java in easy steps: covers java 9

IneasystepsisanimprintofInEasyStepsLimited16HamiltonTerrace·HollyWalk·LeamingtonSpaWarwickshire·CV324LYwww.ineasysteps.com

http://www.ineasysteps.com

SixthEdition

Copyright©2017byInEasyStepsLimited.Allrightsreserved.Nopartofthisbookmaybereproducedortransmittedinanyformorbyanymeans,electronicormechanical,includingphotocopying,recording,orbyanyinformationstorageorretrievalsystem,withoutpriorwrittenpermissionfromthepublisher.

NoticeofLiabilityEveryefforthasbeenmadetoensurethatthisbookcontainsaccurateandcurrentinformation.However,InEasyStepsLimitedandtheauthorshallnotbeliableforanylossordamagesufferedbyreadersasaresultofanyinformationcontainedherein.

TrademarksAlltrademarksareacknowledgedasbelongingtotheirrespectivecompanies.

Contents

1GettingstartedIntroductionInstallingtheJDKWritingafirstJavaprogramCompiling&runningprogramsCreatingavariableRecognizingdatatypesCreatingconstantsAddingcommentsTroubleshootingproblemsSummary

2PerformingoperationsDoingarithmeticAssigningvaluesComparingvaluesAssessinglogicExaminingconditionsSettingprecedenceEscapingliteralsWorkingwithbitsSummary

3MakingstatementsBranchingwithifBranchingalternativesSwitchingbranchesLoopingforLoopingwhiletrueDoingdo-whileloopsBreakingoutofloopsReturningcontrolSummary

4DirectingvaluesCastingtypevaluesCreatingvariablearrays

PassinganargumentPassingmultipleargumentsLoopingthroughelementsChangingelementvaluesAddingarraydimensionsCatchingexceptionsSummary

5ManipulatingdataExploringJavaclassesDoingmathematicsRoundingnumbersGeneratingrandomnumbersManagingstringsComparingstringsSearchingstringsManipulatingcharactersSummary

6CreatingclassesFormingmultiplemethodsUnderstandingprogramscopeFormingmultipleclassesExtendinganexistingclassCreatinganobjectclassProducinganobjectinstanceEncapsulatingpropertiesConstructingobjectvaluesSummary

7ImportingfunctionsHandlingfilesReadingconsoleinputReadingfilesWritingfilesSortingarrayelementsMakingarraylistsManagingdatesFormattingnumbersSummary

8BuildinginterfacesCreatingawindowAddingpushbuttonsAddinglabels

AddingtextfieldsAddingitemselectorsAddingradiobuttonsArrangingcomponentsChangingappearanceSummary

9RecognizingeventsListeningforeventsGeneratingeventsHandlingbuttoneventsHandlingitemeventsReactingtokeyboardeventsRespondingtomouseeventsAnnouncingmessagesRequestinginputSummary

10DeployingprogramsProducinganapplicationDistributingprogramsBuildingJavaarchivesDeployingapplicationsCreatingAndroidprojectsExploringprojectfilesAddingresources&controlsInsertingJavacodeTestingtheapplicationDeployingAndroidappsSummary

Preface

Thecreationofthisbookhasprovidedme,MikeMcGrath,awelcomeopportunitytoupdatemypreviousbooksonJavaprogrammingwiththelatesttechniques.AllexamplesIhavegiveninthisbookdemonstrateJavafeaturessupportedbycurrentcompilersonbothWindowsandLinuxoperatingsystems,andthebook’sscreenshotsillustratetheactualresultsproducedbycompilingandexecutingthelistedcode,orbyimplementingcodesnippetsintheJavashell.

Conventionsinthisbook

Inordertoclarifythecodelistedinthestepsgivenineachexample,Ihaveadoptedcertaincolorizationconventions.ComponentsoftheJavalanguageitselfarecoloredblue;programmer-specifiednamesarered;numericandstringvaluesareblack;andcommentsaregreen,likethis:

//Storethenoutputatextstringvalue.Stringmessage=“WelcometoJavaprogramming!”;System.out.println(message);

Additionally,inordertoidentifyeachsourcecodefiledescribedinthesteps,acolorediconandfilenameappearsinthemarginalongsidethesteps,likethese:

App.java

App.class

App.jar

App.xml

Grabbingthesourcecode

Forconvenience,IhaveplacedsourcecodefilesfromtheexamplesfeaturedinthisbookintoasingleZIParchive.Youcanobtainthecompletearchivebyfollowingtheseeasysteps:

Browsetowww.ineasysteps.comthennavigatetoFreeResourcesandchoosetheDownloadssectionFindJavaineasysteps,6thEditioninthelist,thenclickonthehyperlinkentitledAllCodeExamplestodownloadthearchiveNow,extractthearchivecontentstoanyconvenientlocationonyourcomputer

IsincerelyhopeyouenjoydiscoveringtheprogrammingpossibilitiesofJavaandhaveasmuchfunwithitasIdidinwritingthisbook.

MikeMcGrath

http://www.ineasysteps.com

1

Gettingstarted

WelcometotheexcitingworldofJavaprogramming.Thischaptershowshowtocreateandexecute

simpleJavaprograms,anddemonstrateshowtostoredatawithinprograms.

IntroductionInstallingtheJDKWritingafirstJavaprogramCompiling&runningprograms

CreatingavariableRecognizingdatatypesCreatingconstantsAddingcomments

TroubleshootingproblemsSummary

Introduction

TheJava™programminglanguagewasfirstdevelopedin1990byanengineeratSunMicrosystemsnamedJamesGosling.HewasunhappyusingtheC++programminglanguagesohecreatedanewlanguagethathenamed“Oak”,aftertheoaktreethathecouldseefromhisofficewindow.

AsthepopularityoftheWorldWideWebgrew,SunrecognizedthatGosling’slanguagecouldbedevelopedfortheinternet.Consequently,Sunrenamedthelanguage“Java”(simplybecausethatnamesoundedcool)andmadeitfreelyavailablein1995.Developersaroundtheworldquicklyadoptedthisexcitingnewlanguageand,becauseofitsmodulardesign,wereabletocreatenewfeaturesthatcouldbeaddedtothecorelanguage.ThemostendearingadditionalfeatureswereretainedinsubsequentreleasesofJavaasitdevelopedintothecomprehensiveversionoftoday.

TheessenceofJavaisalibraryoffilescalled“classes”,whicheachcontainsmallpiecesofready-madeprovencode.Anyoftheseclassescanbeincorporatedintoanewprogram,likebricksinawall,sothatonlyarelativelysmallamountofnewcodeeverneedstobewrittentocompletetheprogram.Thissavestheprogrammeravastamountoftime,andlargelyexplainsthehugepopularityofJavaprogramming.Additionally,thismodulararrangementmakesiteasiertoidentifyanyerrorsthaninasinglelargeprogram.

Javatechnologyisbothaprogramminglanguageandaplatform.InJavaprogramming,thesourcecodeisfirstwrittenashuman-readableplaintextfilesendingwiththe.javaextension.Thesearecompiledintomachine-readable.classfilesbythejavaccompiler.ThejavainterpretercanthenexecutetheprogramwithaninstanceoftheJavaVirtualMachine(JavaVM):

TheNewiconpicturedaboveindicatesaneworenhancedfeatureintroducedwiththelatestversionofJava.

AstheJavaVMisavailableonmanydifferentoperatingsystems,thesame.classfilesarecapableofrunningonWindows,LinuxandMacoperatingsystems–soJavaprogrammerstheoreticallyenjoythecross-platformabilityto“writeonce,runanywhere”.

InordertocreateJavaprograms,theJavaclasslibrariesandthejavaccompilerneedtobeinstalledonyourcomputer.InordertorunJavaprograms,theJava™RuntimeEnvironment(JRE)needstobeinstalledtosupplythejavainterpreter.AllofthesecomponentsarecontainedinafreelyavailablepackagecalledtheJava™Platform,StandardEditionDevelopmentKit(JDK).

TheJavaprogramsinthisbookuseversionJDK9,whichincorporatesboththeDevelopmentKititselfandtheRuntimeEnvironment,andcanbedownloadedfromtheOracle®websiteatwww.oracle.com/technetwork/java/javase/downloads

http://www.oracle.com/technetwork/java/javase/downloads

TheOracledownloadpagealsofeaturesotherpackages,butonlytheJDK9packageisrequiredtogetstartedwithJavaprogramming.

TheJDK9packageisavailableinversionsfor32-bitand64-bitvariantsoftheLinux,Mac,SolarisandWindowsplatforms–accepttheOracleLicenseAgreement,thenselecttheappropriateversionforyourcomputertodownloadtheJavaDevelopmentKit.

ThereisnotruthintherumorthatJAVAstandsfor“JustAnotherVagueAcronym”.

InstallingtheJDKSelecttheappropriateJavaDevelopmentKit(JDK)packageforyoursystemfromtheOracle®downloadspage,andthenfollowthesestepstoinstallJavaonyourcomputer:

UninstallanypreviousversionsoftheJDKand/orJavaRuntimeEnvironment(JRE)fromyoursystem

StarttheinstallationandaccepttheLicenseAgreementWhenthe“CustomSetup”dialogappears,eitheracceptthesuggestedinstallationlocationorclicktheChangebuttontochooseyourpreferredlocation–suchasC:\JavaforWindowssystemsorusrJavaforLinuxsystems

EnsurethatthePublicJREandDevelopmentToolsfeaturesareselectedfromthelist.Optionally,youmaydeselecttheotherfeaturesastheyarenotrequiredtostartprogrammingwiththisbook

ClicktheNextbuttontoinstallallthenecessaryJavaclasslibrariesandtoolsatthechosenlocation

ApreviousversionoftheJREmaybeinstalledsoyourwebbrowsercanrunJavaapplets.ItisbesttouninstallthistoavoidconfusionwiththenewerversioninJDK9.

Youcanstartoutbyinstallingjusttheminimumfeaturestoavoidconfusion.

ThetoolstocompileandrunJavaprogramsarenormallyoperatedfromacommand-linepromptandarelocatedinthebinsub-directoryoftheJavadirectory.Theycanbemadeavailablesystem-widebyaddingtheirlocationtothesystempath:

• OnWindows,navigatethroughControlPanel,System,AdvancedSystemSettings,Advancedtab,EnvironmentVariables,thenselectthesystemvariablenamed“Path”.ClicktheEditbuttonandaddtheaddressofJava’sbinssub-directorytothelist(e.g.C:\Java\bin),thenclickOKtoapplythechange.

• OnLinux,addthelocationofJava’sbinsub-directorytothesystempathbyeditingthe.bashrcfileinyourhomedirectory.Forinstance,addPATH=$PATH:usrJava/binthensavethefile.

PathsthatcontainspacesmustbeenclosedwithindoublequotesandterminatedbyasemicolononolderversionsofWindows.Forexample,withthepath“C:\ProgramFiles\Java\jdk-9\bin”;

Youarenowabletotesttheenvironment:

Youarenowabletotesttheenvironment:

Openacommand-linepromptwindow,suchasWindowsPowerShellorLinuxTerminalTypethecommandjava-versionthenhittheEnterkeytoseetheJavainterpreter’sversionnumberNext,typethecommandjavac-versionthenhittheEnterkeytoseetheJavacompiler’sversionnumberNow,typethecommandjshell-versionthenhittheEnterkeytoseetheJavashellversionnumberEnsurethatallversionnumbersmatch(9),andyou’rereadytobeginJavaprogramming

Ifthe.bashrcfileisnotvisibleinyourLinuxhomedirectorychooseView,ShowHiddenFilestorevealit.

TheJavashelljshellisanewfeatureinJava9.Thisinteractivetoolletsyouquicklytestsnippetsofcode,withouttheneedtofirstcompilethecode.Itisusedinthenextchaptertodemonstratethevarious“operators”availableinJavaprogramming.

WritingafirstJavaprogramAllJavaprogramsstartastextfilesthatarelaterusedtocreate“class”files,whicharetheactualrunnableprograms.ThismeansthatJavaprogramscanbewritteninanyplaintexteditor,suchastheWindowsNotepadapplication.

FollowthesestepstocreateasimpleJavaprogramthatwilloutputthetraditionalfirstprogramgreeting:

Hello.java

Openaplaintexteditor,likeNotepad,andtypethiscodeexactlyasitislisted–tocreateaclassnamed“Hello”classHello

{

}

BetweenthecurlybracketsoftheHelloclass,insertthiscode–tocreatea“main”methodfortheHelloclasspublicstaticvoidmain(String[]args)

{

}

Betweenthecurlybracketsofthemainmethod,insertthislineofcode–statingwhattheprogramwilldoSystem.out.println(“HelloWorld!”);

Savethefileatanyconvenientlocation,butbesuretonameitpreciselyasHello.java–thecompleteprogramshouldnowlooklikethis:

Javaisacase-sensitivelanguagewhere“Hello”and“hello”aredistinctlydifferent–traditionally,Javaprogramnamesshouldalwaysbeginwithanuppercaseletter.

Javaprogramsarealwayssavedastheirexactprogramnamefollowedbythe“.java”extension.

Theseparatepartsoftheprogramcodeontheoppositepagecanbeexaminedindividuallytounderstandeachpartmoreclearly:

TheProgramContainer

classHello{}

Theprogramnameisdeclaredfollowingtheclasskeyword,andfollowedbyapairofcurlybrackets.AlloftheprogramcodethatdefinestheHelloclasswillbecontainedwithinthesecurlybrackets.

Allstand-aloneJavaprogramsmusthaveamainmethod.Javaappletsaredifferent,andtheirformatisexplainedlater.

TheMainMethod

publicstaticvoidmain(String[]args){}

Thisfearsome-lookinglineisthestandardcodethatisusedtodefinethestartingpointofnearlyallJavaprograms.Itwillbeusedinmostexamplesthroughoutthisbookexactlyasitappearsabove–soitmaybeusefultomemorizeit.

Thecodedeclaresamethodnamed“main”thatwillcontaintheactualprograminstructionswithinitscurlybrackets.

Keywordspublicstaticvoidprecedethemethodnametodefinehowthemethodmaybeused,andareexplainedindetaillater.

Thecode(String[]args)isusefulwhenpassingvaluestothemethod,andisalsofullyexplainedlaterinthisbook.

TheStatement

System.out.println(“HelloWorld!”);

Statementsareactualinstructionstoperformprogramtasks,andmustalwaysendwithasemicolon.Amethodmaycontainmanystatementsinsideitscurlybracketstoforma“statementblock”definingaseriesoftaskstoperform,buthereasinglestatementinstructstheprogramtooutputalineoftext.

Turntoheretodiscoverhowtocompileandrunthisprogram.

Createa“MyJava”directoryinwhichtosaveallyourJavaprogramfiles.OnWindowsusethe

Compiling&runningprogramsBeforeaJavaprogramcanrun,itmustfirstbecompiledintoaclassfilebytheJavacompiler.ThisislocatedinJava’sbinsub-directory,andisanapplicationnamedjavac.Theinstructionsheredescribedhowtoaddthebinsub-directorytothesystempathsothatjavaccanbeinvokedfromanysystemlocation.

Followthesestepstocompiletheprogramhere:

Openacommand-linewindow,thennavigatetothedirectorywhereyousavedtheHello.javasourcecodefileTypejavacfollowedbyaspacethenthefullnameofthesourcecodefileHello.javaandhittheEnterkey

OnWindowsusetheWindowsPowerShellapportheolderCommandPromptapptoprovideacommand-lineprompt,andonLinuxuseaTerminalwindow.

AtaprompttypejavacandhitReturntorevealtheJavacompileroptions.

Ifthejavaccompilerdiscoverserrorsinthecodeitwillhaltanddisplayahelpful

reportindicatingthenatureoftheerror–seeherefortroubleshootingproblems.

Ifthejavaccompilerdoesnotfindanyerrorsitwillcreateanewfilewiththeprogramnameandthe.classfileextension.

Youcanalsocompilethesourcecodefromanotherlocationifyoustatethefile’sfullpathaddresstothejavaccompiler–inthiscase,C:\MyJava\Hello.java

WhentheJavacompilercompletescompilation,thecommand-linepromptwindowfocusreturnstothepromptwithoutanyconfirmationmessage–andtheprogramisreadytorun.

TheJavaprograminterpreterisanapplicationnamedjavathatislocatedinJava’sbinsub-directory–alongsidethejavaccompiler.Asthisdirectorywaspreviouslyaddedtothesystempath,here,thejavainterpretercanbeinvokedfromanylocation.

Followthesestepstoruntheprogramthatwascompiledusingtheproceduredescribedonthepageopposite:

Openacommand-linepromptwindow,thennavigatetothedirectorywheretheHello.classprogramfileislocatedAttheprompt,typejavafollowedbyaspacethentheprogramnameHelloandhittheEnterkey

Donotincludethe.classextensionwhenrunningaprogram–onlyusetheprogramname.

TheHelloprogramrunsandexecutesthetaskdefinedinthestatementwithinitsmainmethod–tooutput“HelloWorld!”.Uponcompletion,focusreturnstothepromptoncemore.

TheprocessofcompilingandrunningaJavaprogramistypicallycombinedinsequentialsteps,andisthesameregardlessofplatform.ThescreenshotbelowillustratestheHelloprogrambeingcompiledandrunincombinedstepsonaLinuxsystem:

CreatingavariableInJavaprogramming,a“variable”issimplyausefulcontainerinwhichavaluemaybestoredforsubsequentusebytheprogram.Thestoredvaluemaybechanged(vary)astheprogramexecutesitsinstructions–hencetheterm“variable”.

Avariableiscreatedbywritingavariable“declaration”intheprogram,specifyingthetypeofdatathatvariablemaycontainandagivennameforthatvariable.Forexample,theStringdatatypecanbespecifiedtoallowavariablenamed“message”tocontainregulartextwiththisdeclaration:Stringmessage;

Variablenamesarechosenbytheprogrammerbutmustadheretocertainnamingconventions.Thevariablenamemayonlybeginwithaletter,dollarsign$,ortheunderscorecharacter_,andmaysubsequentlyhaveonlyletters,digits,dollarsigns,orunderscorecharacters.Namesarecase-sensitive,so“var”and“Var”aredistinctlydifferentnames,andspacesarenotallowedinnames.

VariablenamesshouldalsoavoidtheJavakeywordslistedinthetablebelow,asthesehavespecialmeaningintheJavalanguage.

abstract default goto package synchronized

assert do if private this

boolean double implements protected throw

break else import public throws

byte enum instanceof return transient

case extends int short true

catch false interface static try

char final long strictfp void

class finally native String volatile

const float new super while

continue for null switch

Eachvariabledeclarationmustbeterminatedwithasemicoloncharacter–likeallotherstatements.

Strictlyspeaking,somewordsinthistablearenotactuallykeywords–true,false,andnullareallliterals;Stringisaspecialclassname;constandgotoarereservedwords(currentlyunused).Theseareincludedinthetablebecausetheymustalsobeavoidedwhennamingvariables.

Asgoodpractice,variablesshouldbenamedwithwordsoreasilyrecognizableabbreviations,describingthatvariable’spurpose.Forexample,“button1”or“btn1”todescribebuttonnumberone.Lowercaselettersarepreferredforsingle-wordnames,suchas“gear”,andnamesthatconsistofmultiplewordsshouldcapitalizethefirstletterofeachsubsequentword,suchas“gearRatio”–theso-called“camelCase”namingconvention.

Onceavariablehasbeendeclared,itmaybeassignedaninitialvalueoftheappropriatedatatypeusingtheequalssign=,eitherinthedeclarationorlateronintheprogram,thenitsvaluecanbereferencedatanytimeusingthevariable’sname.

Followthesestepstocreateaprogramthatdeclaresavariable,whichgets

Followthesestepstocreateaprogramthatdeclaresavariable,whichgetsinitializedinitsdeclarationthenchangedlater:

Startanewprogramnamed“FirstVariable”,containingthestandardmainmethodclassFirstVariable

{


}

FirstVariable.java

Betweenthecurlybracketsofthemainmethod,insertthiscodetocreate,initialize,andoutputavariableStringmessage=“Initialvalue”;System.out.println(message);

Addtheselinestomodifyandoutputthevariablevaluemessage=“Modifiedvalue”;System.out.println(message);

SavetheprogramasFirstVariable.java,thencompileandruntheprogram

Ifyouencounterproblemscompilingorrunningtheprogram,youcanget

helpfromTroubleshootingproblemshere.

RecognizingdatatypesThemostfrequently-useddatatypesinJavavariabledeclarationsarelistedinthistable,alongwithabriefdescription:

Datatype: Description: Example:

char AsingleUnicodecharacter ‘a’

String AnynumberofUnicodecharacters “myString”

int Anintegernumber,from-2.14billionto+2.14billion 1000

float Afloating-pointnumber,withadecimalpoint 3.14159265f

boolean Alogicalvalueofeithertrueorfalse true

Duetotheirregularitiesoffloating-pointarithmeticthefloatdatatypeshouldneverbeusedforprecisevalues,suchascurrency–seeherefordetails.

Noticethatchardatavaluesmustalwaysbesurroundedbysinglequotes,andStringdatavaluesmustalwaysbesurroundedbydoublequotes.Also,rememberthatfloatdatavaluesmustalwayshavean“f”suffixtoensuretheyaretreatedasafloatvalue.

Inadditiontothemorecommondatatypesabove,Javaprovidesthesespecializeddatatypesforuseinexactingcircumstances:

Datatype: Description:

byte Integernumberfrom-128to+127

short Integernumberfrom-32,768to+32,767

long Positiveornegativeintegerexceeding2.14billion

double Extremelylongfloating-pointnumber

AlldatatypekeywordsbeginwithalowercaseletterexceptString–whichisaspecialclass.

SpecializeddatatypesareusefulinadvancedJavaprograms–theexamplesinthisbookmostlyusethecommondatatypesdescribedinthetoptable.

FollowthesestepstocreateaJavaprogramthatcreates,initializes,andoutputsvariablesofallfivecommondatatypes:

Startanewprogramnamed“DataTypes”containingthestandardmainmethodclassDataTypes

{


}

DataTypes.java

Betweenthecurlybracketsofthemainmethod,insertthesedeclarationstocreateandinitializefivevariablescharletter=‘M’;Stringtitle=“Javaineasysteps”;intnumber=365;floatdecimal=98.6f;booleanresult=true;

AddtheselinestooutputanappropriatetextStringconcatenatedtothe

valueofeachvariableSystem.out.println(“Initialis”+letter);System.out.println(“Bookis”+title);System.out.println(“Daysare”+number);System.out.println(“Temperatureis”+decimal);System.out.println(“Answeris”+result);

SavetheprogramasDataTypes.java,thencompileandruntheprogram

Noticehowthe+characterisusedheretojoin(concatenate)textstringsandstoredvariablevalues.

TheJavacompilerwillreportanerroriftheprogramattemptstoassignavalueofthewrongdatatypetoavariable–trychangingthevaluesinthisexample,thenattempttorecompiletheprogramtoseetheeffect.

.

CreatingconstantsThe“final”keywordisamodifierthatcanbeusedwhendeclaringvariablestopreventanysubsequentchangestothevaluesthatareinitiallyassignedtothem.Thisisusefulwhenstoringafixedvalueinaprogramtoavoiditbecomingalteredaccidentally.

Variablescreatedtostorefixedvaluesinthiswayareknownas“constants”,anditisconventiontonameconstantswithalluppercasecharacters–todistinguishthemfromregularvariables.Programsthatattempttochangeaconstantvaluewillnotcompile,andthejavaccompilerwillgenerateanerrormessage.

FollowthesestepstocreateaJavaprogramfeaturingconstants:

Startanewprogramnamed“Constants”containingthestandardmainmethodclassConstants

{


}

Constants.java

Betweenthecurlybracketsofthemainmethod,insertthiscodetocreateandinitializethreeintegerconstantsfinalintTOUCHDOWN=6;finalintCONVERSION=1;finalintFIELDGOAL=3;

Now,declarefourregularintegervariablesinttd,pat,fg,total;

Initializetheregularvariables–usingmultiplesoftheconstantvaluestd=

4*TOUCHDOWN;pat=3*CONVERSION;fg=2*FIELDGOAL;total=(td+pat+fg);

AddthislinetodisplaythetotalscoreSystem.out.println(“Score:”+total);

SavetheprogramasConstants.java,thencompileandruntheprogramtoseetheoutput,Score:33(4x6=24,3x1=3,2x3=6,so24+3+6=33).

The*asteriskcharacterisusedheretomultiplytheconstantvalues,andparenthesessurroundtheiradditionforclarity

AddingcommentsWhenprogramminginanylanguage,itisgoodpracticetoaddcommentstoprogramcodetoexplaineachparticularsection.Thismakesthecodemoreeasilyunderstoodbyothers,andbyyourself,whenrevisitingapieceofcodeafteraperiodofabsence.

InJavaprogramming,commentscanbeaddedacrossmultiplelinesbetween/*and*/commentidentifiers,oronasinglelineaftera//commentidentifier.Anythingappearingbetween/*and*/,oronalineafter//,iscompletelyignoredbythejavaccompiler.

WhencommentshavebeenaddedtotheConstants.javaprogram,described

opposite,thesourcecodemightlooklikethis:

Constants.java(commented)

/*

Aprogramtodemonstrateconstantvariables.

