloopiterates.Hereisashortprogramthatillustratestheforloop:

Theoutputgeneratedbytheprogramisshownhere:

Inthisexample,countistheloopcontrolvariable.Itissettozerointheinitializationportionofthefor.Atthestartofeachiteration(includingthefirstone),theconditionaltestcount



Theoutputgeneratedbythisprogramisshownhere:

Inthiscase,thetargetoftheifstatementisablockofcodeandnotjustasinglestatement.Iftheconditioncontrollingtheifistrue(asitisinthiscase),thethreestatementsinsidetheblockwillbeexecuted.Trysettingitozeroandobservetheresult.Youwillseethattheentireblockisskipped.

AsktheExpert

Q:Doestheuseofacodeblockintroduceanyruntimeinefficiencies?Inotherwords,doesJavaactuallyexecutethe{and}?

A:No.Codeblocksdonotaddanyoverheadwhatsoever.Infact,becauseoftheirabilitytosimplifythecodingofcertainalgorithms,theirusegenerallyincreasesspeedandefficiency.Also,the{and}existonlyinyourprogramssourcecode.Javadoesnot,perse,executethe{or}.

Asyouwillseelaterinthisbook,blocksofcodehaveadditionalpropertiesanduses.However,themainreasonfortheirexistenceistocreatelogicallyinseparableunitsofcode.

SemicolonsandPositioning

InJava,thesemicolonisaseparatorthatisusedtoterminateastatement.Thatis,eachindividualstatementmustbeendedwithasemicolon.Itindicatestheendofonelogicalentity.

Asyouknow,ablockisasetoflogicallyconnectedstatementsthataresurroundedbyopeningandclosingbraces.Ablockisnotterminatedwithasemicolon.Sinceablockisagroupofstatements,withasemicolonaftereachstatement,itmakessensethatablockisnotterminatedbyasemicoloninstead,theendoftheblockisindicatedbytheclosingbrace.

Javadoesnotrecognizetheendofthelineasaterminator.Forthisreason,itdoesnotmatterwhereonalineyouputastatement.Forexample,

isthesameasthefollowing,toJava:

Furthermore,theindividualelementsofastatementcanalsobeputonseparatelines.Forexample,thefollowingisperfectlyacceptable:

Breakinglonglinesinthisfashionisoftenusedtomakeprogramsmorereadable.Itcanalsohelppreventexcessivelylonglinesfromwrapping.

IndentationPractices

Youmayhavenoticedinthepreviousexamplesthatcertainstatementswereindented.Javaisafreeformlanguage,meaningthatitdoesnotmatterwhereyouplacestatementsrelativetoeachotheronaline.However,overtheyears,acommonandacceptedindentationstylehasdevelopedthatallowsforveryreadableprograms.Thisbookfollowsthatstyle,anditisrecommendedthatyoudosoaswell.Usingthisstyle,youindentonelevelaftereachopeningbrace,andmovebackoutonelevelaftereachclosingbrace.Certainstatementsencouragesomeadditionalindentingthesewillbecoveredlater.

TryThis12



ImprovingtheGallonstoLitersConverter

Youcanusetheforloop,theifstatement,andcodeblockstocreateanimprovedversionofthegallonstolitersconverterthatyoudevelopedinthefirstproject.Thisnewversionwillprintatableofconversions,beginningwith1gallonandendingat100gallons.Afterevery10gallons,ablanklinewillbeoutput.Thisisaccomplishedthroughtheuseofavariablecalledcounterthatcountsthenumberoflinesthathavebeenoutput.Payspecialattentiontoitsuse.

1.CreateanewfilecalledGalToLitTable.java.

2.Enterthefollowingprogramintothefile:

3.Compiletheprogramusingthefollowingcommandline:

4.Runtheprogramusingthiscommand:

Hereisaportionoftheoutputthatyouwillsee:

TheJavaKeywords

FiftykeywordsarecurrentlydefinedintheJavalanguage(seeTable11).Thesekeywords,combinedwiththesyntaxoftheoperatorsandseparators,formthedefinitionoftheJavalanguage.Thesekeywordscannotbeusedasnamesforavariable,class,ormethod.