*/

classConstants

{

publicstaticvoidmain(Stringargs[])

{

//Constantscorevalues.finalintTOUCHDOWN=6;finalintCONVERSION=1;finalintFIELDGOAL=3;

//Calculatepointsscored.

inttd,pat,fg,total;td=4*TOUCHDOWN;//4x6=24pat=3*CONVERSION;//3x1=3fg=2*FIELDGOAL;//2x3=6total=(td+pat+fg);//24+3+6=33

//Outputcalculatedtotal.System.out.println(“Score:“+total);

}

}

Savedwithcomments,theprogramcompilesandrunsasnormal:

Youcanaddastatementthatattemptstochangethevalueofaconstant,thentrytorecompiletheprogramtoseetheresultingerrormessage.

TroubleshootingproblemsSometimes,thejavaccompilerorjavainterpreterwillcomplainabouterrors,soit’susefultounderstandtheircauseandhowtoquicklyresolvetheproblem.Inordertodemonstratesomecommonerrorreports,thiscodecontainssomedeliberateerrors:

Test.java

classtest

{

publicstaticvoidmain(String[]args)

{

Stringtext;System.out.println(“Test”+text)

}

}

AfirstattempttocompileTest.javathrowsupthiserrorreport:

• Cause–thejavaccompilercannotbefound.

• Solution–editthesystemPATHvariable,asdescribedhere,oruseitsfull

pathaddresstoinvokethecompiler.

Thepathaddressmustbeenclosedwithinquotationmarksifitcontainsanyspaces,suchasthepathaddress“C:\ProgramFiles\Java”.

• Cause–thefileTest.javacannotbefound.

• Solution–navigatetothedirectorywherethefileislocated,orusethefullpathaddresstothefileinthecommand.

• Cause–thestatementisnotterminatedcorrectly.

• Solution–inthesourcecodeaddasemicolonattheendofthestatement,thensavethefiletoapplythechange.

• Cause–theprogramnameandclassnamedonotmatch.

• Solution–inthesourcecodechangetheclassnamefromtesttoTest,thensavethefiletoapplythechange.

• Cause–thevariabletexthasnovalue.

• Solution–inthevariabledeclarationassignthevariableavalidStringvalue,forinstance=“success”,thensavethefile.

Youmustruntheprogramfromwithinitsdirectory–youcannotuseapathaddressastheJavalauncherrequiresaprogramname,notafilename.

Summary• Javaisbothaprogramminglanguageandaruntimeplatform.

• Javaprogramsarewrittenasplaintextfileswitha.javafileextension.

• TheJavacompilerjavaccreatescompiled.classprogramfilesfromoriginal.javasourcecodefiles.

• TheJavainterpreterjavaexecutescompiledprogramsusinganinstanceoftheJavaVirtualMachine.

• TheJavaVMisavailableonmanyoperatingsystemplatforms.

• AddingJava’sbinsub-directorytothesystemPATHvariableallowsthejavaccompilertobeinvokedfromanywhere.

• Javaisacase-sensitivelanguage.

• ThestandardmainmethodistheentrypointforJavaprograms.

• TheSystem.out.println()statementoutputstext.

• AJavaprogramfilenamemustexactlymatchitsclassname.

• Javavariablescanonlybenamedinaccordancewithspecifiednamingconventions,andmustavoidtheJavakeywords.

• InJavaprogramming,eachstatementmustbeterminatedbyasemicoloncharacter.

• ThemostcommonJavadatatypesareString,int,char,floatandboolean.

• Stringvaluesmustbeenclosedindoublequotes;charvaluesinsinglequotes;andfloatvaluesmusthavean“f”suffix.

• Thefinalkeywordcanbeusedtocreateaconstantvariable.

• CommentscanbeaddedtoJavasourcecodebetween/*and*/,ononeormorelines,orafter//onasingleline.

• Errorreportsidentifycompilerandruntimeproblems.

2

Performingoperations

ThischapterdemonstratesthevariousoperatorsthatareusedtocreateexpressionsinJavaprograms.

DoingarithmeticAssigningvalues

ComparingvaluesAssessinglogicExaminingconditionsSettingprecedence

EscapingliteralsWorkingwithbitsSummary

Doingarithmetic

Arithmeticaloperators,listedinthetablebelow,areusedtocreateexpressionsinJavaprogramsthatreturnasingleresultingvalue.Forexample,theexpression4*2returnsthevalue8.

Operator: Operation:

+Addition(andconcatenatesStringvalues)

-Subtraction

-

*Multiplication

/Division

%Modulus

++Increment

--Decrement

Divisionofintvalueswilltruncateanyfractionalpart.Forexample,11/4=2,whereasdivisionoffloatvalues11/4=2.75.

Theincrementoperator++anddecrementoperator--returntheresultofmodifyingasinglegivenoperandbyavalueofone.Forexample,4++returnsthevalue5,and4--returnsthevalue3.

Allotherarithmeticoperatorsreturntheresultofanoperationperformedontwogivenoperands,andactasyouwouldexpect.Forexample,theexpression5+2returns7.

Themodulusoperatordividesthefirstoperandbythesecondoperandandreturnstheremainderoftheoperation.Forexample,32%5returns2–fivedividesinto32sixtimes,with2remainder.

Theoperationperformedbytheadditionoperator+dependsonthetypeofitsgivenoperands.Wherebothoperandsarenumericvaluesitwillreturnthetotalsumvalueofthosenumbers,butwheretheoperandsareStringvaluesitwillreturnasingleconcatenatedString–combiningthetextineachStringoperand.

Forexample,“Java”+“Arithmetic”returns“JavaArithmetic”.

Incrementanddecrementoperatorsaretypicallyusedtocounttheiterationsintheforloopconstructs,introducedhere.

FollowthesestepstoexploretheJavaarithmeticoperatorsintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeytolaunchtheJavashellNext,enterstatementstoinitializethreevariablesintnum=100;intfactor=20;intsum=0;

Next,separatelyenterstatementstoperformadditionandsubtractionoperations,displayingeachresultsum=num+factor;sum=num-factor;

Now,separatelyenterstatementstoperformmultiplicationanddivisionoperations,displayingeachresultsum=num*factor;sum=num/factor;

JavamustbeinstalledonyoursystempathtolaunchtheJavashellfromanyprompt–seeInstallinghereandTroubleshootingherefordetails.

TheJavashelljshellisanewfeatureinJava9.Optionally,thesemicoloncharactermaybeomittedattheendofsinglestatementsenteredintotheshellbutthesearerequiredwhenwritingJavaprogramsforcompilation.Semicolonsareincludedintheshellexamplesinthischaptertoaidcodeconsistency.

Assigningvalues

Assignmentoperators,listedinthetablebelow,areusedtoassigntheresultofanexpression.Allexceptthesimple=operatoraretheshorthandformofalongerequivalentexpression:

Operator: Example: Equivalent:

=a=b a=b

+=a+=b a=a+b

-= a-=b a=a-b

*=a*=b a=a*b

/=a/=b a=a/b

%=a%=b a=a%b

Itisimportanttoregardthe=operatortomean“assign”,ratherthan“equals”,toavoidconfusionwiththe==equalityoperator.

Intheexamplea=b,thevaluestoredinthevariablenamedbisassignedtothevariablenameda,sothatvaluebecomesthenewvaluestoredina–replacinganyvalueitpreviouslycontained.

The+=operatorisusefultoaddavalueontoanexistingvaluestoredinavariable–keepinga“runningtotal”.

Theexamplea+=bfirstcalculatesthesumtotalofthevaluesstoredinthevariablesnamedaandb,thenassignstheresultingtotaltovariablea.Aprogrammightthencontainafurtherassignmenta+=cthatcalculatesthetotalstoredinvariablesnamedaandc,thenassignsthatnewtotaltovariablea–addingthevalueofctothevalueitpreviouslycontained.

Alltheotherassignmentoperatorsworkinthesamewaybyfirstperformingthearithmeticalcalculationonthetwostoredvalues,thenassigningtheresulttothefirstvariable–tobecomeitsnewstoredvalue.

The==equalityoperatorcomparesvalues,andisfullyexplainedhere.

FollowthesestepstoexploretheJavaassignmentoperatorsintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeyto

launchtheJavashellNext,enterstatementstoinitializetwoStringvariablesStringtxt=“Super”;Stringlang=“Java”;

Now,separatelyenterstatementstoaddandassignaStringvalue,thendisplaytheconcatenatedstringresulttxt+=lang;txt;

Then,enterstatementstoinitializetwointegervariablesintsum=10;intnum=20;

Separatelyenterstatementstoaddandassignanintvalue,thendisplaythetotaledintegerresultsum+=num;sum;

ThenewJavashellfeature,introducedinJava9,createsinternal$-prefixednumberedvariablescontainingtheresultofanevaluation.Here,internalvariables$3and$7containevaluationresults.

Assignmentofthewrongdatatypetoavariablewillcauseanerror.

Comparingvalues

Comparisonoperators,listedinthetablebelow,areusedtocomparetwovaluesinanexpressionandreturnasingleBooleanvalueoftrueorfalse–describingtheresultofthatcomparison.

Operator: Comparison:

==Equality

!=Inequality

> Greaterthan

>= Greaterthan,orequalto

< Lessthan

<= Lessthan,orequalto

The==equalityoperatorcomparestwooperands,andwillreturntrueifbothareexactlyequalinvalue.Ifbotharethesamenumbertheyareequal,orifbothareStringvaluescontainingthesamecharactersinthesameordertheyareequal.Booleanoperandsthatarebothtrue,orthatarebothfalse,areequal.

Conversely,the!=inequalityoperatorreturnstrueiftwooperandsarenotequal–applyingthesamerulesastheequalityoperator.

Equalityandinequalityoperatorsareusefulintestingthestateoftwovariablestoperform“conditionalbranching”ofaprogram–proceedingindifferentdirectionsaccordingtothecondition.

The>“greaterthan”operatorcomparestwooperands,andwillreturntrueifthe

firstisgreaterinvaluethanthesecond.

The<“lessthan”operatormakesthesamecomparison,butreturnstrueifthefirstoperandislessinvaluethanthesecond.

Addingthe=assignmentoperatorafterthe>“greaterthan”operator,orafterthe<“lessthan”operator,makesitalsoreturntruewhenthetwooperandsareexactlyequalinvalue.

The<lessthanoperatoristypicallyusedtotestacountervalueinaloop–anexampleofthiscanbefoundhere.

FollowthesestepstoexploretheJavacomparisonoperatorsintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeytolaunchtheJavashellNext,enterstatementstoinitializetwoStringvariablesStringtxt=“Super”;Stringlang=“Java”;

Now,separatelyenterstatementstoinitializeabooleanvariableanddisplaytheresultofStringvaluecomparisonsforequalityandinequalitybooleanstate=(txt==lang);state=(txt!=lang);

Inasimilarway,separatelyenterthesestatementstodisplaytheresultofintvaluecomparisonsforgreaterandlessnumericvalueintdozen=12;intscore=20;state=(dozen>score);state=(dozen<score);

YoucandiscovermoreoptionswithintheJavashellbyenteringthe/helpcommand.

Hereit’suntrue(false)thattheStringvaluesareequal,butitistruethattheyareunequal.

Noticehowanexpressioncanbecontainedinparenthesesforbetterreadability.

Assessinglogic

Logicaloperators,listedinthetablebelow,areusedtocombinemultipleexpressionsthateachreturnaBooleanvalue–intoacomplexexpressionthatreturnsasingleBooleanvalue.

Operator: Operation:

&& LogicalAND

||LogicalOR

! LogicalNOT

LogicaloperatorsareusedwithoperandsthathavetheBooleanvaluesoftrueorfalse,orvaluesthatcanconverttotrueorfalse.

Thelogical&&ANDoperatorwillevaluatetwooperandsandreturntrueonlyifbothoperandsarethemselvestrue.Otherwise,thelogical&&operatorwillreturnfalse.Thisevaluationcanbeusedinconditionalbranching,whereaprogramwillonlyperformacertainactionwhentwotestedconditionsarebothtrue.

Unlikethelogical&&operatorthatneedstwooperandstobetrue,thelogical||ORoperatorwillevaluateitstwooperandsandreturntrueifeitheroneoftheoperandsistrue–itwillonlyreturnfalsewhenneitheroperandistrue.ThisisusefulinJavaprogrammingtoperformacertainactionwheneitheroneoftwotestconditionshasbeenmet.

Thelogical!NOToperatorisa“unary”operatorthatisusedbeforeasingleoperand.ItreturnstheinverseBooleanvalueofthegivenoperand–reversingtruetofalse,andfalsetotrue.It’susefulinJavaprogramstotogglethevalueofavariableinsuccessiveloopiterationswithastatementlikegoState=!goState.Thisensuresthatoneachpassoftheloopthevalueischanged,likeflickingalightswitchonandoff.

Theterm“Boolean”referstoasystemoflogicalthoughtdevelopedbytheEnglishmathematicianGeorgeBoole(1815-1864).

ThenewJavashellfeature,introducedinJava9,isalsoknownasa“REPL”–anacronymforRead,Evaluate,Print,Loopthatdescribesthistypeofinteractivetool.

FollowthesestepstoexplorelogicaloperatorsintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeytolaunchtheJavashellNext,enterstatementstoinitializetwobooleanvariablesbooleanyes=true;booleanno=false;

Enterstatementstotestifbothtwoconditionsaretruebooleanresult=(yes&&yes);result=(yes&&no);

Enterstatementstotestifeitheroftwoconditionsistrueresult=(yes||yes);result=(yes||no);

result=(no||no);

Enterstatementstoshowanoriginalandinversevalueresult=yes;result=!yes;

Noticethatfalse&&falsereturnsfalse,nottrue–demonstratingthemaximthat“twowrongsdon’tmakearight”.

Thevaluereturnedbythe!NOTlogicaloperatoristheinverseofthestoredvalue–thestoredvalueitselfremainsunchanged.

Examiningconditions

Possiblytheall-timefavoriteoperatoroftheJavaprogrammeristhe?:conditionaloperatorthatmakesapowerfulstatementveryconcisely.Itsunusualsyntaxcanseemtrickytounderstandatfirst,butitiswellworthgettingtoknowthisusefuloperator.

Theconditionaloperatorfirstevaluatesanexpressionforatrueorfalsevalue,thenreturnsoneoftwogivenoperandsdependingontheresultoftheevaluation.Itssyntaxlookslikethis:

(boolean-expression)?if-true-return-this:if-false-return-this;

Theconditionaloperatorisalsoknownasthe“ternary”operator.

EachspecifiedoperandalternativeallowstheprogramtoprogressaccordingtotheBooleanvaluereturnedbythetestedexpression.Forinstance,thealternativesmightreturnaStringvalue:status=(quit==true)?“Done!”:“Continuing...”;

Inthiscase,whenthequitvariableistruetheconditionaloperatorassignsthevalueofitsfirstoperandtothestatusvariable;otherwise,itassignsitssecondoperandvalueinstead.

AshorthandavailablewhencodingJavaprogramsallowsexpressionstooptionallyomit==truewhenevaluatingasimpleBooleanvalue,sotheexampleabovecanbewrittensimplyas:

status=(quit)?“Done!”:“Continuing...”;

Theconditionaloperatorcanreturnvaluesofanydatatypeandemployanyvalidtestexpression.Forinstance,theexpressionmightusethegreaterthan>operatortoevaluatetwonumericvaluesthenreturnaBooleanvaluedependingontheresult:

busted=(speed>speedLimit)?true:false;

Similarly,theconditionaloperatormightemploytheinequality!=operatortoevaluateaStringvaluethenreturnanumericvaluedependingontheresult:bodyTemperature=(scale!=“Celsius”)?98.6:37.0;

YoucanalsostarttheJavashellwiththecommandjshell--feedbackverbosetoreceivedescriptiveoutputaftereachevaluation.

FollowthesestepstoexploretheJavaconditionaloperatorintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeytolaunchtheJavashellNext,enterstatementstoinitializetwointvariablesintnum1=1357;intnum2=2468;

DeclareafurthervariabletostoreatestresultStringStringresult;

Enterthisstatementtodeterminewhetherthefirstintegervalueisanoddorevennumberresult=(num1%2!=0)?“Odd”:“Even”;

Now,enterthisstatementtodeterminewhetherthesecondintegervalueisanoddorevennumberresult=(num2%2!=0)?“Odd”:“Even”;

NoticethatanuninitializedStringvariablereturnsaspecialnullvalue–indicatingthatitcontainsnothingwhatsoever.

Here,theexpressionevaluatesastruewhenthereisanyremainder.

Settingprecedence

Complexexpressions,whichcontainmultipleoperatorsandoperands,canbeambiguousunlesstheorderinwhichtheoperationsshouldbeexecutedisclear.Thislackofclaritycaneasilycausedifferentresultstobeimpliedbythesameexpression.Forexample,considerthiscomplexexpression:num=8+4*2;

Workinglefttoright8+4=12,and12*2=24,sonum=24.Butworkingrighttoleft2*4=8,and8+8=16,sonum=16.

TheJavaprogrammercanexplicitlyspecifywhichoperationshouldbeexecutedfirstbyaddingparenthesestosignifywhichoperatorhasprecedence.Inthiscase,(8+4)*2ensuresthattheadditionisperformedbeforethemultiplication–sotheresultis24,not16.Conversely,8+(4*2)performsthemultiplicationfirst–sotheresultis16,not24.

WhereparenthesesdonotexplicitlyspecifyoperatorprecedenceJavafollowsthedefaultprecedenceorderlistedinthetablebelow,fromfirstatthetoptolastatthebottom:

Operator: Description:

++--! Increment,Decrement,LogicalNOT

*/%Multiplication,Division,Modulus

+-Addition,Subtraction

>>=<<= Greaterthan,GreaterthanorequaltoLessthan,Lessthanorequalto

==!=Equality,Inequality

&& LogicalAND

||LogicalOR

?:Conditional

=+=-=*=/=%=Assignment

Operatorsofequalprecedencearehandledintheordertheyappearintheexpression–fromlefttoright.

FollowthesestepstoexploreoperatorprecedenceintheJavashell:

Openacommand-linepromptwindow,thentypejshellandhittheEnterkeytolaunchtheJavashellNext,enterastatementtodisplaytheresultofevaluatinganexpressionthatusesdefaultoperatorprecedenceintsum=32-8+16*2;

Now,enterastatementtodisplaytheresultofevaluatingthesameexpression–butgivingadditionandsubtractionprecedenceovermultiplicationsum=(32-8+16)*2;

Finally,enterastatementtodisplaytheresultofevaluatingthesameexpressiononcemore–butnowwhereoperationprecedenceorderisfirstaddition,thensubtraction,andthenmultiplicationsum=(32-(8+16))*2;

Whereexpressionshavemultiplenestedparentheses,theinnermosttakesprecedence.

Howitworks–Step2...

16x2=32,+24=56

Step3...

24+16=40,x2=80

Step4...

32-24=8,x2=16

ThischapterhassofarusedtheJavashelljshelltoexplorethevariousJavaoperatorsbyevaluatingcodesnippets.EnsuingexampleswillusetheJavacompilerjavacandJavaruntimejavatocreateandexecuteprograms.YoucanquittheJavashelltoreturntoaregularpromptwiththecommand/exit.

Escapingliterals

ThenumericalandtextvaluesinJavaprogramsareknownas“literals”–theyrepresentnothingbutare,literally,whatyousee.

LiteralsarenormallydetachedfromthekeywordsoftheJavalanguage,butwheredoublequotes,orsinglequotes,arerequiredwithinaStringvalueitisnecessarytoindicatethatthequotecharacteristobetreatedliterallytoavoidprematurelyterminatingtheString.Thisiseasilyachievedbyimmediatelyprefixingeachnestedquotecharacterwiththe\escapeoperator.Forexample,includingaquotewithinaStringvariable,likethis:Stringquote=“\”Fortunefavorsthebrave.\”saidVirgil”;

Additionally,the\escapeoperatoroffersavarietyofusefulescapesequencesfor

simpleoutputformatting:

Escape: Description:

\n Newline

\t Tab

\b Backspace

\r Carriagereturn

\f Formfeed

\\Backslash

\’ Singlequotemark

\” Doublequotemark

Singlequotescanbenestedwithindoublequotesasanalternativetoescapingquotecharacters.

The\nnewlineescapesequenceisfrequentlyusedwithinlongStringvaluestodisplaytheoutputonmultiplelines.Similarly,the\ttabescapesequenceisfrequentlyusedtodisplaytheoutputincolumns.Usingacombinationof\nnewlineand\ttabescapesequencesallowstheoutputtobeformattedinbothrowsandcolumns–toresembleatable.

FollowthesestepstocreateaJavaprogramusingescapesequencestoformattheoutput:

Startanewprogramnamed“Escape”containingthestandardmainmethodclassEscape

{


}

Escape.java

Betweenthecurlybracketsofthemainmethod,insertthiscodetobuildaStringcontainingaformattedtabletitleandcolumnheadingsStringheader=“\n\tNEWYORK3-DAYFORECAST:\n”;header+=“\n\tDay\t\tHigh\tLow\tConditions\n”;header+=“\t---\t\t----\t---\t----------\n”;

AddtheselinestobuildaStringcontainingformattedtablecelldataStringforecast=“\tSunday\t\t68F\t48F\tSunny\n”;forecast+=“\tMonday\t\t69F\t57F\tSunny\n”;forecast+=“\tTuesday\t\t71F\t50F\tCloudy\n”;

Now,addthislinetooutputbothformattedStringvaluesSystem.out.print(header+forecast);

SavetheprogramasEscape.java,thencompileandruntheprogram

Inthiscase,escapesequencesaddnewlinessotheprint()methodisusedhere–ratherthantheprintln()methodthatautomaticallyaddsanewlineafteroutput.

Workingwithbits

Inadditiontotheregularoperatorsdescribedearlierinthischapter,Javaprovidesspecialoperatorsforbinaryarithmetic.Thesearelesscommonlyusedthanotheroperators,butarebrieflydiscussedheretosimplyprovideanawarenessoftheirexistence.

TheJava“bitwise”operatorscanbeusedwiththeintintegerdatatypetomanipulatethebitsofthebinaryrepresentationofavalue.Thisrequiresanunderstandingofbinarynumbering,whereeightbitsinabyterepresentdecimalvalueszeroto255.Forexample,53isbinary00110101(0x128,0x64,1x32,1x16,0x8,1x4,0x2,1x1).

Binaryadditionoperationsareperformedlikedecimalarithmetic:

53=

00110101

+7

=00000111

60=

00111100

Thebitwiseoperators,listedbelow,allowmorespecializedoperationstobeperformedinbinaryarithmetic.

Operator: Operation: Example: Result:

& AND a&b 1ifbothbitsare1

|OR a|b 1ifeitherbitis1

^XOR a^b 1ifbothbitsdiffer

~NOT ~a Invertsthebits

<< Leftshift n<<p Movesnbitspleft

>> Rightshift n>>p Movesnbitspright

Forexample,usingthebitwise&operatorinbinaryarithmetic:

53=

00110101

& 7=

00000111

5=

00000101

Don’tconfusethelogicalANDoperator&&withthebitwise&operator,orthelogicalORoperator||withthebitwise|operator.

Acommonuseofbitwiseoperatorscombinesseveralvaluesinasinglevariableforefficiency.Forinstance,aprogramwitheight“flag”intvariables,withvaluesof1or0(representingonandoffstates),requires32bitsofmemoryforeachvariable–256bitsintotal.Thesevaluesonlyreallyrequireasinglebit,however,soeightflagscanbecombinedinasinglebytevariable–usingonebitperflag.Thestatusofeachflagcanberetrievedwithbitwiseoperations:

Startanewprogramnamed“Bitwise”containingthestandardmainmethodclassBitwise

{


}

Bitwise.java

Betweenthecurlybracketsofthemainmethod,insertthiscodetodeclareandinitializeabytevariablewithavaluerepresentingthetotalstatusofuptoeightflagsbytefs=53;//Combinedflagstatusof00110101

AddtheselinestoretrievethestatusofeachflagSystem.out.println(“Flag1:“+(((fs&1)>0)?“ON”:“off”));System.out.println(“Flag2:“+(((fs&2)>0)?“ON”:“off”));System.out.println(“Flag3:“+(((fs&4)>0)?“ON”:“off”));System.out.println(“Flag4:“+(((fs&8)>0)?“ON”:“off”));System.out.println(“Flag5:“+(((fs&16)>0)?“ON”:“off”));System.out.println(“Flag6:“+(((fs&32)>0)?“ON”:“off”));System.out.println(“Flag7:“+(((fs&64)>0)?“ON”:“off”));System.out.println(“Flag8:“+(((fs&128)>0)?“ON”:“off”));

SavetheprogramasBitwise.javathencompileandruntheprogram:

Howitworks–Thebinaryrepresentationof53is00110101sothesetbitsare...1+4+16+32=53

Here,thebitwise&operationreturnsoneorzerotodetermineeachflag’sstatus.

Summary

• Arithmeticaloperatorscanformexpressionswithtwooperandsforaddition+,subtraction–,multiplication*,division/,ormodulus%.

• Increment++anddecrement--operatorsmodifyasingleoperandbyavalueofone.

• Theassignment=operatorcanbecombinedwithanarithmeticaloperatortoperformanarithmeticalcalculationthenassignitsresult.

• Comparisonoperatorscanformexpressionscomparingtwooperandsforequality==,inequality!=,greater>,orlesser<values.

• Theassignment=operatorcanbecombinedwiththegreaterthan>orlesserthan<operatortoalsoreturntruewhenequal.

• Logical&&and||operatorsformexpressionsevaluatingtwooperandstoreturnaBooleanvalueofeithertrueorfalse.

• Thelogical!operatorreturnstheinverseBooleanvalueofasingleoperand.

• Aconditional?:operatorevaluatesagivenBooleanexpressionandreturnsoneoftwooperands,dependingonitsresult.

• ExpressionsevaluatingaBooleanexpressionforatruevaluemayoptionally

omit==true.

• Itisimportanttoexplicitlysetoperatorprecedenceincomplexexpressionsbyaddingparentheses().

• Thebackslashescape\operatorcanbeusedtoprefixquotecharacterswithinStringvaluestopreventsyntaxerrors.

• Escapesequences\nnewlineand\ttabprovidesimpleoutputformatting.

• Bitwiseoperatorscanbeusefultoperformbinaryarithmeticinspecializedsituations.

3

Makingstatements

ThischapterdemonstratesthevariouskeywordsthatareusedtocreatebranchinginJavaprograms.

BranchingwithifBranchingalternatives

SwitchingbranchesLoopingforLoopingwhiletrueDoingdo-whileloops

BreakingoutofloopsReturningcontrolSummary

Branchingwithif

TheifkeywordperformsaconditionaltesttoevaluateanexpressionforaBooleanvalue.Astatementfollowingtheexpressionwillonlybeexecutedwhentheevaluationistrue,otherwisetheprogramproceedsontosubsequentcode–pursuingthenext“branch”.Theifstatementsyntaxlookslikethis:

if(test-expression)code-to-be-executed-when-true;Thecodetobeexecutedcancontainmultiplestatementsiftheyareenclosedwithincurlybracketstoforma“statementblock”:

Startanewprogramnamed“If”containingthestandardmainmethodclassIf

{


}

If.java

Betweenthecurlybracketsofthemainmethod,insertthissimpleconditionaltestthatexecutesasinglestatementwhenonenumberisgreaterthananotherif(5>1)System.out.println(“Fiveisgreaterthanone.”);

Addasecondconditionaltest,whichexecutesanentirestatementblockwhenonenumberislessthananotherif(2<4)

{

System.out.println(“Twoislessthanfour.”);System.out.println(“Testsucceeded.”);

}

SavetheprogramasIf.javathencompileandruntheprogramtoseeallstatementsgetexecuted–becausebothtestsevaluateastrueinthiscase:

Expressionscanutilizethetrueandfalsekeywords.Thetestexpression(2<4)isshorthandfor(2<4==true).

AconditionaltestcanalsoevaluateacomplexexpressiontotestmultipleconditionsforaBooleanvalue.Parenthesesencloseeachtestconditiontoestablishprecedence–sotheygetevaluatedfirst.TheBoolean&&ANDoperatorensuresthecomplexexpressionwillonlyreturntruewhenbothtestedconditionsaretrue:if((test-condition1)&&(test-condition2))execute-this-code;TheBoolean||ORoperatorensuresacomplexexpressionwillonlyreturntruewheneitheroneofthetestedconditionsistrue:if((test-condition1)||(test-condition2))execute-this-code;Acombinationofthesecanformlongercomplexexpressions:

InsidethemainmethodofIf.javainsertthislinetodeclareandinitializeanintegervariablenamednumintnum=8;

Addathirdconditionaltestthatexecutesastatementwhenthevalueofthenumvariableiswithinaspecifiedrange,orwhenit’sexactlyequaltoaspecifiedvalueif(((num>5)&&(num<10))||(num==12))System.out.println(“Numberis6-9inclusive,or12”);

Recompiletheprogram,andrunitoncemoretoseethestatementafterthecomplexexpressiongetexecuted

Changethevalueassignedtothenumvariablesoitisneitherwithinthespecifiedrange6-9,orexactly12.Recompiletheprogram,andrunitagaintonowseethestatementafterthecomplexexpressionisnotexecuted

Therangecanbeextendedtoincludetheupperandlowerlimitsusingthe>=and<=operators.

Thecomplexexpressionusesthe==equalityoperatortospecifyanexactmatch,notthe=assignmentoperator.

Branchingalternatives

Theelsekeywordisusedinconjunctionwiththeifkeywordtocreateifelsestatementsthatprovidealternativebranchesforaprogramtopursue–accordingtotheevaluationofatestedexpression.Initssimplestform,thismerelynominatesanalternativestatementforexecutionwhenthetestfails:if(test-expression)code-to-be-executed-when-true;

else

code-to-be-executed-when-false;Eachalternativebranchmaybeasinglestatementorastatementblockofmultiplestatements–enclosedwithincurlybrackets.

Morepowerfulifelsestatementscanbeconstructedthatevaluateatestexpressionforeachalternativebranch.Theseemploynestedifstatementsaftereachelsekeywordtospecifyeachfurthertest.Whentheprogramdiscoversanexpressionthatevaluatesastrue,itexecutesthestatementsassociatedwithjustthattestthenexitstheifelsestatementwithoutexploringanyfurtherbranches:

Startanewprogramnamed“Else”containingthestandardmainmethodclassElse

{


}

Else.java

Insidethemainmethod,insertthislinetodeclareandinitializeanintegervariablenamedhrsinthrs=11;

Insertthissimpleconditionaltest,whichexecutesasinglestatementwhenthevalueofthehrsvariableisbelow13if(hrs<13)

{

System.out.println(“Goodmorning:”+hrs);

}

SavetheprogramasElse.javathencompileandruntheprogramtoseethestatementgetexecuted

Noticethatthefirststatementisterminatedwithasemicolon,asusual,beforetheelsekeyword.

Changethevalueassignedtothehrsvariableto15,thenaddthisalternativebranchrightaftertheifstatementelseif(hrs<18)

{

System.out.println(“Goodafternoon:”+hrs);

}

Savethechanges,recompile,andruntheprogramagaintoseejustthealternativestatementgetexecuted

Itissometimesdesirabletoprovideafinalelsebranch,withoutanestedifstatement,tospecifya“default”statementtobeexecutedwhennotestedexpressionevaluatesastrue:

Changethevalueassignedtothehrsvariableto21,thenaddthisdefaultbranchtotheendoftheifelsestatementelseSystem.out.println(“Goodevening:”+hrs);

Savethechanges,recompile,andruntheprogramoncemoretoseejustthedefaultstatementgetexecuted

Conditionalbranchingisthefundamentalprocessbywhichcomputerprogramsproceed.

Switchingbranches

Lengthyifelsestatements,whichoffermanyconditionalbranchesforaprogramtopursue,canbecomeunwieldy.Wherethetestexpressionsrepeatedlyevaluatethesamevariablevalue,amoreelegantsolutionisoftenprovidedbyaswitchstatement.

Thesyntaxofatypicalswitchstatementblocklookslikethis:switch(test-variable){

casevalue1:code-to-be-executed-when-true;break;casevalue2:code-to-be-executed-when-true;break;casevalue3:code-to-be-executed-when-true;break;default:code-to-be-executed-when-false;}

Theswitchstatementworksinanunusualway.Ittakesaspecifiedvariablethenseekstomatchitsassignedvaluefromamonganumberofcaseoptions.Statementsassociatedwiththeoptionwhosevaluematchesarethenexecuted.

Optionally,aswitchstatementcanincludeafinaloptionusingthedefaultkeywordtospecifystatementstoexecutewhennocaseoptionsmatchthevalueassignedtothespecifiedvariable.

Eachoptionbeginswiththecasekeywordandavaluetomatch.Thisisfollowedbya:coloncharacterandthestatements,ifany,tobeexecutedwhenthematchismade.

Itisimportanttorecognizethatthestatement,orstatementblock,associatedwitheachcaseoptionmustbeterminatedbythebreakkeyword.Otherwise,theprogramwillcontinuetoexecutethestatementsofothercaseoptionsafterthematchedoption.Sometimes,thisisdesirabletospecifyanumberofcaseoptionsthatshouldeachexecutethesamestatementsifmatched.Forexample,onestatementforeachblockofthreeoptionslikethis:switch(test-variable){

casevalue1:casevalue2:casevalue3:code-A-to-be-executed-when-true;break;casevalue4:casevalue5:casevalue6:code-B-to-be-executed-when-true;break;}

Missingbreakkeywordsarenotsyntaxerrors–ensurethatallintendedbreaksarepresentinswitchblockstoavoidunexpectedresults.

Startanewprogramnamed“Switch”containingthestandardmainmethodclassSwitch

{


}

Switch.java

Insidethemainmethod,declareandinitializethreeintegervariablesintmonth=2,year=2018,num=31;

Addaswitchstatementblocktotestthevalueassignedtothemonthvariableswitch(month)

{

}

}

Insidetheswitchblock,insertcaseoptionsassigninganewvaluetothenumvariableformonths4,6,9and11case4:case6:case9:case11:num=30;break;

Insertacaseoptionassigninganewvaluetothenumvariableformonth2,accordingtotheyearvaluecase2:num=(year%4==0)?29:28;break;

Aftertheswitchblock,attheendofthemainmethod,addthislinetooutputallthreeintegervaluesSystem.out.println(month+“/”+year+“:“+num+“days”);

SavetheprogramasSwitch.javathencompileandruntheprogramtoseetheoutput

Noticehowallthreeintegervariablesaredeclaredandinitializedinlinehereusingconvenientshorthand.

Theconditionaloperatorisusedtogoodeffectinstep5.Youcancheckbacktoheretoberemindedhowitworks.

Loopingfor

Aloopisablockofcodethatrepeatedlyexecutesthestatementsitcontainsuntil

Aloopisablockofcodethatrepeatedlyexecutesthestatementsitcontainsuntilatestedconditionismet–thentheloopendsandtheprogramproceedsontoitsnexttask.

Themostfrequently-usedloopstructureinJavaprogrammingemploystheforkeywordandhasthissyntax:for(initializer;test-expression;updater){

statements-to-be-executed-on-each-iteration;}

Theparenthesesaftertheforkeywordmustcontainthreecontrolsthatestablishtheperformanceoftheloop:• Initializer–assignsaninitialvaluetoacountervariable,whichwillkeepcountofthenumberofiterationsmadebythisloop.Thevariableforthispurposemaybedeclaredhere,anditistraditionallya“trivial”integervariablenamedi.

• Testexpression–evaluatedatthestartofeachiterationoftheloopforaBooleantruevalue.Whentheevaluationreturnstruetheiterationproceedsbutwhenitreturnsfalsetheloopisimmediatelyterminated,withoutcompletingthatiteration.

• Updater–changesthecurrentvalueofthecountervariable,startedbytheinitializer,keepingtherunningtotalofthenumberofiterationsmadebythisloop.Typically,thiswillusei++forcountingup,ori--forcountingdown.

Thecodeexecutedoneachiterationoftheloopcanbeasinglestatement,astatementblock,orevenanother“nested”loop.

Everyloopmust,atsomepoint,enablethetestexpressiontoreturnfalse–otherwise,aninfiniteloopiscreatedthatwillrelentlesslyexecuteitsstatements.Commonly,thetestexpressionwillevaluatethecurrentvalueofthecountervariabletoperformaspecifiednumberofiterations.Forexample,withacounteriinitializedatoneandincrementedbyoneoneachiteration,atestexpressionofi<11becomesfalseafter10iterations–sothatloopwillexecuteitsstatements10timesbeforetheloopends.

Theupdaterisoftenreferredtoasthe“incrementer”asitmoreoftenincrements,ratherthandecrements,thecountervariable

Startanewprogramnamed“For”containingthestandardmainmethodclassFor

{


}

For.java

Insidethemainmethod,declareandinitializeanintegervariabletocountthetotaloverallnumberofiterationsintnum=0;

Addaforlooptoperformthreeiterationsanddisplaythecurrentvalueofitscountervariableioneachiterationfor(inti=1;i<4;i++)

{

System.out.println(“OuterLoopi=”+i);

}

Insidetheforloopblockinsertanestedforlooptoalsoperformthreeiterations,displayingthecurrentvalueofitscountervariablejandtotaloverallnumberofiterationsfor(intj=1;j<4;j++)

{

System.out.print(“\tInnerLoopj=”+j);

System.out.println(“\t\tTotalnum=”+(++num));

}

SavetheprogramasFor.javathencompileandruntheprogramtoseetheoutput

Theincrement++anddecrement--operatorscanprefixavariable,tochangeitsvalueimmediately,orpostfixthevariable–soitsvaluebecomeschangedwhennextreferenced.Trychangingtheincrementoperatorsinthisexampleto++iand++jtoseethedifference.

Loopingwhiletrue

Analternativeloopstructuretothatoftheforloop,describedhere,employsthewhilekeywordandhasthissyntax:while(test-expression){


Liketheforloop,awhilelooprepeatedlyexecutesthestatementsitcontainsuntilatestedconditionismet–thentheloopendsandtheprogramproceedsontoitsnexttask.

Unliketheforloop,theparenthesesafterthewhilekeyworddonotcontainaninitializerorupdaterforaniterationcountervariable.Thismeansthatthetest

expressionmustevaluatesomevaluethatgetschangedintheloopstatementsastheloopproceeds–otherwise,aninfiniteloopiscreatedthatwillrelentlesslyexecuteitsstatements.

ThetestexpressionisevaluatedatthestartofeachiterationoftheloopforaBooleantruevalue.Whentheevaluationreturnstruetheiterationproceedsbutwhenitreturnsfalsetheloopisimmediatelyterminated,withoutcompletingthatiteration.

Notethatifthetestexpressionreturnsfalsewhenitisfirstevaluated,theloopstatementsareneverexecuted.

Awhileloopcanbemadetoresemblethestructureofaforloop,toevaluateacountervariableinitstestexpression,bycreatingacountervariableoutsidetheloopandchangingitsvaluewithinthestatementsitexecutesoneachiteration.Forexample,theouterforloopinthepreviousexamplecanberecreatedasawhileloop,likethis:inti=1;

while(i<4)

{

System.out.println(“OuterLoopi=”+i);i++;

}

Thispositionsthecounterinitializerexternally,beforethewhileloopstructure,anditsupdaterwithinthestatementblock.

Aninfiniteloopwilllocktheprogramasitcontinuestoperformiterations–onWindows,pressCtrl+Ctohalt.

Startanewprogramnamed“While”containingthestandardmainmethodclassWhile

{


}

While.java

Insidethemainmethod,declareandinitializeanintegervariablenamednumintnum=100;

Addawhilelooptodisplaythenumvariable’scurrentvaluewhileitremainsabovezerowhile(num>0)

{

System.out.println(“WhileCountdown:”+num);

}

Insertanupdaterattheendofthewhileloopblocktodecreasethenumvariable’svalueby10oneachiteration–therebyavoidinganinfiniteloopnum-=10;

SavetheprogramasWhile.javathencompileandruntheprogramtoseetheoutput

Theassignmentinthisupdaterisshorthandfornum=(num-10).

Doingdo-whileloopsAvariationofthewhileloopstructure,describedhere,employsthedokeywordtocreatealoopwiththissyntax:

do

{


while(test-expression);Liketheforloopandwhileloop,adowhilelooprepeatedlyexecutesthestatementsitcontainsuntilatestedconditionismet–thentheloopendsandtheprogramproceedstoitsnexttask.

Unliketheforloopandwhileloop,thedowhiletestexpressionappearsaftertheblockcontainingthestatementstobeexecuted.ThetestexpressionisevaluatedattheendofeachiterationoftheloopforaBooleantruevalue.Whentheevaluationreturnstruethenextiterationproceedsbutwhenitreturnsfalsetheloopisimmediatelyterminated.Thismeansthatthestatementsinadowhilelooparealwaysexecutedatleastonce.

Notethatifthetestexpressionreturnsfalsewhenitisfirstevaluated,theloopstatementshavealreadybeenexecutedonce.

Adowhileloopcanbemadetoresemblethestructureofaforloop,toevaluateacountervariableinitstestexpression,bypositioningthecounterinitializeroutsidetheloopstructureanditsupdaterwithinthestatementblock–justaswithawhileloop.

Allfor,while,ordowhileloopstructurescontainingjustonestatementtoexecutemay,optionally,omitthecurlybracketsaroundthestatement.But,ifomitted,youwillneedtoaddcurlybracketsifadditionalstatementsareaddedtothelooplater.

Thechoiceoffor,while,ordowhileloopislargelyamatterofpersonalcodingpreferenceandpurpose.Aforloopstructureconvenientlylocatesthecounter

initializer,testexpression,andupdaterintheparenthesesaftertheforkeyword.Awhileloopstructurecanbemoreconcise–butyoumustremembertoincludeanupdaterintheloop’sstatementstoavoidaninfiniteloop.Adowhileloopsimplyaddsthebenefitofexecutingitsstatementsoncebeforeevaluatingitstestexpression–demonstratedbythedowhileloopdescribedopposite.

Alwaysenclosethestatementstobeexecutedbyaloopwithincurlybrackets–forclarityandimprovedcodemaintainability.

Startanewprogramnamed“DoWhile”containingthestandardmainmethodclassDoWhile

{


}

DoWhile.java

Insidethemainmethod,declareandinitializeanintegervariablenamednumintnum=100;

Addadowhilelooptodisplaythenumvariable’scurrentvaluewhileitisbelow10

do

{

System.out.println(“DoWhileCountup:”+num);

}

}

while(num<10);

Insertanupdaterattheendofthedowhileloopblocktochangethenumvariable’svalueoneachiteration–therebyavoidinganinfiniteloopnum+=10;

SavetheprogramasDoWhile.javathencompileandruntheprogram–seethatthenumvariablenevermeetsthetestcondition,butthestatementexecutesonceanyway

Theassignmentinthisupdaterisshorthandfornum=(num+10).

Breakingoutofloops

Thebreakkeywordcanbeusedtoprematurelyterminatealoopwhenaspecifiedconditionismet.Thebreakstatementissituatedinsidetheloopstatementblock,andisprecededbyatestexpression.Whenthetestreturnstrue,theloopendsimmediatelyandtheprogramproceedsontoitsnexttask.Forexample,inanestedloopitproceedstothenextiterationofitsouterloop.

Startanewprogramnamed“Break”containingthestandardmainmethodclassBreak

{


}

}

Break.java

Insidethemainmethod,createtwonestedforloopsthatdisplaytheircountervaluesoneachofthreeiterationsfor(inti=1;i<4;i++)

{

for(intj=1;j<4;j++)

{

System.out.println(“Runningi=”+i+“j=”+j);

}

}

SavetheprogramasBreak.javathencompileandruntheprogramtoseetheoutput

Thisprogrammakesthreeiterationsoftheouterloop,whichexecutestheinnerlooponeachiteration.Abreakstatementcanbeaddedtostopthesecondexecutionoftheinnerloop.

Addthisbreakstatementtothebeginningoftheinnerloopstatementblock,tobreakoutoftheinnerloop–thenrecompileandre-runtheprogramif(i==2&&j==1)

{

{

System.out.println(“BreaksinnerLoopwheni=”+i+“j=”+j);

break;

}

Here,thebreakstatementhaltsallthreeiterationsoftheinnerloopwhentheouterlooptriestorunitthesecondtime.

Thecontinuekeywordcanbeusedtoskipasingleiterationofaloopwhenaspecifiedconditionismet.Thecontinuestatementissituatedinsidetheloopstatementblockandisprecededbyatestexpression.Whenthetestreturnstrue,thatiterationends.

Addthiscontinuestatementtothebeginningoftheinnerloopstatementblock,toskipthefirstiterationoftheinnerloop–thenrecompileandre-runtheprogramif(i==1&&j==1)

{

System.out.println(“ContinuesinnerLoopwheni=”+i+“j=”+j);

continue;

}

Here,thecontinuestatementskipsjustthefirstiterationoftheinnerloopwhentheouterlooptriestorunitforthefirsttime.

Returningcontrol

Thedefaultbehaviorofthebreakandcontinuekeywordscanbechangedtoexplicitlyspecifythatcontrolshouldreturntoalabeledouterloopbystatingitslabelname.

Startanewprogramnamed“Label”containingthestandardmainmethodclassLabel

{


}

Label.java

Insidethemainmethod,createtwonestedforloopsthatdisplaytheircountervaluesoneachofthreeiterationsfor(inti=1;i<4;i++)

{

{

for(intj=1;j<4;j++)

{

System.out.println(“Runningi=”+i+“j=”+j);

}

}

SavetheprogramasLabel.javathencompileandruntheprogramtoseetheoutput

Thesyntaxtolabelalooprequiresalabelname,followedbya:coloncharacter,toprecedethestartoftheloopstructure

Editthestartoftheouterlooptolabelit“outerLoop”outerLoop:for(inti=1;i<4;i++)

Toexplicitlyspecifythattheprogramshouldproceedintheouterloop,statethatloop’slabelnameafterthecontinuekeyword

Addthiscontinuestatementtothebeginningoftheinnerloopstatementblock,toproceedatthenextiterationoftheouterloop–thenrecompileandre-runtheprogramif(i==1&&j==1)

{

System.out.println(“ContinuesouterLoopwheni=”+i+“j=”+j);continueouterLoop;

}

}

Herethecontinuestatementhaltsallthreeiterationsoftheinnerloop‘sfirstrun–byreturningcontroltotheouterloop.

Toexplicitlyspecifythattheprogramshouldexitfromtheouterloop,statethatloop’slabelnameafterthebreakkeyword

Addthisbreakstatementtothebeginningoftheinnerloopstatementblock,toexittheouterloop–thenrecompileandre-runtheprogramif(i==2&&j==3)

{

System.out.println(“BreaksouterLoopwheni=”+i+“j=”+j);breakouterLoop;

}

Herethebreakstatementhaltsallfurtheriterationsoftheentireloopstructure–byexitingfromtheouterloop.

Summary

• TheifkeywordperformsaconditionaltesttoevaluateanexpressionforaBooleanvalueoftrueorfalse.

• Anifstatementblockcancontainoneormorestatements,whichareonlyexecutedwhenthetestexpressionreturnstrue.

• Theelsekeywordspecifiesalternativestatementstoexecutewhenthetestperformedbytheifkeywordreturnsfalse.

• Combinedifelsestatementsenableaprogramtoproceedbytheprocessofconditionalbranching.

• Aswitchstatementcanoftenprovideanelegantsolutiontounwieldyifelsestatementsbyofferingcaseoptions.

• Eachcaseoptioncanbeterminatedbythebreakkeywordsoonlystatementsassociatedwiththatoptionwillbeexecuted.

• Thedefaultkeywordcanspecifystatementstobeexecutedwhenallcaseoptionsreturnfalse.

• Alooprepeatedlyexecutesthestatementsitcontainsuntilatestedexpressionreturnsfalse.

• Theparenthesesthatfollowtheforkeywordspecifytheloop’scounterinitializer,testexpression,andcounterupdater.

• Statementsinawhileloopandadowhileloopmustchangeavalueusedintheirtestexpressiontoavoidaninfiniteloop.

• Thetestexpressionisevaluatedatthestartofforloopsandwhileloops–beforethefirstiterationoftheloop.

• Thetestexpressionisevaluatedattheendofdowhileloops–afterthefirst

iterationoftheloop.

• Aloopiterationcanbeskippedusingthecontinuekeyword.

• Aloopcanbeterminatedusingthebreakkeyword.

• Nestedinnerloopscanuselabelswiththebreakandcontinuekeywordstoreferencetheouterloop.

4

Directingvalues

ThischapterdemonstrateshowtodirectdatavaluesusingvariousJavaprogrammingconstructs.

CastingtypevaluesCreatingvariablearrays

PassinganargumentPassingmultipleargumentsLoopingthroughelementsChangingelementvalues

AddingarraydimensionsCatchingexceptionsSummary

Castingtypevalues

HandlingvaluesinJavaprogrammingrequirescorrectdatatypingtobecloselyobservedtoavoidcompilererrors.Forexample,sendingafloattypevaluetoamethodthatrequiresaninttypevaluewillproduceacompilererror.Thismeansitisoftennecessarytoconvertavaluetoanotherdatatypebeforeitcanbeprocessed.

Numericvaluescanbeeasily“cast”(converted)intoanothernumericdatatypeusingthissyntax:

(data-type)valueSomelossofprecisionwilloccurwhencastingfloatfloating

pointvaluesintoanintdatatype,asthenumberwillbetruncatedatthedecimalpoint.Forexample,castingafloatvalueof9.9intoanintvariableproducesanintegervalueofnine.

Interestingly,charactervaluesofthechardatatypecanautomaticallybeusedasintvaluesbecausetheyeachhaveauniqueintegerrepresentation.ThisistheirnumericcodevalueintheASCIIcharacterset,whichissupportedbyJava.TheuppercaseletterA,forinstance,hasthecodevalueof65.

NumericvaluescanbeconvertedtotheStringdatatypeusingthetoString()methodofthatvalue’sdatatypeclass.Thistakesthenumericvalueasitsargument,withintheparentheses.Forexample,convertanintnumvariabletoaStringwithInteger.toString(num).Similarly,convertafloatnumvariabletoaStringwithFloat.toString(num).Inpractice,thistechniqueisnotalwaysrequiredbecauseJavaautomaticallyconvertsconcatenatedvariablestoaStringifanyoneofthevariableshasaStringvalue.