Table11TheJavaKeywords

Thekeywordsconstandgotoarereservedbutnotused.IntheearlydaysofJava,severalotherkeywordswerereservedforpossiblefutureuse.However,thecurrentspecificationforJavadefinesonlythekeywordsshowninTable11.

Inadditiontothekeywords,Javareservesthefollowing:true,false,andnull.ThesearevaluesdefinedbyJava.Youmaynotusethesewordsforthenamesofvariables,classes,andsoon.

IdentifiersinJava

InJavaanidentifierisanamegiventoamethod,avariable,oranyotheruserdefineditem.Identifierscanbefromonetoseveralcharacterslong.Variablenamesmaystartwithanyletterofthealphabet,anunderscore,oradollarsign.Nextmaybeeitheraletter,adigit,adollarsign,oranunderscore.Theunderscorecanbeusedtoenhancethereadabilityofavariablename,asinline_count.Uppercaseandlowercasearedifferentthatis,toJava,myvarandMyVarareseparatenames.Herearesomeexamplesofacceptableidentifiers:

Remember,youcantstartanidentifierwithadigit.Thus,12xisinvalid,forexample.

YoucannotuseanyoftheJavakeywordsasidentifiernames.Also,youshouldnotusethenameofanystandardmethod,suchasprintln,asanidentifier.Beyondthesetworestrictions,goodprogrammingpracticedictatesthatyouuseidentifiernamesthatreflectthemeaningorusageoftheitemsbeingnamed.

TheJavaClassLibraries

ThesampleprogramsshowninthischaptermakeuseoftwoofJavasbuiltinmethods:println()andprint().ThesemethodsareaccessedthroughSystem.out.SystemisaclasspredefinedbyJavathatisautomaticallyincludedinyourprograms.Inthelargerview,theJavaenvironmentreliesonseveralbuiltinclasslibrariesthatcontainmanybuiltinmethodsthatprovidesupportforsuchthingsasI/O,stringhandling,networking,andgraphics.Thestandardclassesalsoprovidesupportforagraphicaluserinterface(GUI).Thus,JavaasatotalityisacombinationoftheJavalanguageitself,plusitsstandardclasses.Asyouwillsee,theclasslibrariesprovidemuchofthefunctionalitythatcomeswithJava.Indeed,partofbecomingaJavaprogrammerislearningtousethestandardJavaclasses.Throughoutthisbook,variouselementsofthestandardlibraryclassesandmethodsaredescribed.However,theJavalibraryissomethingthatyouwillalsowanttoexploremoreonyourown.

Chapter1SelfTest

1 .WhatisbytecodeandwhyisitimportanttoJavasusefor

Internetprogramming?

2 .Whatarethethreemainprinciplesofobjectoriented

programming?

3 .WheredoJavaprogramsbeginexecution?

4 .Whatisavariable?

5 .Whichofthefollowingvariablenamesisinvalid?

A.count

B.$count

C.count27

D.67count

6 .Howdoyoucreateasinglelinecomment?Howdoyou

createamultilinecomment?

7 .Showthegeneralformoftheifstatement.Showthegeneral



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formoftheforloop.

8 .Howdoyoucreateablockofcode?

9 .Themoonsgravityisabout17percentthatofearths.Write

aprogramthatcomputesyoureffectiveweightonthemoon.

10.AdaptTryThis12sothatitprintsaconversiontableofinchestometers.Display12feetofconversions,inchbyinch.Outputablanklineevery12inches.(Onemeterequalsapproximately39.37inches.)

11.Ifyoumakeatypingmistakewhenenteringyourprogram,whatsortoferrorwillresult?

12.Doesitmatterwhereonalineyouputastatement?