Morefrequently,youwillwanttoconvertaStringvaluetoanumericdatatypesotheprogramcanusethatvaluearithmetically.AStringvaluecanbeconvertedtoanintvalueusingtheInteger.parseInt()method.ThistakestheStringvalueasitsargument,withintheparentheses.Forexample,convertaStringmsgvariabletoanintwithInteger.parseInt(msg).Similarly,convertaStringmsgvariabletoafloatwithFloat.parseFloat(msg).WhenconvertingaStringvaluetoanumericdatatype,theStringmayonlycontainavalidnumericvalue,orthecompilerwillreportanerror.

Allnumericclasseshaveaparse...methodandatoStringmethodallowingconversionbetweenStringvaluesandnumericdatatypes.

Startanewprogramnamed“Convert”containingthestandardmainmethodclassConvert

{


}

}

Convert.java

Insidethemainmethod,declareandinitializeafloatvariableandaStringvariablefloatdaysFloat=365.25f;StringweeksString=“52”;

CastthefloatvalueintoanintvariableintdaysInt=(int)daysFloat;

ConverttheStringvalueintoanintvariableintweeksInt=Integer.parseInt(weeksString);

Performarithmeticontheconvertedvaluesanddisplaytheresultintweek=(daysInt/weeksInt);System.out.println(“Daysperweek:“+week);

SavetheprogramasConvert.javathencompileandruntheprogramtoseetheoutput

Creatingvariablearrays

Anarrayissimplyavariablethatcancontainmultiplevalues–unlikearegularvariablethatcanonlycontainasinglevalue.

Thedeclarationofanarrayfirststatesitsdatatype,usingoneofthedatatypekeywords,followedbysquarebrackets[]todenotethatitwillbeanarray

keywords,followedbysquarebrackets[]todenotethatitwillbeanarrayvariable.Next,thedeclarationstatesthearrayvariablename,adheringtothenormalnamingconventions.

Anarraycanbeinitializedinitsdeclarationbyassigningvaluesoftheappropriatedatatypeasacomma-delimitedlist,enclosedwithincurlybrackets.Forexample,thedeclarationofanintegerarrayvariableinitializedwiththreevaluesmightlooklikethis:

int[]numbersArray={1,2,3};

Thearrayiscreatedofthelengthoftheassignedlist,allowingone“element”pervalue–inthiscase,anarrayofthreeelements.

Storedvaluesareindexedstartingatzero,andeachvaluecanbeaddressedbyitselementindexposition.Thesyntaxtodosorequiresthearraynametobefollowedbysquarebracketscontainingtheelementindex.Forinstance,numbersArray[0]wouldaddressthefirstvaluestoredintheexampleabove(1).

Althoughthevaluesstoredineachelementcanbechangedassimplyasthoseofregularvariables,thesizeofanarrayisdeterminedbyitsdeclarationandcannotbechangedlater.Usefully,thetotalnumberofelementsinanarrayisstoredasanintegerinthelengthpropertyofthatarray.Thesyntaxtoaddressthisfigurejusttacksaperiodand“length”ontothearrayname.Forexample,numbersArray.lengthwouldreturnthesizeofthearrayintheexampleabove–inthiscase,theinteger3.

Arrayscanalsobedeclaredwithoutassigningalistofinitialvaluesbyusingthenewkeywordtocreateanemptyarray“object”ofaspecifiedsize.Thenumberofrequiredemptyelementsisstatedintheassignmentwithinsquarebracketsaftertheappropriatedatatype.Forexample,thedeclarationofanemptyintegerarrayvariablewiththreeelementsmightlooklikethis:int[]numbersArray=newint[3];

Theelementsareassigneddefaultvaluesofzeroforintandfloatdatatypes,nullforStringdatatypes,\0forchardatatypes,andfalseforbooleandatatypes.

Rememberthatarrayindexingstartsatzero.Thismeansthatindex[2]addressesthethirdelementinthearray,notitssecondelement.

Startanewprogramnamed“Array”containingthestandardmainmethodclassArray

{


}

Array.java

Insidethemainmethod,declareandinitializeaStringarraywiththreeelementsString[]str={“Much”,“More”,“Java”};

Declareanemptyintegerarraywiththreeelementsint[]num=newint[3];

Assignvaluestothefirsttwointegerarrayelementsnum[0]=100;

num[1]=200;

AssignanewvaluetothesecondStringarrayelementstr[1]=“Better”;

OutputthelengthofeacharrayandthecontentofallelementsineacharraySystem.out.println(“Stringarraylengthis“+str.length);System.out.println(“Integerarraylengthis“+num.length);System.out.println(num[0]+”,”+num[1]+”,”+num[2]);System.out.println(str[0]+str[1]+str[2]);

SavetheprogramasArray.javathencompileandruntheprogramtoseetheoutput

Stringvaluesneedtobeenclosedwithinquotes.

Passinganargument

ThestandardJavacodethatdeclarestheprogram’smainmethodincludesanargumentwithinitsparenthesesthatcreatesaStringarray,traditionallynamed“args”:publicstaticvoidmain(String[]args){}

Thepurposeoftheargs[]arrayistoallowvaluestobepassedtotheprogramwhenitiscalledupontorun.Atthecommandline,avaluetobepassedtotheprogramisaddedafterasinglespacefollowingtheprogramname.Forexample,thecommandtopasstheString“Java”toaprogramnamed“Run”wouldbeRunJava.

Asinglevaluepassedtoaprogramisautomaticallyplacedintothefirstelementoftheargs[]array,soitcanbeaddressedbytheprogramasargs[0].

Itisimportanttorecognizethattheargs[]arrayisoftheStringdatatype–soanumericvaluepassedtoaprogramwillbestoredasaStringrepresentationofthatnumber.Thismeansthattheprogramcannotusethatvaluearithmeticallyuntilithasbeenconvertedtoanumericaldatatype,suchasanintvalue.Forexample,Run4passesthenumberfourtotheprogram,whichstoresitastheString“4”,notastheint4.Consequently,outputofargs[0]+3producestheconcatenatedString“43”,notthesum7.TheargumentcanbeconvertedwiththeInteger.parseInt()methodsothatInteger.parseInt(args[0])+3doesproducethesum7.

AStringcontainingspacescanbepassedtoaprogramasasingleStringvaluebyenclosingtheentireStringwithindoublequotesonthecommandline.Forexample,Run“JavaInEasySteps”.

Passinganargumenttoaprogramismostusefultodeterminehowtheprogramshouldrunbyindicatinganexecutionoption.TheoptionispassedtotheprogramasaStringvalueinargs[0]andcanbeevaluatedusingtheString.equals()method.Thesyntaxforthisjusttacksaperiodand“equals()”ontothearrayname,withacomparisonStringwithintheparentheses.Forexample,args[0].equals(“b”)evaluatestheargumentfortheStringvalue“b”.

Startanewprogramnamed“Option”containingthestandardmainmethodclassOption

{


}

Option.java

Insidethemainmethod,writeanifstatementtoseekanargumentof“-en”if(args[0].equals(“-en”))

{

System.out.println(“Englishoption”);

}

Addanelsealternativeontotheifstatementtoseekanargumentof“-es”elseif(args[0].equals(“-es”))

{

System.out.println(“Spanishoption”);

}

}

AddanotherelsealternativeontotheifstatementtoprovideadefaultresponseelseSystem.out.println(“Unrecognizedoption”);

SavetheprogramasOption.javathencompileandruntheprogramtoseetheoutput

ThisexamplewillthrowanArrayIndexOutOfBoundsexceptionifyouattempttoexecutetheprogramwithoutanyargument.Seeherefordetailsonhowtocatchexceptions.

Passingmultiplearguments

Multipleargumentscanbepassedtoaprogramatthecommandline,followingtheprogramnameandaspace.Theargumentsmustbeseparatedbyatleastonespaceandtheirvaluesareplaced,inorder,intotheelementsoftheargs[]array.Eachvaluecanthenbeaddressedbyitsindexnumberaswithanyotherarray–args[0]forthefirstargument,args[1]forthesecondargument,andsoon.

Theprogramcantestthelengthpropertyoftheargs[]arraytoensuretheuserhasenteredtheappropriatenumberofarguments.Whenthetestfails,thereturnkeywordcanbeusedtoexitthemainmethod,therebyexitingtheprogram:

Startanewprogramnamed“Args”containingthestandardmainmethodclassArgs

{


}

Args.java

Insidethemainmethod,writeanifstatementtooutputadviceandexittheprogramwhentherearenottherequirednumberofarguments–inthiscase,threeif(args.length!=3)

{

System.out.println(“Wrongnumberofarguments”);return;

}

Belowtheifstatement,createtwointvariables–initializedwiththevaluesofthefirstargumentandthirdargumentrespectivelyintnum1=Integer.parseInt(args[0]);intnum2=Integer.parseInt(args[2]);

AddaStringvariable,initializedwithaconcatenationofallthreeargumentsStringmsg=args[0]+args[1]+args[2]+“=”;

Thereturnkeywordexitsthecurrentmethod.Itcanalsoreturnavaluetothepointwherethemethodwascalled.Seehereformoredetails.

AddthisifelsestatementtoperformarithmeticontheargumentsandappendtheresulttotheStringvariableif(args[1].equals(“+”))msg+=(num1+num2);elseif(args[1].equals(“-”))msg+=(num1-num2);elseif(args[1].equals(“x”))msg+=(num1*num2);elseif(args[1].equals(“/”))msg+=(num1/num2);elsemsg=“Incorrectoperator”;

InsertthislineattheendofthemainmethodtodisplaytheappendedStringSystem.out.println(msg);

SavetheprogramasArgs.javathencompileandruntheprogramwiththreearguments–aninteger,anyarithmeticalsymbol+-x/,andanotherinteger

Now,runtheprogramwithanincorrectsecondargumentandwiththewrongnumberofarguments

Thisprogramwillreportanerrorifnon-numericvaluesareentered.Seeherefordetailsonhowtocatcherrors.

Loopingthroughelements

Alltypesofloopcanbeusedtoeasilyreadallthevaluesstoredinsidetheelementsofanarray.Theloopcountershouldstartwiththeindexnumberofthefirstelementthenproceedonuptothefinalindexnumber.Theindexnumberofthelastelementinanarraywillalwaysbeonelessthanthearraylength–becausetheindexstartsatzero.

Itisusefultosetthearraylengthpropertyastheloop’sconditionaltestdeterminingwhentheloopshouldend.Thismeansthattheloopwillcontinueuntilthecountervalueexceedstheindexnumberofthearray’sfinalelement.

Startanewprogramnamed“Loops”containingthestandardmainmethodclassLoops

{


}

Loops.java

Insidethemainmethod,writeanifstatementtotestwhetheranyargumentvalueshavebeenenteredintotheargs[]arrayfromthecommandlineif(args.length>0){}

Insertaforloopinsidethecurlybracketsoftheifstatementtooutputthevaluestoredineachelementfor(inti=0;i<args.length;i++)

{

System.out.println(“args[“+i+“]is|“+args[i]);

}

SavetheprogramasLoops.javathencompiletheprogramandrunitwiththeargumentsJavaineasysteps

EditLoops.javatoaddaStringarrayandawhilelooptooutputthevaluestoredineachelementString[]htm={“HTML5”,“in”,“easy”,“steps”};

intj=0;while(j<htm.length)

{

System.out.println(“htm[“+j+“]is|“+htm[j]);j++;

}

Savethechanges,thenrecompileandre-runtheprogram

EditLoops.javatoaddanotherStringarrayandadowhilelooptooutputthevaluestoredineachelementString[]xml={“XML”,“in”,“easy”,“steps”};

intk=0;if(xml.length>0)do

{

System.out.println(“\t\txml[“+k+“]is|“+xml[k]);k++;

}while(k<xml.length);

Savethechanges,thenrecompileandre-runtheprogram

Noticethatthedostatementisprecededbyaconditionaltesttoensurethearrayisnotemptybeforeattemptingtooutputthevalueofthefirstelement.

Changingelementvalues

Thevaluestoredinanarrayelementcanbechangedbyassigninganewvaluetothatparticularelementusingitsindexnumber.Additionally,anytypeofloopcanbeusedtoefficientlypopulatealltheelementsinanarrayfromvaluesstoredinotherarrays.Thisisespeciallyusefultocombinedatafrommultiplearraysintoasinglearrayoftotaleddata.

Startanewprogramnamed“Elements”containingthestandardmainmethodclassElements

{


}

Elements.java

Inthemainmethod,addinitializedintarraysrepresentingmonthlykiosksalesfromallfourquartersofayearint[]kiosk_q1={42000,48000,50000};int[]kiosk_q2={52000,58000,60000};int[]kiosk_q3={46000,49000,58000};int[]kiosk_q4={50000,51000,61000};

Addinitializedintarraysrepresentingmonthlyoutletsalesfromallfourquartersofayearint[]outlet_q1={57000,63000,60000};int[]outlet_q2={70000,67000,73000};int[]outlet_q3={67000,65000,62000};int[]outlet_q4={72000,69000,75000};

Now,createanemptyintarrayof12elementsinwhichtocombineallthemonthlysalesfiguresandanintvariableinwhichtorecordtheirgrandtotalvalueint[]sum=newint[12];inttotal=0;

Addaforlooptopopulateeachelementoftheemptyarraywithcombinedvalues

fromtheotherarraysfor(inti=0;i<kiosk_q1.length;i++)

{

sum[i]=kiosk_q1[i]+outlet_q1[i];sum[i+3]=kiosk_q2[i]+outlet_q2[i];sum[i+6]=kiosk_q3[i]+outlet_q3[i];sum[i+9]=kiosk_q4[i]+outlet_q4[i];

}

Next,addasecondforlooptooutputeachofthecombinedmonthlysalestotals,andtocalculatetheirgrandtotalfor(inti=0;i<sum.length;i++)

{

System.out.println(“Month“+(i+1)+”sales:\t”+sum[i]);total+=sum[i];

}

InsertafinalstatementattheendofthemainmethodtooutputthegrandtotalSystem.out.println(“TOTALYEARSALES\t”+total);

SavetheprogramasElements.javathencompiletheprogramandrunittoseetheoutput

Thecounternumbergetsincreasedbyonetoproducethemonthnumbers1-12.

Addingarraydimensions

Arrayscanbecreatedtostoremultiplesetsofelementvalues,eachhavingtheirownindexdimension.Individualvaluesareaddressedinamulti-dimensionalarrayusingtheappropriateindexnumbersofeachdimension.Forexample,num[1][3].

Atwo-dimensionalarraymightbeusedtorecordanintegervalueforeachdayofabusinessyear,organizedbyweek.Thisrequiresanarrayof52elements(oneperweek)thateachhaveanarrayofsevenelements(oneperday).Itsdeclarationlookslikethis:

int[][]dailyRecord=newint[52][7];

Avoidusingmorethanthreedimensionsinarrays–itwillbeconfusing.

This“arrayofarrays”providesanelementforeachbusinessday.Valuesareassignedtoamulti-dimensionalarraybystatingtheappropriateindexnumbersofeachdimension.Withtheexampleabove,forinstance,avaluecanbeassignedtothefirstdayofthesixthweeklikethis:dailyRecord[5][0]=5000;

Eacharrayhasitsownlengthpropertythatcanbeaccessedbyspecifyingthedimensionrequired.Fortheexampleabove,thesyntaxdailyRecord.lengthreturnsavalue52–thesizeofthefirstdimension.Toaccessthesizeoftheseconddimension,thesyntaxdailyRecord[0].lengthreturnsthevalueofseven.

Two-dimensionalarraysareoftenusedtostoregridcoordinates,whereonedimensionrepresentstheXaxisandtheotherdimensionrepresentstheYaxis.Forexample,point[3][5].

Three-dimensionalarrayscanbeusedtostoreXYZcoordinatesinasimilarway,butitcanbedifficulttovisualizepoint[4][8][2].

Nestedloopsareperfectlysuitedtomulti-dimensionalarrays,aseachlooplevelcanaddresstheelementsofeacharraydimension.

Startanewprogramnamed“Dimensions”containingthestandardmainmethodclassDimensions

{


}

Dimensions.java

Inthemainmethod,createatwo-dimensionalarraytostoreBooleanflatsrelatingtoXYcoordinatesboolean[][]points=newboolean[5][20];

DefineoneYpointoneachXaxispoints[0][5]=true;points[1][6]=true;points[2][7]=true;points[3][8]=true;points[4][9]=true;

Addaforlooptoiteratethroughthefirstarrayindex,addinganewlinecharacterattheendofeachiterationfor(inti=0;i<points.length;i++)

{

System.out.print(“\n”);

}

Withinthecurlybracketsoftheforloop,insertasecondforlooptoiteratethroughthesecondarrayindexfor(intj=0;j<points[0].length;j++){}

Withinthecurlybracketsofthesecondforloop,insertastatementtooutputacharacterforeachelementaccordingtothatelement’sBooleanvaluecharmark=(points[i][j])?‘X’:‘-’;System.out.print(mark);

SavetheprogramasDimensions.javathencompileandruntheprogramtoseetheoutput

Booleanvariablesarefalsebydefault.

Catchingexceptions

Aprogrammayencounteraruntimeproblemthatcausesan“exception”error,

Aprogrammayencounteraruntimeproblemthatcausesan“exception”error,whichhaltsitsexecution.Often,thiswillbecreatedbyunexpecteduserinput.Awell-writtenprogramshould,therefore,attempttoanticipateallpossiblewaystheusermightcauseexceptionsatruntime.

Codewhereexceptionsmightarisecanbeidentifiedandenclosedwithinatrycatchstatementblock.Thisallowstheprogramtohandleexceptionswithouthaltingexecutionandlookslikethis:

try

{

statementswhereanexceptionmayarise}catch(Exceptione)

{

statementsrespondingtoanexception}

Theparenthesesfollowingthecatchkeywordspecifytheclassofexceptiontobecaughtandassignittothevariable“e”.Thetop-levelExceptionclasscatchesallexceptions.Responsescanbeprovidedforspecificexceptions,however,usingmultiplecatchstatementstoidentifydifferentlower-levelexceptionclasses.

ThemostcommonexceptionsaretheNumberFormatException,whichariseswhentheprogramencountersavaluethatisnotoftheexpectednumerictype,andtheArrayIndexOutOfBoundsException,whichariseswhentheprogramattemptstoaddressanarrayelementnumberthatisoutsidetheindexsize.Itishelpfultocreateaseparateresponseforeachoftheseexceptionstoreadilynotifytheuseraboutthenatureoftheproblem.

Optionally,atrycatchstatementblockcanbeextendedwithafinallystatementblock,containingcodethatwillalwaysbeexecuted–irrespectiveofwhethertheprogramhasencounteredexceptions.

Thee.getMessage()methodreturnsfurtherinformationaboutsomecapturedexceptions.

Startanewprogramnamed“Exceptions”containingthestandardmainmethodclassExceptions

{


}

Exceptions.java

Insidethemainmethod,writeatrystatementtooutputasingleintegerargument

try

{

intnum=Integer.parseInt(args[0]);System.out.println(“Youentered:“+num);

}

Addacatchstatementtohandletheexceptionthatariseswhentheprogramisrunwithoutanargumentcatch(ArrayIndexOutOfBoundsExceptione){System.out.println(“Integerargumentrequired.”);}

Addacatchstatementtohandletheexceptionthatariseswhentheprogramisrunwithanon-integerargumentcatch(NumberFormatExceptione){System.out.println(“Argumentiswrongformat.”);}

Addafinallystatementattheendoftheprogramfinally{System.out.println(“Programends.”);}

SavetheprogramasExceptions.javathencompileandruntheprogram,tryingtocauseexceptions

Summary

• Numericvaluescanbeconvertedtoothernumericdatatypesbycasting,andtotheStringtypeusingthetoString()method.

• AStringvaluecanbeconvertedtoanintvalueusingtheInteger.parseInt()method,andtoafloatusingFloat.parseFloat().

• Anarrayisavariablethatcancontainmultiplevalues,initializedasalistwithincurlybracketsinitsdeclaration.

• Anemptyarrayobjectcanbecreatedusingthenewkeyword.

• Thelengthpropertyofanarraystoresaninteger,whichisthenumberofelementsinthatarray.

• Eachelementofanarraycanbeaddressedbyitsindexnumber.

• Aprogram’smainmethodcreatesaStringarray,traditionallynamed“args”,tostorecommandlinearguments.

• Thefirstcommandlineargumentgetsautomaticallystoredintheargs[0]element–asaStringdatatype.

• Multipleargumentsbeingpassedtoaprogramfromthecommandlinemusteachbeseparatedbyaspace.

• Loopsareanidealwaytoreadallthevaluesstoredwithinarrayelements.

• Datafrommultiplearrayscanbecombinedtoformanewarrayoftotaleddataineachelement.

• Multi-dimensionalarrayscanstoremultiplesetsofelementvalues,eachhavingtheirownindexdimension.

• Atrycatchstatementblockisusedtoanticipateandhandleruntimeexceptionsthatmayarise.

• TheExceptionclasscatchesallexceptionerrors,includingNumberFormatExceptionandArrayIndexOutOfBoundsException.

• Atrycatchstatementcanbeextendedwithafinallystatementblock,containingcodethatwillalwaysbeexecuted.

5

Manipulatingdata

ThischapterdemonstrateshowtomanipulateprogramdatausingvariousJavalibrarymethods.

ExploringJavaclassesDoingmathematics

RoundingnumbersGeneratingrandomnumbersManagingstringsComparingstrings

SearchingstringsManipulatingcharactersSummary

ExploringJavaclasses

Javahasavastlibraryofpre-testedcodepackages,whicharearrangedinmodules.ThoseprovidingfunctionalitythatisfundamentaltotheJavalanguageitselfarecontainedinthejava.langpackage,withinthejava.basemodule.TheseareautomaticallyaccessibletotheJavaAPI(ApplicationProgrammingInterface).Thismeansthatthepropertiesandmethodsprovidedbythejava.langpackagearereadilyavailablewhencreatingprograms.Forexample,themathematicfunctionalityprovidedbytheabs()methodoftheMathclass,whichispartofthejava.langpackage,inthejava.basemodule.

ModulesareanewfeatureintroducedinJava9toimprovescalabilityandincreaseperformance.

Packagecontentsarearrangedinhierarchicalorder,allowinganyitemtobeaddressedusingdotnotation.Forexample,theSystemclasscontainsanoutproperty(field),whichinturncontainsaprintln()method–socanbeaddressedasSystem.out.println().

TheJavadocumentationprovidesinformationabouteveryitemavailable,andcanbeusedtoexploretheJavaclasses.Itisavailableonlineatdocs.oracle.com/javase/9/docs/apiorcanbedownloadedforofflinereference.Thedocumentationisunderstandablylarge,butfamiliaritywithitisvaluable.AgoodstartingpointistheAPIOverviewpagecontainingalistofeverymoduleineachofthreesections,togetherwithabriefdescriptionofeachmodule:

StartawebbrowserandopentheAPIOverviewpageatdocs.oracle.com/javase/9/docs/api

SeetheModuleslistedalphabeticallyineachsection–scrolldownthepagetothe“JavaSE”sectionandfindthejava.basemodule,thenclickitshyperlink

YoucanclickontheFrameslinktoseeamulti-paneviewofthedocumentation.

https://docs.oracle.com/javase/9/docs/api/overview-summary.html

https://docs.oracle.com/javase/9/docs/api/overview-summary.html

Seethemodule’sPackageslistedalphabeticallyineachsection–scrolldownthepagetothe“Exports”sectionandfindthejava.langpackage,thenclickitshyperlink

Seethepackage’sClasseslistedalphabeticallyineachsection–scrolldownthepagetothe“ClassSummary”sectiontofindtheMathclass,thenclickitshyperlink

Seetheclass’sMethodslistedalphabeticallyinthe“MethodSummary”section–clickonanyhyperlinktodiscoverthepurposeofthatmethodanditssyntax

YoucanalsousetheSearchboxtofindinformationonanyitem.

Examinetheinformationavailableviaotheritemsonthepagemenutobecomemorefamiliarwiththedocumentation.

Doingmathematics

TheMathclasswithinthejava.langpackageprovidestwoconstantvaluesthatareoftenusefultoperformmathematicalcalculations.Math.PIstoresthevalueofPi,andMath.Estoresthevaluethatisthebaseofnaturallogarithms.Boththeseconstantvaluesarestoredasdoubleprecisiondatatypeswith15decimalplaces.

Startanewprogramnamed“Pi”containingthestandardmainmethodclassPi

{


}

Pi.java

Insidethemainmethod,declareandinitializeafloatvariablefromacommandlineargument,andcastthedoubleMath.PIconstantintoasecondfloatvariablefloatradius=Float.parseFloat(args[0]);floatshortPi=(float)Math.PI;

Performmathematicalcalculationsusingthecastvalue,assigningtheresultstomorefloatvariablesfloatcirc=shortPi*(radius+radius);floatarea=shortPi*(radius*radius);

OutputthevalueofMath.PIanditscastfloatequivalent,followedbytheresultsofthecalculationsSystem.out.print(“WithPicommutedfrom“+Math.PI);System.out.println(“to“+shortPi+“...”);System.out.println(“Acircleofradius“+radius+“cm”);System.out.printIn(“hasacircumferenceof“+circ+“cm”);System.out.println(“andanareaof“+area+“sq.cm”);

SavetheprogramasPi.javathencompileandruntheprogramtoseetheoutput

ThecommutedvalueofPiusuallyprovidessufficientprecision.

TheMathclasswithinthejava.langpackageprovidesmanymethodsthatareusefultoperformmathematicalcalculations.UsingMath.pow(),agivennumbercanberaisedtoaspecifiedpower.Theparenthesesrequirethenumberasitsfirstargumentandthepowerbywhichitistoberaisedasitssecondargument.TheMath.sqrt()methodreturnsthesquarerootofthenumberspecifiedasitssoleargument.Bothmethodsreturnadoubletype.

Startanewprogramnamed“Power”containingthestandardmainmethodclassPower

{


}

Power.java

Insidethemainmethod,declareandinitializeanintvariablefromapassedcommandlineargumentintnum=Integer.parseInt(args[0]);

Performmathematicalcalculations,castingtheresultsintomoreintvariablesintsquare=(int)Math.pow(num,2);intcube=(int)Math.pow(num,3);intsqrt=(int)Math.sqrt(num);

OutputtheresultsofthecalculationsSystem.out.println(num+”squaredis“+square);System.out.println(num+”cubedis“+cube);System.out.println(“Squarerootof“+num+”is“+sqrt);

SavetheprogramasPower.javathencompileandruntheprogramtoseetheoutput

Boththeseexamplescouldbeimprovedbyaddingtrycatchstatement

blockstoanticipateusererrors–seeherefordetails.

Roundingnumbers

TheMathclasswithinthejava.langpackageprovidesthreemethodstoroundfloating-pointnumberstothenearestinteger.SimplestoftheseistheMath.round()methodthatroundsanumberstatedasitsargumentup,ordown,totheclosestinteger.

TheMath.floor()methodroundsdowntotheclosestintegerbelow,andMath.ceil()roundsuptotheclosestintegerabove.

WhiletheMath.round()methodreturnsanintdatatype,bothMath.floor()andMath.ceil()methodsreturnadoubledatatype.

Startanewprogramnamed“Round”containingthestandardmainmethodclassRound

{


}

Round.java

Insidethemainmethod,declareandinitializeafloatvariablefloatnum=7.25f;

OutputtheroundedfloatvalueasanintvalueSystem.out.println(num+”roundedis“+Math.round(num));

OutputtheroundedfloatvalueasdoublevaluesSystem.out.println(num+”flooredis“+Math.floor(num));System.out.println(num+“ceilingis“+Math.ceil(num));

SavetheprogramasRound.javathencompileandruntheprogramtoseetheoutput

Bydefault,Math.round()willroundup–so7.5wouldberoundedupto8.

TheMathclasswithinthejava.langpackageprovidestwomethodstocomparetwonumericalvalues.TheMath.max()methodandtheMath.min()methodeachrequiretwonumberstobestatedastheirarguments.Math.max()willreturnthegreaternumberandMath.min()willreturnthesmallernumber.

Thenumberstobecomparedcanbeofanynumericaldatatype,buttheresultwillbereturnedasadoubledatatype.

Startanewprogramnamed“Compare”containingthestandardmainmethodclassCompare

{


}

Compare.java

Insidethemainmethod,declareandinitializeafloatvariableandanintvariable

floatnum1=24.75f;intnum2=25;

OutputthegreatervalueSystem.out.println(“Mostis“+Math.max(num1,num2));

OutputthelesservalueSystem.out.println(“Leastis“+Math.min(num1,num2));

SavetheprogramasCompare.javathencompileandruntheprogramtoseetheoutput

Generatingrandomnumbers

TheMathclasswithinthejava.langpackageprovidestheabilitytogeneraterandomnumberswithitsMath.random()method,whichreturnsadoubleprecisionrandomnumberbetween0.0and0.999.Multiplyingtherandomnumberwillspecifyawiderrange.Forexample,multiplyingby10willcreatearandomnumberintherangeof0.0to9.999.NowroundingtherandomnumberupwithMath.ceil()willensureitfallswithintherangeof1-10inclusive.

Startanewprogramnamed“Random”containingthestandardmainmethodclassRandom

{


}

Random.java

Insidethemainmethod,assignarandomnumbertoafloatvariable,andoutputitsvaluefloatrandom=(float)Math.random();System.out.println(“Randomnumber:“+random);

Assignamultiplicationoftherandomnumbertoasecondfloatvariable,andoutputitsvaluefloatmultiplied=random*10;System.out.println(“Multipliednumber:“+multiplied);

Assignaroundedintegerofthemultipliedrandomnumbertoanintvariable,andoutputitsvalueintrandomInt=(int)Math.ceil(multiplied);System.out.println(“RandomInteger:“+randomInt);

SavetheprogramasRandom.javathencompileandruntheprogramtoseetheoutput

TheLotteryprogramdescribedoppositecombinesallthreestepsfromthisexampleintoasinglestatement.

Asequenceofsixnon-repeatingrandomnumberswithintherange1-59inclusivecanbegeneratedusingMath.random()toproducearandomlotteryselection.

Startanewprogramnamed“Lottery”containingthestandardmainmethodclassLottery

{


}

Lottery.java

Insidethemainmethod,createanintarrayof60elements,thenfillelements1-59withintegers1-59int[]nums=newint[60];for(inti=1;i<60;i++){nums[i]=i;}

Shufflethevaluesinelements1-59for(inti=1;i<60;i++)

{

intr=(int)Math.ceil(Math.random()*59);inttemp=nums[i];nums[i]=nums[r];nums[r]=temp;}

Outputonlythosevaluescontainedinelements1-6for(inti=1;i<7;i++)

{

System.out.print(Integer.toString(nums[i])+““);

}

SavetheprogramasLottery.javathencompileitandruntheprogramthreetimestoseethreedifferentsequences

ThisprogramisrevisitedwithagraphicaluserinterfaceinChapter10.

Managingstrings

InJavaprogramming,aStringiszeroormorecharactersenclosedwithinquotationmarks.So,theseareallvalidStringvalues:Stringtxt1=“MyFirstString”;

Stringtxt2=“”;

Stringtxt3=“2”;

Stringtxt4=“null”;

Array.lengthisapropertybutString.length()isamethod–soitmusthavetrailingparentheses.

Theemptyquotesoftxt2initializethevariableasanemptyStringvalue.Thenumericvalueassignedtotxt3isaStringrepresentationofthenumber.TheJavanullkeyword,whichnormallyrepresentstheabsenceofanyvalue,issimplyaStringliteralwhenitisenclosedwithinquotes.

Essentially,aStringisacollectionofcharacters;eachcharactercontainingitsowndata–justlikeelementsinadefinedarray.Itis,therefore,logicaltoregardaStringasanarrayofcharactersandapplyarraycharacteristicswhendealingwithStringvalues.

TheStringclassispartofthefundamentaljava.langpackageandprovidesalength()methodthatwillreturnthesizeofaString,muchlikethelengthpropertyofanarray.EachStringvariableiscreatedasan“instance”oftheStringclasssoitsmethodscanbeusedbytackingtheirnameontothevariablenameusingdotnotation.Forexample,thesyntaxtoreturnthesizeofaStringvariablenamedtxtistxt.length().

TheStringclasswithinthejava.langpackagealsoprovidesanalternativetothe+concatenationoperatorforjoiningStringvaluestogether.Itsconcat()methodrequiresasingleargumentspecifyingthesecondStringtobeappended.InuseitistackedontothevariablenameofthefirstStringusingdotnotation.Forexample,appendtxt2ontotxt1usingtxt1.concat(txt2).

Startanewprogramnamed“StringLength”containingthestandardmainmethodclassStringLength

{


}

StringLength.java

Insidethemainmethod,createandinitializetwoStringvariablesStringlang=“Java”;Stringseries=“ineasysteps”;

AddanotherStringvariableandassignittheconcatenatedvalueoftheothertwoStringvariablesStringtitle=lang.concat(series);

OutputtheconcatenatedStringwithinquotationmarks,togetherwithitssizeSystem.out.print(“\””+title+“\”has“);

System.out.println(title.length()+“characters”);

SavetheprogramasStringLength.javathencompileandruntheprogramtoseetheoutput

SpacesarepartoftheStringsoareincludedinthecharactercount–butthequotationmarksarenotincluded.

Comparingstrings

TheStringclasswithinthejava.langpackageprovidestheusefulequals()methodthatwasintroducedheretoevaluateacommandlineargumentintheargs[0]element.ThiscanalsobeusedtocompareanytwoStringvaluesbytackingthemethodnameontothefirstStringvariableusingdotnotation,andspecifyingtheStringtobecomparedasitsargument.Forexample,thesyntaxtocomparetxt2totxt1istxt1.equals(txt2).WhenbothStringvalueshaveidenticalcharacters,inthesameorder,themethodreturnstrue–otherwise,itreturnsfalse.

Besuretoobservecorrectcapitalizationusingacapital“C”inthe

toUpperCaseandtoLowerCasemethods.

Stringvaluesthatusedifferentlettercase,suchas“Java”and“JAVA”,arenotconsideredequalbecausetheASCIIcodevaluesofthecharactersdiffer.Forinstance,thevalueofuppercase“A”is65,whereaslowercase“a”is97.

TocompareaninputStringvalue,wherethelettercaseenteredbytheuserisuncertain,againstaStringvalueintheprogramitisoftenusefultotransformtheinputintoaparticularcase.Forthispurpose,theStringclassprovidesatoUpperCase()methodandatoLowerCase()method.TheinputStringisspecifiedastheargument,andthemethodreturnsthetransformedString.

Atypicalexamplemightforceauser-inputpasswordStringtolowercasebeforecomparingittothecorrectpasswordstoredinalllowercaseinaStringvariablewithintheprogram.Thiswouldallowtheusertoentertheirpasswordinuppercase,lowercase,oramixtureofbothcaseswherecase-insensitivepasswordsarepermissible.

Dotnotationallowsmethodstobetackedontoothermethodssotheiroperationscanbeperformedinsequence.ThismeansthattoLowerCase().equals()canbeusedtotransformaStringvaluetolowercaseandthencomparethatlowercaseversionagainstaspecifiedargument.

Startanewprogramnamed“StringComparison”containingthestandardmainmethodclassStringComparison

{


}

StringComparison.java

Insidethemainmethod,createandinitializeaStringvariablewithacorrectlowercasepasswordStringpassword=“bingo”;

Addatrycatchstatementtocatchtheexceptionthatoccurswhennopassword

argumentisenteredtry{}catch(Exceptione)

{

System.out.println(“Passwordrequired.”);

}

Insertthisifelsestatementintothetrystatementblocktoevaluatethepasswordargumententeredbytheuserif(args[0].toLowerCase().equals(password))

{

System.out.println(“Passwordaccepted.”);

}

else

{

System.out.println(“Incorrectpassword.”);

}

SavetheprogramasStringComparison.javathencompileandruntheprogramwithvariousarguments

Searchingstrings

TheStringclasswithinthejava.langpackageprovidesstartsWith()andendsWith()methodstocompareportionsofaStringvalue.TheseareespeciallyusefultocompareanumberofStringvaluesandselectthosewithcommonbeginningsorcommonendings.WhentheStringsectionmatchesthespecifiedargument,themethodreturnstrue–otherwise,itreturnsfalse.

AportionofaStringvaluecanbecopiedbystatingthepositionnumberofthefirstcharactertobecopiedastheargumenttoitssubstring()method.ThiswillreturnasubstringoftheoriginalStringvalue,startingatthespecifiedstartpositionandendingattheendoftheoriginalString.

Optionally,thesubstring()methodcantakeasecondargumenttospecifythepositionnumberofthefinalcharactertobecopied.ThiswillreturnasubstringoftheoriginalStringvalue,startingatthespecifiedstartpositionandendingatthespecifiedendposition.

AStringvaluecanbesearchedtofindacharacterorsubstringspecifiedastheargumenttoitsindexOf()method.Unusually,thismethodreturnsthenumericpositionofthefirstoccurrenceofthematchedcharacterorsubstringwithinthesearchedStringvalue.Wherenomatchisfound,themethodreturnsthenegativeintegervalueof-1.

Startanewprogramnamed“StringSearch”containingthestandardmainmethodclassStringSearch

{


}

StringSearch.java

Insidethemainmethod,createaninitializedStringarrayofbooktitlesString[]books={“Javaineasysteps”,“XMLineasysteps”,“SQLineasysteps”,”CSSineasysteps”,“GoneWiththeWind”,“DroptheDefense”};

Createandinitializethreeintcountervariablesintcounter1=0,counter2=0,counter3=0;

AddaforlooptoiteratethroughtheStringarray,listingasoutputthefirstfourcharactersofeachtitlefor(inti=0;i<books.length;i++)

{

System.out.print(books[i].substring(0,4)+“|“);

}

Insertastatementintheforloopblocktocountthetitlesfoundwithaspecifiedendingif(books[i].endsWith(“ineasysteps”))counter1++;

Insertastatementintheforloopblocktocountthetitlesfoundwithaspecifiedbeginningif(books[i].startsWith(“Java”))counter2++;

Insertastatementintheforloopblocktocountthetitlesfoundnotcontainingaspecifiedsubstringif(books[i].indexOf(“easy”)==-1)counter3++;

Attheendofthemainmethod,addthesestatementstooutputtheresultsofeachsearchSystem.out.println(“\nFound“+counter1+“titlesfromthisseries”);System.out.println(“Found“+counter2+“Javatitle”);System.out.println(“Found“+counter3+“othertitles”);

SavetheprogramasStringSearch.javathencompileandruntheprogramtoseetheoutput

The!NOToperatorcannotbeusedtotestiftheindexOf()methodhasfailed–becauseitreturnsanintegervalue,notaBooleanvalue.

Manipulatingcharacters

TheStringclasswithinthejava.langpackageprovidesthetrim()methodthatisusedtoremoveanywhitespacefromthebeginningandendoftheStringspecifiedasitsargument.Thismethodwillremoveallextremespaces,newlines,andtabs,returningthetrimmedversionofthatString.

AnindividualcharacterinaStringcanbeaddressedbystatingitsindexpositionwithinthatStringastheargumenttoitscharAt()method.ThismethodtreatstheStringasanarrayofcharacterswherethefirstcharacterisatpositionzero–justlikeotherarrayswhoseelementsareindexedstartingatzero.ThefirstcharacterinaStringcanbeaddressedascharAt(0),thesecondcharacterascharAt(1),andsoon.

Ascharacterindexingbeginsatzero,thefinalcharacterinaStringwillalwayshaveanindexnumberthatisonelessthanthetotalnumberofcharactersintheString.ThismeansthatthefinalcharacterinanyStringhastheindexnumberequivalenttolength()-1.ThefinalcharacterinaStringnamed“str”can,therefore,beaddressedasstr.charAt(str.length()-1).

AlloccurrencesofaparticularcharacterinaStringcanbereplacedbyanothercharacterusingitsreplace()method.Thismethodrequirestwoargumentsthatspecifythecharactertobereplacedandthecharacterthatistotakeitsplace.Forexample,toreplacealloccurrencesoftheletter“a”withtheletter“z”,thesyntaxwouldbereplace(‘a’,‘z’).

TheisEmpty()methodcanbeusedtodiscoverifaStringcontainsnocharacters.ThismethodwillreturntrueiftheStringisabsolutelyempty,otherwiseitwillreturnfalse.

Startanewprogramnamed“CharacterSwap”containingthestandardmainmethodclassCharacterSwap

{


}

CharacterSwap.java

Insidethemainmethod,declareandinitializeanemptyStringvariableStringtxt=““;

AssignsomecharacterstotheStringvariable,ifitisindeedempty,withbothleadingandtrailingspacesif(txt.isEmpty())txt=“Borrocudo“;

OutputtheStringvalueandthenumberofcharactersitcontainsSystem.out.println(“String:“+txt);System.out.println(“OriginalStringLength:“+txt.length());

Removetheleadingandtrailingspaces,thenoutputtheStringvalueanditssizeagaintxt=txt.trim();System.out.println(“String:“+txt);System.out.println(“StringLength:“+txt.length());

OutputthefirstcharacterintheStringcharinitial=txt.charAt(0);System.out.println(“FirstLetter:“+initial);

Now,outputthelastcharacterintheStringinitial=txt.charAt((txt.length()-1));System.out.println(“LastLetter:“+initial);

Replacealloccurrencesoftheletter“o”withletter“a”txt=txt.replace(‘o’,’a’);System.out.println(“String:“+txt);

SavetheprogramasCharacterSwap.javathencompileandruntheprogramtoseetheoutput

Summary

• TheJavadocumentationprovidesinformationaboutthemethodsandpropertiesineachJavaclass.

• JavaclassesthatarefundamentaltotheJavalanguagearecontainedinthejava.langpackage,inthejava.basemodule.

• TheMathclassprovidesMath.PIandMath.Econstants.

• Math.pow()raisestoaspecifiedpowerandMath.sqrt()returnsthesquarerootofaspecifiednumber.

• NumberscanberoundedtoanintegervaluewithMath.round(),Math.floor(),andMath.ceil().

• NumberscanbecomparedwithMath.max()andMath.min().

• Math.random()returnsadoubleprecisionrandomnumberbetween0.0and0.999999999999999.

• AStringiszeroormorecharactersenclosedinquotemarks.

• Thelength()methodreturnsthesizeofitsString,muchlikethelengthpropertyofanarray.

• Theconcat()methodofaStringappendsanotherStringvalue.

• Theequals()methodofaStringonlyreturnstruewhentwoStringvalueshaveidenticalcharacters,inthesameorder.

• CharactercaseofaStringcanbechangedusingitstoUpperCase()methodandtoLowerCase()method.

• StringvaluescanbecomparedusingthestartsWith()andendsWith()methodsofaString.

• AsubstringcanbesoughtinaStringusingitsindexOf()andsubstring()methods.

• TheisEmpty()methodonlyreturnstruewhentheStringcontainsabsolutelynothing.

• CharacterscanbemanipulatedwithinaStringvalueusingitstrim(),charAt(),andreplace()methods.

6

Creatingclasses

ThischapterdemonstrateshowtocreateJavaprogramsthatemploymultiplemethodsandclasses.

FormingmultiplemethodsUnderstandingprogramscope

FormingmultipleclassesExtendinganexistingclassCreatinganobjectclassProducinganobjectinstance

EncapsulatingpropertiesConstructingobjectvaluesSummary

Formingmultiplemethods

Programsaretypicallysplitintoseparatemethodsinordertocreatemodulesofcodethateachperformtasks,andthatcanbecalledrepeatedlythroughouttheprogramasrequired.Splittingtheprogramintomultiplemethodsalsomakesiteasiertotrackdownbugs,aseachmethodcanbetestedindividually.Furthermethodsmaybedeclared,insidethecurlybracketsthatfollowtheclassdeclaration,usingthesamekeywordsthatareusedtodeclarethemainmethod.Eachnewmethodmustbegivenaname,followingtheusualnamingconventions,andmayoptionallyspecifyargumentsintheparenthesesafteritsname.

Startanewprogramnamed“Methods”containingthestandardmainmethodclassMethods

{


}

Methods.java

Betweenthecurlybracketsofthemainmethod,insertstatementstooutputamessageandtocallasecondmethodnamed“sub”System.out.println(“Messagefromthemainmethod.”);sub();

Afterthemainmethod,beforethefinalcurlybracketoftheclass,addthesecondmethodtooutputamessagepublicstaticvoidsub()

{

System.out.println(“Messagefromthesubmethod.”);

}

SavetheprogramasMethods.javathencompileandruntheprogramtoseetheoutput

Thesyntaxtocallamethodwithoutargumentsjustneedsthemethodname,followedbyparentheses.

Aclassmayevencontainmultiplemethodsofthesamenameprovidingtheyeachhavedifferentarguments–requiringadifferentnumberofarguments,orargumentsofdifferentdatatypes.Thisusefulfeatureisknownasmethod“overloading”.

Startanewprogramnamed“Overload”containingthestandardmainmethodclassOverload

{


}

Overload.java

Betweenthecurlybracketsofthemainmethod,insertthreestatementscallingdifferentoverloadedmethodsandpassingthemargumentvaluesSystem.out.println(write(12));System.out.println(write(“Twelve”));System.out.println(write(4,16));

Afterthemainmethod,beforethefinalcurlybracketoftheclass,addthethreeoverloadedmethodstoeachreturnaStringtothecallerpublicstaticStringwrite(intnum){return(“Integerpassedis“+num);}publicstaticStringwrite(Stringnum){return(“Stringpassedis“+num);}publicstaticStringwrite(intnum1,intnum2){return(“SumTotalis“+(num1*num2));}

SavetheprogramasOverload.javathencompileandruntheprogramtoseetheoutput

ThedeclarationforeachoftheoverloadedmethodsmustindicatethatthemethodreturnsaStringvalue,notvoid.

Understandingprogramscope

Avariablethatisdeclaredinsideamethodisonlyaccessiblefrominsidethatmethod–its“scope”ofaccessibilityisonlylocaltothemethodinwhichitisdeclared.Thismeansthatavariableofthesamenamecanbedeclaredinanothermethodwithoutconflict.

Startanewprogramnamed“Scope”containingthestandardmainmethodclassScope

{


}

Scope.java

BetweenthecurlybracketsofthemainmethoddeclareandinitializealocalStringvariable,thenoutputitsvalueStringtxt=“Thisisalocalvariableinthemainmethod”;System.out.println(txt);

Afterthemainmethod,beforethefinalcurlybracketoftheclass,addanothermethodnamed“sub”publicstaticvoidsub(){}

Betweenthecurlybracketsofthesubmethod,declareandinitializealocalStringvariableofthesamenameasthevariableinthemainmethodStringtxt=“Thisisalocalvariableinthesubmethod”;System.out.println(txt);

Insertacalltothesubmethodattheendofthemainmethodsub();

SavetheprogramasScope.javathencompileandruntheprogramtoseetheoutput

Acountervariabledeclaredinaforloopcannotbeaccessedoutsidetheloop–itsscopeislimitedtotheforstatementblock.

Thestatickeywordthatisusedinmethoddeclarationsensuresthatthemethodisa“classmethod”–globallyaccessiblefromanyothermethodintheclass.

Similarly,a“classvariable”canbedeclaredwiththestatickeywordtoensureitisgloballyaccessiblethroughouttheclass.Itsdeclarationshouldbemadebeforethemainmethoddeclaration,rightafterthecurlybracketfollowingtheclassdeclaration.

Aprogrammayhaveaglobalclassvariableandlocalmethodvariableofthe

Aprogrammayhaveaglobalclassvariableandlocalmethodvariableofthesamename.Thelocalmethodvariabletakesprecedenceunlesstheglobalclassvariableisexplicitlyaddressedbytheclassnameprefixusingdotnotation,orifalocalvariableofthatnamehasnotbeendeclared.

EditScope.javabyinsertingaglobalclassStringvariableconstantofthesamenameasthelocalmethodvariablesfinalstaticStringtxt=“ThisisaglobalvariableoftheScopeclass”;

AddastatementattheendofthemainmethodtooutputthevalueoftheglobalclassvariableSystem.out.println(Scope.txt);

Commentoutthelinethatdeclaresthelocalvariableinthesubmethod–sotheoutputstatementwillnowaddresstheglobalvariableofthesamename//Stringtxt=“Thisisalocalvariableinthesubmethod”;

Savethechanges,thenrecompiletheprogramandrunitoncemoretoseetherevisedoutput

Uselocalmethodvariableswhereverpossibletoavoidconflicts–globalclassvariablesaretypicallyonlyusedforconstants.

Formingmultipleclasses

Inthesamewaythataprogrammayhavemultiplemethods,largerprogramsmayconsistofseveralclasses–whereeachclassprovidesspecificfunctionality.Thismodularformatisgenerallypreferabletowritingtheentireprogramina

Thismodularformatisgenerallypreferabletowritingtheentireprograminasingleclassasitmakesdebuggingeasierandprovidesbetterflexibility.

Thepublickeywordthatappearsindeclarationsisan“accessmodifier”thatdetermineshowvisibleanitemwillbetootherclasses.Itcanbeusedintheclassdeclarationtoexplicitlyensurethatclasswillbevisibletoanyotherclass.Ifitisomitted,thedefaultaccesscontrollevelallowsaccessfromotherlocalclasses.Thepublickeywordmustalwaysbeusedwiththeprogram’smainmethod,however,sothatmethodwillbevisibletothecompiler.

Startanewprogramnamed“Multi”containingthestandardmainmethod–includingthepublickeywordasusualclassMulti

{


}

Multi.java

Betweenthecurlybracketsofthemainmethod,declareandinitializeaStringvariable,thenoutputitscontentsStringmsg=“ThisisalocalvariableintheMulticlass”;System.out.println(msg);

OutputthecontentsofaclassStringvariableconstantnamed“txt”fromaclassnamed“Data”System.out.println(Data.txt);

Callamethodnamed“greeting”fromtheDataclassData.greeting();

Callamethodnamed“line”fromaclassnamed“Draw”Draw.line();

SavetheprogramasMulti.java

Thecompilerwillautomaticallyfindclassesinadjacentexternal.javafiles–andcreatecompiled.classfilesforeachone.

StartanewfilecreatingtheDataclassclassData

{

}

Data.java

DeclareandinitializeapublicclassvariableconstantpublicfinalstaticStringtxt=“ThisisaglobalvariableintheDataclass”;

Addapublic“greeting”classmethodpublicstaticvoidgreeting()

{

System.out.print(“Thisisaglobalmethod“);System.out.println(“oftheDataclass”);

}

SavethefileasData.javainthesamedirectoryastheMulti.javaprogramStartanewfilecreatingaDrawclassandaclass“line”methodfordefaultaccess–withoutthepublickeywordclassDraw

{

staticvoidline()

{

System.out.println(“”);

}

}

Draw.java

SavethefileasDraw.javainthesamedirectoryastheMulti.javaprogram,thencompileandruntheprogramtoseetheoutput

Thepublickeywordallowsaccessfromanyotherclass,butdefaultaccessonlyallowsaccessfromclassesinthesamepackage.

Extendinganexistingclass

Aclasscaninheritthefeaturesofanotherclassbyusingtheextendskeywordintheclassdeclarationtospecifythenameoftheclassfromwhichitshouldinherit.Forexample,thedeclarationclassExtraextendsBaseinheritsfromtheBaseclass.

Theinheritingclassisdescribedasthe“sub”class,andtheclassfromwhichit

inheritsisdescribedasthe“super”class.Intheexampledeclarationabove,theBaseclassisthesuperclassandtheExtraclassisthesubclass.

Methodsandvariablescreatedinasuperclasscangenerallybetreatedasiftheyexistedinthesubclassprovidingtheyhavenotbeendeclaredwiththeprivatekeyword,whichdeniesaccessfromoutsidetheoriginalclass.

Amethodinasubclasswilloverrideamethodofthesamenamethatexistsinitssuperclassunlesstheirargumentsdiffer.Themethodinthesuperclassmaybeexplicitlyaddressedusingitsclassnameanddotnotation.Forexample,SuperClass.run().

Itshouldbenotedthatatrycatchstatementinamethodwithinasuperclassdoesnotcatchexceptionsthatoccurinasubclass–thecallingstatementmustbeenclosedwithinitsowntrycatchstatementtocatchthoseexceptions.

Startanewclassnamed“SuperClass”classSuperClass{}

SuperClass.java

Betweenthecurlybracketsoftheclass,addamethodthatoutputsanidentifyingStringpublicstaticvoidhello()

{

System.out.println(“HellofromtheSuperClass”);

}

Addasecondmethodthatattemptstooutputapassedargument,thensavethefileasSuperClass.javapublicstaticvoidecho(Stringarg)

{

try{System.out.println(“Youentered:“+arg);}catch(Exceptione){System.out.println(“Argumentrequired”);}

}

Startanewprogramnamed“SubClass”thatextendstheSuperClassclassclassSubClassextendsSuperClass

{


}

SubClass.java

Afterthemainmethod,addamethodthatoutputsanidentifyingString,overridingtheinheritedmethodofthesamenamepublicstaticvoidhello()

{

System.out.println(“HellofromtheSubClass”);

}

Betweenthecurlybracketsofthemainmethod,insertacalltotheoverridingmethodandthenexplicitlycallthemethodofthesamenameinthesuperclasshello();SuperClass.hello();

Addacalltotheotherinheritedmethodecho(args[0]);

SavetheprogramasSubClass.javathencompileandruntheprogramwithoutacommandlineargument

EditSubClass.javatoenclosethemethodcallinStep7,toplaceitwithinitsowntrycatchstatementtocatchexceptions,thenrecompileandre-runtheprogramtoseetheproblemresolved

Youcanfindmoreinformationaboutcatchingexceptionshere.

Creatinganobjectclass

Real-worldobjectsareallaroundus,andtheyeachhaveattributesandbehaviorsthatwecandescribe:

• Attributesdescribethefeaturesthatanobjecthas• BehaviorsdescribeactionsthatanobjectcanperformForexample,acarmightbedescribedwithattributesof“red”and“coupe”,alongwithan“accelerates”behavior.

ThesefeaturescouldberepresentedinJavaprogrammingwithaCarclasscontainingvariablepropertiesofcolorandbodyType,alongwithanaccelerate()method.

JavaissaidtobeanObjectOrientedProgramming(OOP)languagebecauseit

JavaissaidtobeanObjectOrientedProgramming(OOP)languagebecauseitmakesextensiveuseofobjectattributesandbehaviorstoperformprogramtasks.

ObjectsarecreatedinJavabydefiningaclassasatemplatefromwhichdifferentcopies,or“instances”,canbemade.

Eachinstanceoftheclasscanbecustomizedbyassigningattributevaluesandbehaviorstodescribethatobject.

TheCarclassiscreatedasaclasstemplateinthestepsdescribedopposite–withthedefaultattributesandbehavioroutlinedabove.AninstanceoftheCarclassiscreatedinthestepsdescribedhere,inheritingthesamedefaultattributesandbehavior.

Startanewtemplateclassnamed“Car”classCar

{

}

FirstObject.java

BetweenthecurlybracketsoftheCarclass,declareandinitializetwoglobalStringconstantsdescribingattributespublicfinalstaticStringcolor=“Red”;publicfinalstaticStringbodyType=“Coupe”;

AddaglobalmethoddescribingabehaviorpublicstaticStringaccelerate()

{

Stringmotion=“Accelerating...”;returnmotion;

}

AftertheCarclass,startanewprogramclassnamed“FirstObject”containingthestandardmainmethod

classFirstObject

{


}

Betweenthecurlybracketsofthemainmethod,insertstatementstooutputthevalueofeachCarattributeandcallitsbehaviormethodSystem.out.println(“Paintis“+Car.color);System.out.println(“Styleis“+Car.bodyType);System.out.println(Car.accelerate());

SavetheprogramasFirstObject.javathencompileandruntheprogramtoseetheoutput

Thestatickeyworddeclaresclassvariablesandclassmethods–inthiscase,asmembersoftheCarclass.

Objectclassesarenormallycreatedbeforetheprogramclasscontainingthemainmethod.

Producinganobjectinstance

Eachclasshasabuilt-in“constructor”methodthatcanbeusedtocreateanewinstanceofthatclass.Theconstructormethodhasthesamenameastheclass,andisinvokedwiththenewkeyword.

Eachinstanceofaclassinheritstheobject’sattributesandbehaviors.TheprincipleofinheritanceisusedthroughoutJavasothatprogramscanuseready-madeproperties.

Tobemoreflexible,objectclasstemplatescanbedefinedinafileotherthanthatcontainingtheprogram.Thismeanstheycanbereadilyusedbymultipleprograms.

Startanewfile,repeatingtheCarclassobjecttemplatefromthepreviousexamplehereclassCar

{

publicfinalstaticStringcolor=“Red”;publicfinalstaticStringbodyType=“Coupe”;publicstaticStringaccelerate()

{


}

}

Car.java

SavethefileasCar.java

Startanewprogramnamed“FirstInstance”containingthestandardmainmethodclassFirstInstance

{


}

FirstInstance.java

Betweenthecurlybracketsofthemainmethod,insertstatementstooutputthevalueofeachattributeoftheCarclassandcallitsbehaviormethodSystem.out.println(“Carpaintis“+Car.color);System.out.println(“Carstyleis“+Car.bodyType);System.out.println(Car.accelerate());

Now,addastatementtocreateaPorscheinstanceoftheCarclassCarPorsche=newCar();

AddstatementstooutputtheinheritedvalueofeachPorscheattributeandcallitsbehaviormethodSystem.out.println(“Porschepaintis“+Porsche.color);System.out.println(“Porschestyleis“+Porsche.bodyType);System.out.println(Porsche.accelerate());

SavetheprogramasFirstInstance.javaalongsidetheCar.javatemplatefile,thencompileandruntheprogramtoseetheoutput

Youcannotaddressthemotionvariabledirectly–itisoutofscopewithinthemethoddeclaration.

AvirtualclassiscreatedforthenewPorscheobjectthatreplicatestheoriginalCarclass.Boththeseobjectscontainstatic“classvariables”anda“classmethod”,whichareaddressedusingtheclassnameanddotnotation–asthesemembersaregloballyaccessible,thisisnotconsideredgoodprogrammingpractice.

Whilstthisexampledemonstrateshowinstancesofanobjectinheritpropertiesoftheoriginalclass,itisimprovedinthenextexampleherethatusesnon-staticmemberstocreate“instancevariables”andan“instancemethod”,whichcannotbeaddressedfromoutsidethatclass–asthesemembersarenotgloballyaccessible,thisisconsideredgoodprogrammingpractice.

ThecompilerautomaticallyfindstheCarclassintheexternal.javafile–andcreatesacompiled.classfileforit.

Encapsulatingproperties

Theprivatekeywordcanbeusedwhendeclaringobjectvariablesandmethodstoprotectthemfrommanipulationbyexternalprogramcode.Theobjectshouldthenincludepublicmethodstoretrievethevaluesandcallthemethods.Thistechniqueneatlyencapsulatesthevariablesandmethodswithintheobjectstructure.Itisdemonstratedinthefollowingstepsthatreproducethepreviousexample–butwithencapsulatedattributesandmethod:

Startanewclassnamed“Car”classCar

{

{

}

SafeInstance.java

Betweenthecurlybracketsoftheclass,declarethreeprivateStringvariablestostoreobjectattributesprivateStringmaker;privateStringcolor;privateStringbodyType;

AddaprivatemethoddescribingabehaviorprivateStringaccelerate()

{


}

AddapublicmethodtoassignpassedargumentvaluestoeachprivatevariablepublicvoidsetCar(Stringbrand,Stringpaint,Stringstyle)

{

maker=brand;color=paint;bodyType=style;

}

AddanotherpublicmethodtoretrievetheprivatevariablevaluesandtocalltheprivatemethodpublicvoidgetCar()

{

System.out.println(maker+”paintis“+color);

System.out.println(maker+“styleis“+bodyType);System.out.println(maker+“is“+accelerate()+”\n”);

}

AftertheendoftheCarclass,startanotherclassnamed“SafeInstance”containingthestandardmainmethodclassSafeInstance

{


}

Betweenthecurlybracketsofthemainmethod,insertastatementtocreateaninstanceoftheCarclassCarPorsche=newCar();

AddastatementthatcallsapublicmethodoftheCarclasstoassignvaluestoitsprivatevariablesPorsche.setCar(“Porsche”,”Red”,”Coupe”);

NowaddastatementtocalltheotherpublicmethodoftheCarclasstoretrievethestoredattributevaluesandcalltheprivatebehaviormethodPorsche.getCar();

Createanotherinstance,assigningandretrievingvaluesCarBentley=newCar();Bentley.setCar(“Bentley”,”Green”,”Saloon”);Bentley.getCar();

SavetheprogramasSafeInstance.javathencompileandruntheprogramtoseetheoutput

AnuninitializedStringvariablehasanullvalue–socallingthegetCar()methodbeforesetCar()willreturnanullfromeachvariable.

Constructingobjectvalues

Anobject’sconstructormethodcanbecalleddirectlyintheobjectclasstoinitializeobjectvariables.Thishelpstokeepthedeclarationsandassignmentsseparate,andisconsideredtobegoodprogrammingstyle.Itisdemonstratedinthefollowingstepsthatreproducethepreviousexampleherewithencapsulatedattributesandmethod–togetherwithinitializationbytheconstructor:

Startanewclassnamed“Car”classCar

{

}

Constructor.java

Betweenthecurlybracketsoftheclass,declarethreeprivateStringvariablestostoreobjectattributesprivateStringmaker;privateStringcolor;privateStringbodyType;

AddaconstructormethodthatinitializesallthreevariableswithattributevaluespublicCar()

{

maker=“Porsche”;

color=“Silver”;bodyType=“Coupe”;

}

AddaprivatemethoddescribingabehaviorprivateStringaccelerate()

{


}

AddapublicmethodtoassignpassedargumentvaluestoeachprivatevariablepublicvoidsetCar(Stringbrand,Stringpaint,Stringstyle)

{

maker=brand;color=paint;bodyType=style;

}

Constructormethoddeclarationsdonotstateanyreturndatatype.

AddanotherpublicmethodtoretrievetheprivatevariablevaluesandtocalltheprivatemethodpublicvoidgetCar()

{

System.out.println(maker+”paintis“+color);System.out.println(maker+“styleis“+bodyType);System.out.println(maker+“is“+accelerate()+”\n”);

}

}

AftertheendoftheCarclass,startanotherclassnamed“Constructor”containingthestandardmainmethodclassConstructor

{


}

Betweenthecurlybracketsofthemainmethod,insertstatementstocreateaninstanceoftheCarclassandretrievetheinitialdefaultvaluesCarPorsche=newCar();Porsche.getCar();

Createanotherinstance,assigningandretrievingvaluesCarFerrari=newCar();Ferrari.setCar(“Ferrari”,”Red”,”Sport”);Ferrari.getCar();

SavetheprogramasConstructor.javathencompileandruntheprogramtoseetheoutput

Summary

• Splittingprogramsintomultiplemethods,whichcanbecalleduponwhenrequired,increasesflexibilityandmakesiteasiertotrackdownbugs.

• Overloadedmethodshavethesamenamebuttakedifferentarguments.

• Variablesdeclaredwithinamethodhavelocalscope,butclassvariableshave

globalscopethroughoutthatclass.

• Thestatickeywordisusedtodeclareclassmethodsandclassvariables–havingglobalscopethroughoutthatclass.

• Thepublickeywordexplicitlyallowsaccessfromanyclass.

• Aclassdeclarationcanincludetheextendskeywordtonominateasuperclassfromwhichitwillinherit.

• Theclassnameanddotnotationcanbeusedtoexplicitlyaddressaparticularclassmethodorclassvariable.

• Real-worldobjectshaveattributesandbehaviorsthatcanberepresentedinprogramsbyvariablesandmethods.

• Javaobjectsarecreatedasatemplateclassfromwhichinstancecopiescanbemade.

• Eachclasshasaconstructormethodthatcanbeinvokedusingthenewkeywordtocreateaninstancecopyofthatclass.

• Instancesinherittheattributesandbehaviorsoftheclassfromwhichtheyarederived.

• Encapsulationprotectsinstancevariablesandinstancemethodsfrommanipulationbyexternalclasses.

• Theprivatekeyworddeniesaccessfromoutsidetheclasswherethedeclarationismade.

• Anobject’sconstructormethodcanbecalledtoinitializevariableattributesofthatobject.

7

Importingfunctions

ThischapterdemonstrateshowtoimportadditionalprogramfunctionalityfromspecializedJavaclasses.

HandlingfilesReadingconsoleinput

ReadingfilesWritingfilesSortingarrayelementsMakingarraylists

ManagingdatesFormattingnumbersSummary

Handlingfiles

Javacontainsapackagenamedjava.iothatisdesignedtohandlefileinputandoutputprocedures.Thepackagecanbemadeavailabletoaprogrambyincludinganimportstatementattheverybeginningofthe.javafile.Thiscanusethe*wildcardcharactertomean“allclasses”inthestatementimportjava.io.*;.

Thejava.iopackagehasaclassnamed“File”thatcanbeusedtoaccessfilesorcompletedirectories.AFileobjectmustfirstbecreatedusingthenewkeywordandspecifyingthefilename,ordirectoryname,astheconstructor’sargument.Forexample,thesyntaxtocreateaFileobjectnamed“info”torepresentalocal

filenamed“info.dat”lookslikethis:Fileinfo=newFile(“info.dat”);

Thisfilewouldbelocatedinthesamedirectoryastheprogram,buttheargumentcouldstatethepathtoafilelocatedelsewhere.NotethatthecreationofaFileobjectdoesnotactuallycreateafile,butmerelythemeanstorepresentafile.

OnceaFileobjecthasbeencreatedtorepresentafile,itsmethodscanbecalledtomanipulatethefile.ThemostusefulFileobjectmethodsarelistedinthistable,togetherwithabriefdescription:

Method: Returns:

exists() trueifthefileexists–falseifitdoesnot

getName() thefilenameasaString

length() numberofbytesinthefile,asalongtype

createNewFile() trueifabletocreatethenewuniquefile

delete() trueifabletosuccessfullydeletethefile

renameTo(File) trueifabletosuccessfullyrenamethefile

list() anarrayoffileorfoldernamesasStrings

Thefilenamespecifiedastheconstructorargumentmustbeenclosedwithinquotes.

Startanewprogramthatimportsthefunctionalityofallthejava.ioclassesimportjava.io.*;

ListFiles.java

Addaclassnamed“ListFiles”containingthestandardmainmethodclassListFiles

{


}

Betweenthecurlybracketsofthemainmethod,insertastatementtocreateaFileobjectforadirectoryfoldernamed“data”Filedir=newFile(“data”);

Addanifstatementtooutputthenamesofallfilesinthatfolder,oramessageifthefolderisemptyif(dir.exists())

{

String[]files=dir.list();System.out.println(files.length+“filesfound...”);for(inti=0;i<files.length;i++)

{

System.out.println(files[i]);

}

}

else{System.out.println(“Foldernotfound.”);}

SavetheprogramasListFiles.javaalongsidea“data”foldercontainingsomefiles,thencompileandruntheprogramtoseethefilenameslistedasoutput

Readingconsoleinput

Thejava.iopackageallowsaprogramtoreadinputfromthecommandline–interactingwiththeuser.JustastheSystem.outfieldcansendoutputtothecommandline,theSystem.infieldcanbeusedtoreadfromitwithanInputStreamReaderobject.Thisreadstheinputasbytes,whichitconvertsintointegervaluesthatrepresentUnicodecharactervalues.

Inordertoreadanentirelineofinputtext,thereadLine()methodofaBufferedReaderobjectreadsthecharactersdecodedbytheInputStreamReader.ThismethodmustbecalledfromwithinatrycatchstatementtocatchanyIOExceptionproblems.

Typically,thereadLine()methodwillassigntheinputtoaStringvariableformanipulationbytheprogram.


ReadString.java

Addaclassnamed“ReadString”containingthestandardmainmethodclassReadString

{


}

Betweenthecurlybracketsofthemainmethod,insertastatementtooutputamessagepromptingtheuserforinputSystem.out.print(“Enterthetitleofthisbook:“);

AddastatementcreatinganInputStreamReaderobject,enablinginputtobereadfromthecommandlineInputStreamReaderisr=newInputStreamReader(System.in);

CreateaBufferedReaderobjecttoreadthedecodedinputBufferedReaderbuffer=newBufferedReader(isr);

DeclareandinitializeanemptyStringvariableinwhichtostoretheinputStringinput=““;

Addatrycatchstatementtoreadtheinputfromthecommandlineandstoreitinthevariable

try

{

input=buffer.readLine();buffer.close();

}

catch(IOExceptione)

{

System.out.println(“Aninputerrorhasoccurred”);

}

OutputamessagethatincludesthestoredvalueSystem.out.println(“\nThanks,youarereading“+input);

SavetheprogramasReadString.javathencompileandruntheprogram

Entertextasprompted,thenhitReturntoseetheoutputmessagecontainingyourinputtext

Itisgoodpracticetocalltheclose()methodoftheBufferedReaderobjectwhenitisnolongerneeded.

Readingfiles

Thejava.iopackagecontainsaclassnamedFileReaderthatisespeciallydesignedtoreadtextfiles.ThisclassisasubclassoftheInputStreamReaderclassthatcanbeusedtoreadconsoleinputbyconvertingabytestreamintointegersthatrepresentUnicodecharactervalues.

AFileReaderobjectiscreatedusingthenewkeyword,andtakesthenameofthefiletobereadasitsargument.Optionally,theargumentcanincludethefullpath

toafileoutsidethedirectorywheretheprogramislocated.

Inordertoefficientlyreadthetextfileline-by-line,thereadLine()methodofaBufferedReaderobjectcanbeemployedtoreadthecharactersdecodedbytheFileReaderobject.ThismethodmustbecalledfromwithinatrycatchstatementtocatchanyIOExceptionproblemsthatmayarise.

ReadingalllinesinatextfilecontainingmultiplelinesoftextisaccomplishedbymakingrepeatedcallstothereadLine()methodinaloop.Attheendofthefilethecallwillreturnanullvalue,whichcanbeusedtoterminatetheloop.

Openaplaintexteditor,suchasWindowsNotepad,andwriteafewlinesoftext–forexample,afamousversefrom“TheBalladofReadingGaol”byOscarWilde

SavethetextfileasOscar.txtthenstartanewprogramthatimportsthefunctionalityofallthejava.ioclassesimportjava.io.*;

ReadFile.java

Addaclassnamed“ReadFile”containingthestandardmainmethodclassReadFile{publicstaticvoidmain(String[]args){}}

Betweenthecurlybracketsofthemainmethod,insertatrycatchstatementtry{}catch(IOExceptione)

{

System.out.println(“Areaderrorhasoccurred”);

}

}

Betweenthecurlybracketsofthetryblock,insertastatementtocreateaFileReaderobjectFileReaderfile=newFileReader(“Oscar.txt”);

CreateaBufferedReaderobjecttoreadthefileBufferedReaderbuffer=newBufferedReader(file);

DeclareandinitializeanemptyStringvariableinwhichtostorealineoftextStringline=““;

Addalooptoreadthetextfilecontentsintothevariableandoutputeachlineoftextwhile((line=buffer.readLine())!=null){System.out.println(line);}

RemembertoclosetheBufferedReaderobjectwhenitisnolongerneededbuffer.close();

SavetheprogramasReadFile.javaalongsidethetextfile,thencompileandruntheprogramtoseetheoutput

ThetextfilespecifiedastheFileReaderargumentmustbeenclosedwithinquotationmarks.

Writingfiles

Inthejava.iopackagetheFileReaderandBufferedReaderclasses,whichareusedtoreadtextfiles,havecounterpartsnamedFileWriterandBufferedWriterthatcanbeusedtowritetextfiles.

AFileWriterobjectiscreatedusingthenewkeyword,andtakesthenameofthefiletobewrittenasitsargument.Optionally,theargumentcanincludethefullpathtoafiletobewritteninadirectoryoutsidethatinwhichtheprogramislocated.

TheBufferedWriterobjectiscreatedwiththenewkeyword,andtakesthenameoftheFileWriterobjectasitsargument.Textcanthenbewrittenwiththewrite()methodoftheBufferedWriterobject,andlinesseparatedbycallingitsnewLine()method.ThesemethodsshouldbecalledfromwithinatrycatchstatementtocatchanyIOExceptionproblemsthatmayarise.

Ifafileofthespecifiednamealreadyexists,itscontentswillbeoverwrittenbythewrite()method,otherwiseanewfileofthatnamewillbecreatedanditscontentswritten.


WriteFile.java

Addaclassnamed“WriteFile”containingthestandardmainmethodclassWriteFile

{


}

Betweenthecurlybracketsofthemainmethod,insertatrycatchstatementtry{}catch(IOExceptione)

{

System.out.println(“Awriteerrorhasoccurred”);

}

Betweenthecurlybracketsofthetryblock,insertastatementtocreateaFileWriter

objectforatextfilenamed“Tam.txt”FileWriterfile=newFileWriter(“Tam.txt”);

CreateaBufferedWriterobjecttowritethefileBufferedWriterbuffer=newBufferedWriter(file);

Addstatementstowritelinesoftextandnewlinecharactersintothetextfile–forexample,atranslatedversefrom“TamO’Shanter”byRobertBurnsbuffer.write(“Thewindblewasifithadblownitslast”);

buffer.newLine();buffer.write(“Therattlingshowersroseonitsblast”);

buffer.newLine();buffer.write(“Thespeedygleamsthedarknessswallowed”);

buffer.newLine();buffer.write(“Loud,deepandlongthethunderbellowed”);

buffer.newLine();buffer.write(“Thatnightachildmightunderstand”);

buffer.newLine();buffer.write(“Thedevilhadbusinessonhishand.”);

RemembertoclosetheBufferedWriterobjectwhenitisnolongerneededbuffer.close();

SavetheprogramasWriteFile.javathencompileandruntheprogramtowritethetextfilealongsidetheprogram

Youcancalltheappend()methodoftheBufferedWriterobjecttoaddtext–ratherthanoverwritingtextwiththewrite()method.

Sortingarrayelements

Javacontainsapackagenamedjava.utilthatprovidesusefulutilitiesforhandlingcollectionsofdata.Thepackagecanbemadeavailabletoaprogrambyincludinganimportstatementattheverybeginningofthe.javafile.Thiscanusethe*wildcardcharactertomean“allclasses”inthestatementimportjava.util.*;.

Thejava.utilpackagehasaclassnamed“Arrays”thathasmethodswhichcanbeusedtomanipulatearrays.Itsfunctionalitycanbemadeavailabletotheprogrambyimportingallclassesfromthejava.utilpackageor,wheretheprogramonlyrequiresasingleclass,theimportstatementcanimportjustthatspecificclass.Forexample,theprogramcanimporttheArraysclasswiththestatementimportjava.util.Arrays;.

TheArraysclasshasasort()methodthatcanrearrangethecontentsofarrayelementsalphabeticallyandnumerically.

Startanewprogramthatimportsthefunctionalityofallmethodsinthejava.util.Arraysclassimportjava.util.Arrays;

Sort.java

Addaclassnamed“Sort”containingthestandardmainmethodclassSort{publicstaticvoidmain(String[]args){}}

Afterthemainmethod,insertamethodtodisplayallelementcontentsofapassedStringarraypublicstaticvoiddisplay(String[]elems)

{

System.out.println(“\nStringArray:”);for(inti=0;i<elems.length;i++)System.out.println(“Element”+i+“is”+elems[i]);

}

Addanoverloadedversionofthedisplay()methodtodisplayallelementcontentsofapassedintarraypublicstaticvoiddisplay(int[]elems)

{

System.out.println(“\nIntegerArray:”);for(inti=0;i<elems.length;i++)System.out.println(“Element”+i+“is”+elems[i]);

}

Seehereformoreonoverloadingmethods.

Betweenthecurlybracketsofthemainmethod,declareandinitializeaStringarrayandanintarrayString[]names={“Mike”,“Dave”,“Andy”};int[]nums={200,300,100};

Outputthecontentsofallelementsineacharraydisplay(names);

display(nums);

SorttheelementcontentsofbotharraysArrays.sort(names);

Arrays.sort(nums);

Outputthecontentsofallelementsineacharrayagaindisplay(names);

display(nums);

SavetheprogramasSort.javathencompileandruntheprogramtoseetheoutput

Theforloopsinthisexampleeachexecuteasinglestatementsonocurlybracketsarerequired–buttheycouldbeaddedforclarity.

Makingarraylists

Thejava.utilpackagecontainsaclassnamedArrayListthatstoresdatainanordered“Collection”(resizablesequence)oflistelements.Thiscanbemadeavailabletoaprogrambyimportingthespecificclasswithimportjava.util.ArrayList;.Alistmaycontainduplicateelements,andanArrayListobjecthasusefulmethodsthatallowmanipulationofstoredvaluesbyspecifyingtheir

elementindexnumber.Forexample,thelist’smethodcallget(0)willretrievethevaluestoredinthefirstelementwhereasremove(1)willremovethesecondlistelement.

Elementvaluescanbemodifiedbyspecifyingtheindexnumberandnewvalueasargumentstothelist’sset()method.Elementscanbeaddedtothelistataparticularpositionbyspecifyingtheindexnumberandvalueasargumentstothelist’sadd()method.Thelistexpandstoaccommodateadditionalelementsbymovingtheelementvaluesalongtheindex.

Youcandiscoverhowmanyelementsalistcurrentlyhasbycallingitssize()method.

AnArrayListobjectissimplycreatedusingthenewkeywordbut,likeotherJavacollections,thestatementmustspecifywhichgenerictypeofitemthelistmaycontain.Typically,alistmaycontainStringitems,soArrayListmusthavea<String>suffix.

Collections,suchasArrayList,haveaforEach()methodthatiteratesovereachelementinthelist.Thismakesiteasytoloopthroughallitemscontainedinthelist.

Eachstoredlistitemcanbeconvenientlyreferencedinturnbyspecifyinga“lambdaexpression”astheargumenttotheforEach()method.Lambdaexpressionsaresimplyshort,anonymous(un-named)methodsthatcanbespecifiedinthelocationtheyaretobeexecuted.Theybeginwithparentheses,tocontainanyarguments,thenhavea->charactersequencefollowedbythestatementblock,withthissyntax:(argument/s)->{statement/s}

Thedatatypeoftheargumentscanbeexplicitlydeclared,oritcanbeinferredfromthecontext–(Stringx)canbesimply(x).Additionally,thecurlybracketscanbeomittedifthelambdaexpressionstatementblockcontainsonlyonestatement.

Withalist’sforEach()methodthevalueofthecurrentelementintheiterationcanbepassedtothelambdaexpressionasitsargument,thendisplayedinoutputby

itsstatement.

LambdaexpressionswereintroducedinJava8toenablesuccinctanonymousmethods.

Startanewprogramthatimportsthefunctionalityofallmethodsinthejava.util.ArrayListclassimportjava.util.ArrayList;

Lists.java

Addaclassnamed“Lists”containingthestandardmainmethodclassLists{publicstaticvoidmain(String[]args){}}

Betweenthecurlybracketsofthemainmethod,insertastatementtocreateaStringArrayListobjectnamed“list”ArrayList<String>list=newArrayList<String>();

Next,addstatementstopopulatethelistelementswithStringvaluesthendisplaytheentirelistlist.add(“Alpha”);list.add(“Delta”);list.add(“Charlie”);System.out.println(“List:”+list);

Now,identifythecurrentvalueinthesecondelementthenreplaceitwithanewStringSystem.out.println(“Replacing:”+list.get(1)+“\n”);list.set(1,“Bravo”);

Finally,iteratethroughthelistanddisplaytheStringvaluenowstoredineachelementlist.forEach((x)->System.out.println(“Item:“+x));

SavetheprogramasLists.javathencompileandruntheprogramtoseetheoutput

Aswithregulararrays,elementsinanArrayListhaveazero-basedindex.

ThegraphicalJavaSwingJComboBoxcomponentthatisintroducedhereholdsadrop-downlistofoptions,somustalsospecifyitsgenericdatatypewhenthatobjectgetscreated.

Managingdates

Thejava.timepackagecontainsaclassnamedLocalDateTimethathasusefulmethodstoextractspecificfieldsfromaLocalDateTimeobjectthatdescribeaparticularpointintime.Thesecanbemadeavailabletoaprogrambyimportingthespecificclasswithimportjava.time.LocalDateTime;orbyimportingallclassesinthispackageusingthewildcardwithimportjava.time.*;.

Thejava.timepackagewasintroducedinJava8tomakeiteasiertoworkwithdatesandtimes.

AnewLocalDateTimeobjectcanbecreatedwithfieldsdescribingthecurrentdateandtimeusingitsnow()method.Thefieldsareinitializedfromthesystemclockforthecurrentlocale.

ThevaluewithinanindividualfieldcanberetrievedusinganappropriatemethodoftheLocalDateTimeobject.Forexample,thevalueoftheyearfieldcanberetrievedusingitsgetYear()method.Similarly,anyfieldcanbechangedusinganappropriatemethodoftheLocalDateTimeobjecttospecifyareplacementvalue.Forexample,thevalueoftheyearfieldcanbechangedbyspecifyinganewyearvalueasanargumenttoitswithYear()method.

Startanewprogramthatimportsthefunctionalityofallmethodsinthejava.time.LocalDateTimeclassimportjava.time.LocalDateTime;

DateTime.java

Addaclassnamed“DateTime”containingthestandardmainmethodclassDateTime

{


}

Betweenthecurlybracketsofthemainmethod,insertastatementtocreateacurrentLocalDateTimeobjectLocalDateTimedate=LocalDateTime.now();

OutputthecurrentdateandtimedetailsSystem.out.println(“\nItisnow”+date);

Incrementtheyear,andoutputthereviseddateandtimedate=date.withYear(2019);System.out.println(“\nDateisnow”+date);

OutputindividualLocalDateTimefieldsofthereviseddateStringfields=

“\nYear:\t\t\t”+date.getYear();

fields+=“\nMonth:\t\t\t”+date.getMonth();

fields+=“\nMonthNumber:\t\t”+date.getMonthValue();

fields+=“\nDay:\t\t\t”+date.getDayOfWeek();

fields+=“\nDayNumber:\t\t”+date.getDayOfMonth();

fields+=“\nDayNumberOfYear:\t”+date.getDayOfYear();

fields+=“\nHour(0-23):\t\t”+date.getHour();

fields+=“\nMinute:\t\t\t”+date.getMinute();

fields+=“\nSecond:\t\t\t”+date.getSecond();

System.out.println(fields);

SavetheprogramasDateTime.javathencompileandruntheprogramtoseetheoutput

ConcatenatingaStringlikethismeanstheprogrammakesjustonecall

toprintln()tooutputfielddetails–thisismoreefficientthancallingprintln()manytimestooutputeachindividualfieldseparately.

YoucanalternativelyusetheZonedDateTimeclassinsteadofLocalDateTimeifyoualsorequireatimezonefield.

Formattingnumbers

Javacontainsapackagenamedjava.textthatprovidesusefulclassesforformattingnumbersandcurrency.Thepackagecanbemadeavailabletoaprogrambyincludinganimportstatementattheverybeginningofthe.javafile.Thiscanusethe*wildcardcharactertomean“allclasses”inthestatementimportjava.text.*;.Alternatively,specificclassescanbeimportedbyname.

Thejava.textpackagehasaclassnamed“NumberFormat”,whichhasmethodsthatcanbeusedtoformatnumericalvaluesforoutput–addinggroupseparators,currencysigns,andpercentagesigns.

ThemethodusedtocreateanewNumberFormatobjectdeterminesitsformattingtype–getNumberInstance()forgroupseparators,getCurrencyInstance()forcurrencysigns,andgetPercentInstance()forpercentagesigns.Formattingisappliedbyspecifyingthenumericalvaluetobeformattedastheargumenttotheformat()methodoftheNumberFormatobject.

Thejava.time.formatpackagewasintroducedinJava8tomakeiteasiertospecifydateformatpatterns.

Thejava.time.formatpackagehasaDateTimeFormatterclassthatcanbeusedtoformatjava.timedatesandtimeobjects.ADateTimeFormatterobjectcontainsaformatterpatternthatisspecifiedasastringargumenttoitsofPattern()method.

Theformattercomprisesletters,definedintheJavadocumentation,andyourchoiceofseparators.Forexample,“M/d/y”specifiesthemonth,day,andyear,separatedbyslashes.Theformatisappliedbyspecifyingtheformatterastheargumenttotheformat()methodofajava.timedateandtimeobject.

StartanewprogramthatimportsthefunctionalityofallmethodsoftheNumberFormatclassinthejava.textpackageandallmethodsoftheDateTimeFormatterclassinthejava.time.formatpackageimportjava.text.NumberFormat;importjava.time.format.DateTimeFormatter;

Formats.java

Addaclassnamed“Formats”containingthestandardmainmethodclassFormats

{

publicstaticvoidmain(String[]args)

{

}

}

Betweenthecurlybracketsofthemainmethod,insertstatementstooutputanumberwithgroupseparatorsNumberFormatnf=NumberFormat.getNumberInstance();System.out.println(“\nNumber:“+nf.format(123456789));

AddstatementstooutputanumberwithacurrencysignNumberFormatcf=NumberFormat.getCurrencyInstance();System.out.println(“\nCurrency:“+cf.format(1234.50f));

AddstatementstooutputanumberwithapercentsignNumberFormatpf=NumberFormat.getPercentInstance();System.out.println(“\nPercent:“+pf.format(0.75f));

AddastatementcreatingacurrentLocalDateTimeobject

java.time.LocalDateTimenow=java.time.LocalDateTime.now();

AddstatementstooutputaformattednumericaldateDateTimeFormatterdf=DateTimeFormatter.ofPattern(“MMMd,yyy”);System.out.println(“\nDate:“+now.format(df));

AddstatementstooutputaformattednumericaltimeDateTimeFormattertf=DateTimeFormatter.ofPattern(“h:ma”);System.out.println(“\nTime:“+now.format(tf));

SavetheprogramasFormats.javathencompileandruntheprogramtoseetheformattedoutput

Astatementcanaddressaclassthathasnotbeenimportedbyusingitsfullpackageaddress–asseenhereinthestatementcreatingaLocalDateTimeobject.

Patternlettersarecasesensitive–refertothedocumentationtodiscover

thefulldetailsofpossiblepatterns.

Summary

• Oneormoreimportstatementscanbeincludedatthestartofaprogramtomakethefunctionalityofotherclassesavailable.

• Animportstatementcanimportallclassesinapackagewitha*wildcardcharacter,orindividualclassesbyname.

• Thejava.iopackagehasclassesthataredesignedtohandleinputandoutputprocedures.

• AFileobjectcanbeusedtoaccessfilesanddirectories.

• TheInputStreamReaderobjectdecodesinputbytesintocharacters,andtheBufferedReaderreadsitsdecodedcharacters.

• AFileReaderobjectcanbeusedtodecodetextfilebytesintocharactersforreadingbyaBufferedReaderobject.

• AFileWriterobjectandBufferedWriterobjectcancreateandupdatetextfiles.

• Thejava.utilpackagecontainsutilitiesforhandlingcollectionsofdata,suchasarraymanipulationwithitsArraysclass.

• Thejava.utilpackagealsocontainsanArrayListclassthathasmethodstoeasilymanipulatesequencedlistitems.

• AnArrayListobjectisaCollectionthatmustspecifythegenerictypeofitemthatlistmaycontain,suchas<String>.

• Alambdaexpressionisananonymousmethodthatcanbespecifiedwhereitistobeexecuted.

• Thejava.timepackagecontainsaLocalDateTimeclassthatprovidesfieldsfordateandtimecomponents.

• Thejava.textpackagecontainsaNumberFormatclassthatcanformatnumbersandcurrency.

• Thejava.time.formatpackagecontainsaDateTimeFormatterclassthatcanspecifypatternstoformatdatesandtimes.

8

Buildinginterfaces

ThischapterdemonstrateshowtouseJavaSwingcomponentstocreateagraphicalprograminterface.

CreatingawindowAddingpushbuttons

AddinglabelsAddingtextfieldsAddingitemselectorsAddingradiobuttons

ArrangingcomponentsChangingappearanceSummary

Creatingawindow

Programscanprovideagraphicaluserinterface(GUI)usingthe“Swing”componentsoftheJavalibrary.Thejavax.swingpackagecontainsclassestocreateavarietyofcomponentsusingthestyleofthenativeoperatingsystem.Thesecanbemadeavailabletoaprogrambyincludingtheinitialstatementimportjavax.swing.*;.

AclassmustbecreatedtorepresenttheGUItowhichcomponentscanbeaddedtobuildtheinterface.ThisiseasilyachievedbydeclaringitasubclassofSwing’sJFrameclassusingtheextendskeyword–therebyinheritingattributes

andbehaviorsthatallowtheusertomove,resize,andclosethewindow.

Remembertheletterxinjavax.swingbythinkingofJAVAeXtra.

Theclassconstructorshouldincludestatementstosettheseminimumrequirements:• Thetitleofthewindow–specifiedasaStringargumenttotheinheritedsuper()methodoftheJFrameclass.

• Thesizeofthewindow–specifiedaswidthandheightinpixelsasargumentstoitssetSize()method.

• Whattodowhentheuserclosesthewindow–specifiedasaconstantargumenttoitssetDefaultCloseOperation()method.

• Displaythewindow–specifiedasaBooleanargumenttoitssetVisible()method.

Additionally,theconstructorcanaddaJPanelcontainercomponenttothewindow,inwhichsmallercomponentscanbeadded,usingtheinheritedadd()methodoftheJFrameclass.

Bydefault,aJPanelcontaineremploysaFlowLayoutlayoutmanagerthatlaysoutcomponentsinleft-to-rightlines,wrappingattherightedgeofthewindow.

ThestepsoppositedescribehowtocreateabasicwindowcontainingaJPanelcontainerwithaFlowLayoutlayoutmanager.ThiswindowisfeaturedinsubsequentexamplesinthisbookthatdemonstratehowtoaddvariouscomponentstotheJPanelcontainer.

Layoutmanagersaredescribedinmoredetailhere.

StartanewprogramthatimportsallSwingcomponentsimportjavax.swing.*;

Window.java

CreateasubclassoftheJFrameclassnamed“Window”containingthestandardmainmethodclassWindowextendsJFrame

{


}

Beforethemainmethod,createaJPanelcontainerobjectJPanelpnl=newJPanel();

Next,insertthisconstructormethodtospecifywindowrequirementsandtoaddtheJPanelobjecttotheJFramepublicWindow()

{

super(“SwingWindow”);setSize(500,200);setDefaultCloseOperation(EXIT_ON_CLOSE);add(pnl);setVisible(true);

}

CreateaninstanceoftheWindowclassbyinsertingthislineintothemainmethodWindowgui=newWindow();

SavetheprogramasWindow.javathencompileandruntheprogramtoseethebasicwindowappear

TheEXIT_ON_CLOSEoperationisaconstantmemberoftheJFrameclass.Itexitstheprogramwhenthewindowgetsclosed.

Noticehowtheadd()methodisusedheretoaddtheJPanelobjecttotheJFramewindow.

Addingpushbuttons

TheSwingJButtonclasscreatesapush-buttoncomponentthatcanbeaddedtoagraphicalinterface.Thisletstheuserinteractwiththeprogrambyclickingonabuttontoperformanaction.

TheJButtonobjectiscreatedwiththenewkeyword,anditsconstructortakesaStringargumentspecifyingtexttobedisplayedonthatbutton.

Imagescanappearonbuttonstoo.AnImageIconobjectmustfirstbecreatedtorepresenttheimage,specifyingtheimagefilenameastheargumenttoitsconstructor.Typically,theimagewillbelocatedalongsidetheprogrambuttheargumentcanincludethepathforimagesoutsidethelocaldirectory.

Detailsofhowtocreateevent-handlermethodstorespondtouseractions,suchasabuttonclick,canbefoundinthenextchapter.

SpecifythenameoftheImageIconobjectastheargumenttotheJButtonconstructortodisplaythatimageonthebutton,orspecifyaStringandImageIconasitstwoargumentstodisplaybothtextandtheimage.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Buttons”

Buttons.java

BeforetheButtons()constructor,createtwoImageIconobjectsImageIcontick=newImageIcon(“Tick.png”);ImageIconcross=newImageIcon(“Cross.png”);

Next,createthreeJButtonobjectstodisplaytext,animage,andbothtextandanimagerespectivelyJButtonbtn=newJButton(“ClickMe”);JButtontickBtn=newJButton(tick);JButtoncrossBtn=newJButton(“STOP”,cross);

InsidetheButtons()constructor,insertthreestatementstoaddtheJButtoncomponentstotheJPanelcontainerpnl.add(btn);pnl.add(tickBtn);pnl.add(crossBtn);

SavetheprogramasButtons.javathencompileandruntheprogramtoseepushbuttonsappearinthewindow

TheJPanelobjecthasanadd()method–toaddcomponentstothatpanel.

DetailsofhowtocreateaJavaArchive(JAR)canbefoundhere.

Thebuttonsrespondgraphicallywhentheyareclicked,butwillnotperformanactionuntiltheprogramprovidesanevent-handlermethodtorespondtoeachclickevent.

WheretheprogramisintendedfordeploymentinasingleJavaarchive(JAR),imageresourcesmustbeloadedbyaClassLoaderobjectbeforecreatingtheImageIconobjectstorepresentthem.

SpecifyingtheresourcefilenameorpathtothegetResource()methodofaClassLoaderreturnsaURL,whichcanbeusedastheargumenttotheImageIconconstructor.Thejava.netpackageprovidesausefulURLclasstowhichthesemayfirstbeassigned.

BeforetheButtons()constructor,createaClassLoaderobjectClassLoaderldr=this.getClass().getClassLoader();

LoadtheURLsoftheimageresourcesjava.net.URLtickURL=ldr.getResource(“Tick.png”);java.net.URLcrossURL=ldr.getResource(“Cross.png”);

EdittheImageIcon()constructorsinStep2oppositetouseURLsImageIcontick=newImageIcon(tickURL);ImageIconcross=newImageIcon(crossURL);

Savethechangesthenrecompileandre-runtheprogram–itwillrunasbeforebutcannowbedeployedinaJAR

NoticehowthegetClass()methodandthiskeywordareusedheretoreferencethisclassobject.

Addinglabels

TheSwingJLabelclasscreatesalabelcomponentthatcanbeaddedtoagraphicalinterface.Thiscanbeusedtodisplaynon-interactivetextorimage,orbothtextandanimage.

TheJLabelobjectiscreatedwiththenewkeyword,anditsconstructortakesaStringargumentspecifyingtexttobedisplayedonthatlabel,orthenameofanImageIconobjectrepresentinganimagetodisplay.Itcanalsotakethreeargumentstospecifytext,image,andhorizontalalignmentasaJLabelconstantvalue.Forexample,JLabel(“text”,img,JLabel.CENTER)alignscentrally.

WhereaJLabelobjectcontainsbothtextandanimage,therelativepositionofthetextcanbedeterminedbyspecifyingaJLabelconstantastheargumentto

setVerticalPosition()andsetHorizontalPosition()methodsoftheJLabelobject.

Additionally,aJLabelobjectcanbemadetodisplayaToolTipwhenthecursorhoversover,byspecifyingatextStringastheargumenttothatobject’ssetToolTipText()method.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Labels”

Labels.java

BeforetheLabels()constructor,createanImageIconobjectImageIconduke=newImageIcon(“Duke.png”);

Next,createthreeJLabelobjectstodisplayanimage,text,andbothtextandanimagerespectivelyJLabellbl1=newJLabel(duke);JLabellbl2=newJLabel(“DukeisthefriendlymascotofJavatechnology.”);JLabellbl3=newJLabel(“Duke”,duke,JLabel.CENTER);

InsidetheLabels()constructor,insertthisstatementtocreateaToolTipforthefirstlabellbl1.setToolTipText(“Duke-theJavaMascot”);

Addthesetwostatementstoalignthetextcentrallybelowthethirdlabellbl3.setHorizontalTextPosition(JLabel.CENTER);lbl3.setVerticalTextPosition(JLabel.BOTTOM);

Now,addthreestatementstoaddtheJLabelcomponentstotheJPanelcontainerpnl.add(lbl1);pnl.add(lbl2);pnl.add(lbl3);

SavetheprogramasLabels.javathencompileandruntheprogram,placingthecursoroverthefirstlabel

JLabelalignmentconstantsincludeLEFT,CENTER,RIGHT,TOPandBOTTOM.

WheretheprogramisintendedfordeploymentinasingleJavaarchive(JAR),theimageresourcemustbeloadedbyaClassLoaderobjectbeforecreatingtheImageIconobjecttorepresentit.

SpecifyingtheresourcefilenameorpathtothegetResource()methodofaClassLoaderreturnsaURL,whichcanbeusedastheargumenttotheImageIconconstructor.

BeforetheLabels()constructor,createaClassLoaderobjectClassLoaderldr=this.getClass().getClassLoader();

EdittheImageIcon()constructorinStep2oppositetoloadtheURLoftheimageresourceusingtheClassLoaderobjectImageIconduke=newImageIcon(ldr.getResource(“Duke.png”));

Savethechanges,thenrecompileandre-runtheprogram–itwillrunasbefore,butcannowbedeployedinaJAR

DetailsofhowtocreateaJavaArchive(JAR)canbefoundhere.

Addingtextfields

TheSwingJTextFieldclasscreatesasingle-linetextfieldcomponentthatcanbeaddedtoagraphicalinterface.Thiscanbeusedtodisplayeditabletext,andallowstheusertoentertexttointeractwiththeprogram.

TheJTextFieldobjectiscreatedwiththenewkeyword,anditsconstructorcantakeaStringargumentspecifyingdefaulttexttobedisplayedinthatfield.Inthiscase,thecomponentwillbesizedtoaccommodatethelengthoftheString.Alternatively,theargumentmaybeanumericvaluetospecifythetextfieldsize.Theconstructorcanalsotaketwoarguments,specifyingbothdefaulttextandthetextfieldsize.

UsetheJPasswordFieldclassinsteadoftheJTextFieldclasswhereinputcharactersareneededtobenotvisible.

Amultiple-linetextfieldcanbecreatedwiththeJTextAreaclass,whoseconstructortakestwonumericalargumentsspecifyingitsnumberoflinesanditswidth.Alternatively,threeargumentscanbesuppliedspecifyingdefaulttext,linenumber,andwidth.TextcanbemadetowrapatwordendingswithinthisfieldbyspecifyingtrueastheargumenttothesetLineWrap()methodandsetWrapStyleWord()methodoftheJTextAreaobject.

WheretextenteredintoaJTextAreacomponentexceedsitsinitialsize,thecomponentwillexpandtoaccommodatethetext.Tomakethecomponentafixedsizewithscrollingcapability,itcanbeplacedinaJScrollPanecontainer.Thisiscreatedwiththenewkeyword,andtakesthenameoftheJTextAreaasitsargument.

Scrollbarswill,bydefault,onlyappearwhenthefieldcontainstextthatexceedsitsinitialsize–buttheycanbemadetoappearconstantlybyspecifyingaJScrollPaneconstantastheargumenttothesnappily-namedsetVerticalScrollBarPolicy()orsetHorizontalScrollBarPolicy()methodsoftheJScrollPaneobject.Forexample,toalwaysdisplayaverticalscrollbarusetheJScrollPane.VERTICAL_SCROLLBAR_ALWAYSconstantastheargument.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“TextFields”

TextFields.java

BeforetheTextFields()constructor,createtwoJTextFieldobjectsJTextFieldtxt1=newJTextField(38);JTextFieldtxt2=newJTextField(“DefaultText”,38);

CreateaJTextAreaobjectfivelineshighJTextAreatxtArea=newJTextArea(5,37);

AddaJScrollPaneobject–tocontaintheJTextAreacreatedinStep3,aboveJScrollPanepane=newJScrollPane(txtArea);

IntheTextFields()constructormethod,insertstatementstoenabletheJTextAreaobjecttowrapatwordendingstxtArea.setLineWrap(true);txtArea.setWrapStyleWord(true);

InsertastatementtoalwaysdisplayaverticalscrollbarfortheJTextAreaobjectpane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

InserttwostatementstoaddtheJTextFieldcomponentstotheJPanelcontainerpnl.add(txt1);pnl.add(txt2);

InsertanotherstatementtoaddtheJScrollPanecontainer,(containingtheJTextAreafield)totheJPanelcontainerpnl.add(pane);

SavetheprogramasTextFields.javathencompileandruntheprogram,enteringsometextintothetextarea

AJTextAreacomponenthasnoscrollingabilityunlessitiscontainedwithinaJScrollPanecomponent.

Addingitemselectors

TheSwingJCheckBoxclasscreatesacheckboxcomponentthatcanbeaddedtoagraphicalinterface.Thiscanbeusedtoallowtheusertoselectordeselectindividualitemsinaprogram.TheJCheckBoxobjectiscreatedwiththenewkeyword,anditsconstructortakesaStringargumentspecifyingtexttobedisplayedalongsidethatcheckbox.Itcanalsotakeasecondtrueargumenttomakethecheckboxbeselectedbydefault.

AchoiceofitemscanbeofferedbytheJComboBoxclassthatcreatesadrop-downlistfromwhichtheusercanselectanysingleitem.Thisobjectiscreatedwiththenewkeyword,anditsconstructortypicallytakesthenameofaStringarrayasitsargument.Eachelementinthearrayprovidesanitemforselectioninthedrop-downlist.Similarly,achoiceofitemscanbeofferedbytheJListclass

thatcreatesafixed-sizelistfromwhichtheusercanselectoneormoreitems.Itiscreatedwiththenewkeyword,anditsconstructoralsotakesanarrayasitsargument,witheachelementprovidinganitemforselection.AsbothJListandJComboBoxare“Collections”theymustspecifythegenerictypetheymaycontainwhentheygetcreated,suchas<String>.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Items”

Items.java

BeforetheItems()constructor,createaStringarrayofitemsforselectionString[]toppings={“Pepperoni”,“Mushroom”,“Ham”,“Tomato”};

Next,createfourJCheckBoxobjectstopresenteacharrayitemforselection–withoneselectedbydefaultJCheckBoxchk1=newJCheckBox(toppings[0]);JCheckBoxchk2=newJCheckBox(toppings[1],true);JCheckBoxchk3=newJCheckBox(toppings[2]);JCheckBoxchk4=newJCheckBox(toppings[3]);

AddasecondStringarrayofitemsforselectionString[]styles={“DeepDish”,“GourmetStyle”,“Thin&Crispy”};

CreateaJComboBoxobjecttopresenteachiteminthesecondarrayforselectionJComboBox<String>box1=newJComboBox<String>(styles);

AddaJListobjecttopresenteachiteminthefirstarrayforselectionfromalistJList<String>lst1=newJList<String>(toppings);

IntheItems()constructormethod,insertstatementstoaddeachJCheckBoxcomponenttotheJPanelcontainerpnl.add(chk1);pnl.add(chk2);pnl.add(chk3);pnl.add(chk4);

InsertstatementstomakeadefaultselectionandtoaddtheJComboBox

componenttotheJPanelcontainerbox1.setSelectedIndex(0);

pnl.add(box1);

Now,insertastatementtoaddtheJListcomponenttotheJPanelcontainerpnl.add(lst1);

SavetheprogramasItems.javathencompileandruntheprogram,selectingitemsfromthelists

OnlyoneitemcanbeselectedfromaJComboBoxcomponent–multipleitemscanbeselectedfromaJListcomponent.

Detailsofhowtocreateevent-handlermethodstorespondtouseractions,suchasanitemselection,canbefoundinChapter9.

Addingradiobuttons

TheSwingJRadioButtonclasscreatesaradiobuttoncomponentthatcanbeaddedtoagraphicalinterface.Thiscanbeusedtoallowtheusertoselectanitemfromagroupofradiobuttons.

TheJRadioButtonobjectiscreatedwiththenewkeyword,anditsconstructortakesaStringargumentspecifyingtexttobedisplayedalongsidethatradiobutton.Itcanalsotakeasecondtrueargumenttomakearadiobuttonbeselectedbydefault.

AButtonGroupobjectlogicallygroupsanumberofradiobuttonssothatonlyonebuttoninthatgroupcanbeselectedatanytime.EachradiobuttonisaddedtotheButtonGroupobjectbyspecifyingitsnameastheargumenttothegroup’sadd()method.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Radios”

Radios.java

BeforetheRadios()constructor,createthreeJRadioButtonobjects–withoneselectedbydefaultJRadioButtonrad1=newJRadioButton(“Red”,true);JRadioButtonrad2=newJRadioButton(“Rosé”);JRadioButtonrad3=newJRadioButton(“White”);

Next,createaButtonGroupobjectwithwhichtogrouptheradiobuttonsButtonGroupwines=newButtonGroup();

IntheRadios()constructormethod,insertstatementstoaddeachJRadioButtoncomponenttotheJButtonGroupwines.add(rad1);wines.add(rad2);wines.add(rad3);

InsertstatementstoaddtheJRadioButtoncomponentstotheJPanelcontainerpnl.add(rad1);pnl.add(rad2);pnl.add(rad3);

SavetheprogramasRadios.javathencompileandruntheprogram,selectinganyoneradiobuttonafterthedefault

TheButtonGroupobjectonlygroupsthebuttonslogically,notphysically.

Detailsofhowtocreateevent-handlermethodstorespondtouseractionscanbefoundinthenextchapter.

TheexamplesonthepreviouspageshavedemonstratedthemostcommonSwingcomponents–JButton,JLabel,JTextField,JCheckBox,JComboBox,JListandJRadioButton.Therearemanymorespecializedcomponentsavailableinthejavax.swingpackage,whosedetailscanbefoundintheJavadocumentation.Forexample,theJSlider,JProgressBar,andJMenuBarcomponentsbelow:

TryusingtheJavadocumentationtoaddaJSlidercomponenttotheRadiosprogram–seeherefordetailsonhowtousethedocumentation.

Arrangingcomponents

Thejava.awtpackage(AbstractWindowToolkit)containsanumberoflayoutmanagerclassesthatcanbeusedtoplacecomponentsinacontainerindifferentways.

Alayoutmanagerobjectiscreatedusingthenewkeyword,andcanthenbespecifiedastheargumenttoaJPanelconstructortohavethepanelusethatlayout.Whencomponentsgetaddedtothepaneltheywillbeplacedaccordingtotherulesofthespecifiedlayoutmanager.

LayoutManager: Rules:

BorderLayoutPlacesNorth,South,East,WestandCenter(thecontentpanedefault)

BoxLayoutPlacesinasingleroworcolumn

CardLayoutPlacesdifferentcomponentsinaspecifiedareaatdifferenttimes

FlowLayoutPlaceslefttorightinawrappingline(theJPaneldefault)

GridBagLayoutPlacesinagridofcells,allowingcomponentstospancells

GridLayoutPlacesinagridofrowsandcolumns

GroupLayoutPlaceshorizontallyandvertically

SpringLayoutPlacesbyrelativespacing

ThetoplevelJFrameobjecthasa“contentpane”containerthatplacescomponentsusingtheBorderLayoutlayoutmanagerbydefault.ThiscanbeusedtoplaceuptofiveJPanelcontainers,whichmayeachusetheirdefaultFlowLayoutlayoutmanager,oranyofthelayoutmanagersinthetableabove.Usingavarietyoflayoutmanagersaccommodatesmostlayoutrequirements.

Thecontentpanecanberepresentedbyajava.awt.Containerobject,whoseadd()methodcanspecifythepositionandnameofacomponenttobeplacedwithinthecontentpane.

Youcanfindfurtherdetailsofeachlayoutmanagerinthejava.awtsectionoftheJavadocumentation.

EditacopyofWindow.javafromhere,changingtheclassdeclaration,constructor,andinstancefrom“Window”to“Layout”,thenaddastatementatthestartoftheprogramtoimportthefunctionalityofthejava.awtpackageimportjava.awt.*;

Layout.java

BeforetheLayout()constructor,createaContainerobjectrepresentingtheJFrame

contentpanecontainerContainercontentPane=getContentPane();

CreateasecondJPanelobjectusingaGridLayoutlayoutmanagerina2x2gridJPanelgrid=newJPanel(newGridLayout(2,2));

IntheLayout()constructormethod,insertstatementsaddingJButtoncomponentstobothJPanelobjectspnl.add(newJButton(“Yes”));pnl.add(newJButton(“No”));pnl.add(newJButton(“Cancel”));grid.add(newJButton(“1”));grid.add(newJButton(“2”));grid.add(newJButton(“3”));grid.add(newJButton(“4”));

Now,insertstatementsaddingbothpanelsandabuttontothecontentpanecontentPane.add(“North”,pnl);contentPane.add(“Center”,grid);contentPane.add(“West”,newJButton(“West”));

SavetheprogramasLayout.javathencompileandruntheprogramtoseethecomponentlayout

WhiletheFlowLayoutmaintainstheJButtonsize,otherlayoutmanagersexpandthecomponentstofilltheirlayoutdesign.

Changingappearance

Thejava.awtpackage(AbstractWindowToolkit)contains“painting”classesthatcanbeusedtocolorinterfacecomponents.Thesecanbemadeavailabletoaprogrambyincludingtheinitialstatementimportjava.awt.*;.

Includedinthejava.awtpackageisaColorclassthathasconstantsrepresentingafewbasiccolors,suchasColor.RED.Additionally,instancesoftheColorclasscanbecreatedusingthenewkeywordtorepresentcustomcolors.Theconstructorcantakethreeintegerargumentsbetweenzeroand255torepresentred,green,andblue(RGB)valuestoformthecustomcolor.

EachcomponenthasasetBackground()methodandasetForeground()methodthattakeaColorobjectastheirargumenttopaintthatcomponentwiththespecifiedcolor.

NotethatthebackgroundofJLabelcomponentsaretransparentbydefault,soitisrecommendedthattheirsetOpaque()methodshouldbecalledtosettheopacitytotruebeforetheyarepainted.

Alsointhejava.awtpackageisaFontclassthatcanbeusedtomodifythefontdisplayingtext.AFontobjectrepresentsafont,anditsconstructorcantakethreeargumentstospecifyname,styleandsize:

• Thespecifiednameshouldbeoneofthethreeplatform-independentnames“Serif”,“SansSerif”or“Monospaced”.

• Thespecifiedstyleshouldbeoneofthefollowingthreeconstants:Font.PLAIN,Font.BOLDorFont.ITALIC

• Thespecifiedsizeshouldbeanintegerofthepointsize.

EachcomponenthasasetFont()methodthattakesaFontobjectasitsargumenttopaintthatcomponentwiththespecifiedfont.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Custom”

Custom.java

Addastatementattheverystartoftheprogramtoimportthefunctionalityofallclassesinthejava.awtpackageimportjava.awt.*;

BeforetheCustom()constructor,createthreeJLabelobjectsJLabellbl1=newJLabel(“CustomBackground”);JLabellbl2=newJLabel(“CustomForeground”);JLabellbl3=newJLabel(“CustomFont”);

Next,createColor,Font,andBoxlayoutobjectsColorcustomColor=newColor(255,0,0);FontcustomFont=newFont(“Serif”,Font.PLAIN,64);Boxbox=Box.createVerticalBox();

IntheCustom()constructormethod,insertstatementstocoloraJLabelbackgroundusingaColorconstantlbl1.setOpaque(true);lbl1.setBackground(Color.YELLOW);

InsertastatementtocoloraJLabelforegroundusingacustomColorobjectlbl2.setForeground(customColor);

InsertastatementtopainttextonaJLabelcomponentusingacustomfontlbl3.setFont(customFont);

Addeachlabeltothelayoutcontainerbox.add(lbl1);box.add(lbl2);box.add(lbl3);

Then,addthelayoutcontainertotheJPanelcontainerpnl.add(box);

SavetheprogramasCustom.javathencompileandruntheprogramtoseetheeffect

Inthiscase,thecustomcolorisequivalenttoColor.REDastheRGBvaluespecifiesthemaximumredvaluewithnogreenorblue.

ABoxobjectisahandylightweightcontainerthatusesBoxLayoutasitslayoutmanager.TheBoxobject’screateVerticalBox()methodindividuallydisplaysitscomponentsfromtoptobottom.

Summary

• Thejavax.swingpackagecontainstheJavaSwingclassesthatareusedtocreateGUIcomponents.

• Awindowiscreatedasatop-levelJFramecontainer.

• TheJFrameconstructorshouldspecifythewindow’stitle,size,defaultclose

operationandvisibility.

• AJPanelcontainerdisplayssmallercomponentsinawrappinglineusingitsdefaultFlowLayoutlayoutmanager.

• TheJButtonconstructorcanspecifytextandimagestobedisplayedonapushbuttoncomponent.

• AnImageIconobjectrepresentsanimagetouseintheprogram.

• ProgramsthataretobedeployedasasingleJavaarchive(JAR)shoulduseaClassLoaderobjecttospecifyanimagesource.

• AJLabelobjectdisplaysnon-interactivetextandimagecontent.

• EditabletextcanbedisplayedinJTextFieldandJTextAreafields.

• AJScrollPaneobjectprovidesscrollbarsforaJTextAreafield.

• ItemsforselectioncanbedisplayedwithJCheckBox,JComboBoxandJListcomponents.

• AButtonGroupobjectlogicallygroupsanumberofJRadioButtoncomponentssoonlyonecanbeselected.

• SpecificRGBcolorscanberepresentedbytheColorclassofthejava.awtpackage.

• Thejava.awtpackagehasaFontclassthatcanbeusedtocreateobjectsrepresentingaparticularfontname,style,andsize.

• MultipleJPanelcontainerscanbeaddedtoaJFramecontainerbyusingtheContainerclassofthejava.awtpackagetorepresentthecontentpaneoftheJFrame.

• WhencreatingaJPanelcontainerobject,itsargumentmayoptionallyspecifyalayoutmanager.

9

Recognizingevents

ThischapterdemonstrateshowtocreateJavaprogramevent-handlersthatrespondtouserinterface

actions.

ListeningforeventsGeneratingevents

HandlingbuttoneventsHandlingitemeventsReactingtokeyboardeventsRespondingtomouseevents

AnnouncingmessagesRequestinginputSummary

Listeningforevents

Ausercaninteractwithaprogramthatprovidesagraphicaluserinterface(GUI)byperformingactionswithamouse,keyboard,orotherinputdevice.Theseactionscause“events”tooccurintheinterface,andmakingaprogramrespondtothemisknownas“event-handling”.

ForaprogramtorecognizeusereventsitneedstohaveoneormoreEventListenerinterfacesaddedfromthejava.awt.eventpackage.Thesecanbemadeavailabletotheprogrambyaddinganinitialstatementtoimportjava.awt.event.*;.

ThedesiredEventListenerinterfacecanbeincludedintheclassdeclarationusingtheimplementskeyword.Forexample,aclassdeclarationtolistenforbuttonclicksmightlooklikethis:classClickextendsJFrameimplementsActionListener{}

TheJavadocumentationdescribesmanyEventListenerinterfacesthatcanlistenoutfordifferentevents,butthemostcommononesarelistedinthetablebelow,togetherwithabriefdescription:

EventListener: Description:

ActionListenerRecognizesactioneventsthatoccurwhenapushbuttonispushedorreleased

ItemListenerRecognizesitemeventsthatoccurwhenalistitemgetsselectedordeselected

KeyListenerRecognizeskeyboardeventsthatoccurwhentheuserpressesorreleasesakey

MouseListenerRecognizesmousebuttonactionsthatoccurwhentheuserpressesorreleasesamousebutton,andwhenthemouseentersorexitsacomponent

MouseMotionListenerRecognizesmotioneventsthatoccurwhentheusermovesthemouse

MultipleEventListenerscanbeincludedaftertheimplementskeywordasacomma-separatedlist.

Generatingevents

ComponentsneedtogenerateeventsthattheEventListenerinterfacescanrecognizeiftheyaretobeuseful.HavingaddedtheappropriateEventListenertotheprogram,asdescribedopposite,aneventgeneratormustbeaddedtothecomponent.

Forexample,inordertohavetheprogramrespondtoabuttonclick,theActionListenerinterfaceisaddedtotheprogramandthebutton’saddActionListener()methodmustbecalled,specifyingthethiskeywordasitsargument.Thismakesthebuttongenerateaneventwhenitgetsclicked,whichcanberecognizedbytheActionListenerinterface.

Statementscreatingabuttonthatgenerateseventslooklikethis:JButtonbtn=newJButton(“ClickMe”);btn.addActionListener(this);

Whentheuserclicksabuttonthatgeneratesanevent,theActionListenerinterfacerecognizestheeventandseeksanevent-handlermethodwithintheprogramtoexecutearesponse.

EachEventListenerinterfacehasanassociatedevent-handlermethodthatiscalledwhenaneventisrecognized.Forexample,whenabuttongetsclicked,theActionListenerinterfacecallsanassociatedmethodnamedactionPerformed()andpassesanActionEventobjectasitsargument.

AnActionEventobjectcontainsinformationabouttheeventandthesourcecomponentfromwhereitoriginated.Mostusefully,ithasagetSource()methodthatidentifiestheobjectthatgeneratedtheevent.Thiscanbeusedtocreateanappropriateresponseforthatcomponent.

Anevent-handlermethodtocreatearesponseforaspecificbuttonclickcouldlooklikethis:

publicvoidactionPerformed(ActionEventevent)

{

if(event.getSource()==btn)

{

Statementstobeexecutedforthisbuttonclickevent}

}

Handlingbuttonevents

ASwingJButtoncomponentthatissettogenerateanActionEventeventwhenitgetsclickedcanberecognizedbytheActionListenerinterface,whichwillpasstheeventtoitsactionPerformed()event-handlermethod.TheActionEventobjecthasagetSource()methodthatidentifiestheoriginatingcomponent,andagetActionCommand()methodthatreturnsaString.Thiswillbethetextlabelforabuttoncomponent,orthecontentforatextfieldcomponent.

OneresponsetoabuttonmightbetodisableacomponentbycallingitssetEnabled()methodwithafalseargument.Conversely,thecomponentcanbeenabledoncemorebyspecifyingatrueargumenttoitssetEnabled()method.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Actions”

Actions.java

Addastatementattheverystartoftheprogramtoimportthefunctionalityofallclassesinthejava.awt.eventpackageimportjava.awt.event.*;

EdittheclassdeclarationtoaddanActionListenerinterfacetotheprogramclassActionsextendsJFrameimplementsActionListener

BeforetheActions()constructor,createtwoJButtonpushbuttonsandaJTextAreatextfieldJButtonbtn1=newJButton(“Button1”);JButtonbtn2=newJButton(“Button2”);JTextAreatxtArea=newJTextArea(5,38);

AddthebuttonsandtextareatotheJPanelcontainerpnl.add(btn1);pnl.add(btn2);

pnl.add(txtArea);

Insertstatementstosettheinitialstateoftwocomponentsbtn2.setEnabled(false);txtArea.setText(“Button2isDisabled”);

IntheActions()constructor,insertstatementstomakeeachbuttongenerateanActionEventeventwhenclickedbtn1.addActionListener(this);btn2.addActionListener(this);

Aftertheconstructormethod,addanevent-handlermethodfortheActionListenerinterface–todisplaytextidentifyingwhichbuttonhasbeenclickedpublicvoidactionPerformed(ActionEventevent)

{

txtArea.setText(event.getActionCommand()+“ClickedandDisabled”);

}

Insertifstatementsintheevent-handlermethod–executingaspecificresponsetoeachbuttonclickif(event.getSource()==btn1){btn2.setEnabled(true);btn1.setEnabled(false);}

if(event.getSource()==btn2){btn1.setEnabled(true);btn2.setEnabled(false);}

SavetheprogramasActions.javathencompileandruntheprogram,clickingthepushbuttons

Thecomponentsaredeclaredbeforetheconstructorsotheyaregloballyaccessibletotheevent-handlermethod.

It’ssometimesusefultodisableacomponentuntiltheuserhasperformedarequiredaction.

Handlingitemevents

SwingJRadioButton,JCheckBoxandJComboBoxcomponentsmaintainstateswhosechangecanberecognizedbytheItemListenerinterface,whichwillpasstheItemEventtoitsitemStateChanged()event-handlermethod.TheItemEventobjecthasagetItemSelectable()methodthatidentifiestheoriginatingcomponentandagetStateChange()methodthatreturnsitsstatus.Thiswilldetermineifthechangeisselectingordeselectinganitem,andcanbecomparedtoanItemEvent.SELECTEDconstant.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“States”.Then,addastatementattheverystartoftheprogramtoimportthefunctionalityofthejava.awt.eventpackageimportjava.awt.event.*;

States.java

EdittheclassdeclarationtoaddanItemListenerinterfacetotheprogramclassStatesextendsJFrameimplementsItemListener

BeforetheStates()constructor,createthesecomponentsString[]styles={“DeepDish”,“GourmetStyle”,“Thin&Crispy”};JComboBox<String>box=newJComboBox<String>(styles);JRadioButtonrad1=newJRadioButton(“White”);JRadioButtonrad2=newJRadioButton(“Red”);ButtonGroupwines=newButtonGroup();JCheckBoxchk=newJCheckBox(“Pepperoni”);JTextAreatxtArea=newJTextArea(5,38);

IntheStates()constructor,insertstatementstogroupthetwoJRadioButtoncomponentswines.add(rad1);wines.add(rad2);

InsertstatementstoaddthecomponentstotheJPanelcontainerpnl.add(rad1);pnl.add(rad2);pnl.add(chk);pnl.add(box);pnl.add(txtArea);

Notehowthisexampleusestheappend()methodtoaddfurthertexttothetextarea.

InsertstatementstomakeselectablecomponentsgenerateanItemEvent

eventwhenanitemisselectedordeselectedrad1.addItemListener(this);rad2.addItemListener(this);chk.addItemListener(this);box.addItemListener(this);

Aftertheconstructormethod,addanevent-handlermethodforthe

ItemListenerinterface–identifyingitemsselectedbytheJRadioButtoncomponentspublicvoiditemStateChanged(ItemEventevent)

{

if(event.getItemSelectable()==rad1)txtArea.setText(“Whitewineselected”);

if(event.getItemSelectable()==rad2)txtArea.setText(“Redwineselected”);

}

Addanifstatementtotheevent-handlermethodtoindicatethestatusoftheJCheckBoxcomponentif((event.getItemSelectable()==chk)&&

(event.getStateChange()==ItemEvent.SELECTED))txtArea.append(“\nPepperoniselected\n”);

Addanifstatementtotheevent-handlermethodtoindicatethestatusoftheJComboBoxcomponentif((event.getItemSelectable()==box)&&

(event.getStateChange()==ItemEvent.SELECTED))txtArea.append(event.getItem().toString()+“selected”);

SavetheprogramasStates.javathencompileandruntheprogram,selectingvariousitemsfromlefttoright

TheJComboBoxfirestwoItemEventswhenanitemgetsselected–one

selectingtheitemandonedeselectingthepreviouslyselecteditem.Thatiswhysteps8&9mustidentifyboththeoriginatingcomponentandthetypeofItemEvent.

NoticethatthegetItem()methodreturnstheitemaffectedbythechange.

Reactingtokeyboardevents

SwingcomponentsthatallowtheusertoinputtextcanrecognizeuserkeystrokeswiththeKeyListenerinterface,whichwillpasstheKeyEventeventtothesethreeevent-handlermethods:

Event-handler: Description:

keyPressed(KeyEvent) Calledwhenakeyispressed

keyTyped(KeyEvent) Calledafterakeyispressed

keyReleased(KeyEvent) Calledwhenakeyisreleased

WhenaprogramimplementstheKeyListenerinterfaceitmustdeclarethesethreemethods–evenifnotallareactuallyused.

TheKeyEventobjecthasagetKeyChar()method,whichreturnsthecharacterforthatkey,andagetKeyCode()method,whichreturnsanintegerUnicodevaluerepresentingthatkey.Additionally,agetKeyText()methodtakesthekeycodevalueasitsargumentandreturnsadescriptionofthatkey.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Keystrokes”.Then,addaninitialstatementtoimportthefunctionalityofthejava.awt.eventpackageimportjava.awt.event.*;

Keystrokes.java

EdittheclassdeclarationtoaddaKeyListenerinterfacetotheprogramclassKeystrokesextendsJFrameimplementsKeyListener

BeforetheKeystrokes()constructor,createaJTextFieldcomponentandaJTextAreacomponentJTextFieldfield=newJTextField(38);JTextAreatxtArea=newJTextArea(5,38);

InsertstatementstoaddthesetwocomponentstotheJPanelcontainerpnl.add(field);pnl.add(txtArea);

IntheKeystrokes()constructor,insertastatementtomaketheJTextFieldcomponentgenerateKeyEventeventsfield.addKeyListener(this);

Aftertheconstructormethod,addanevent-handlerthatacknowledgeswhenakeygetspressedpublicvoidkeyPressed(KeyEventevent)

{

txtArea.setText(“KeyPressed”);

}

Addasecondevent-handlerthatdisplaysthekeycharacterafterthekeyhasbeenpressedpublicvoidkeyTyped(KeyEventevent)

{

txtArea.append(“\nCharacter:”+event.getKeyChar());

}

Addathirdevent-handlerthatdisplaysthekeycodeandkeytextwhenthekeygetsreleasedpublicvoidkeyReleased(KeyEventevent)

{

intkeyCode=event.getKeyCode();txtArea.append(“\nKeyCode:”+keyCode);txtArea.append(“\nKeyText:”+event.getKeyText(keyCode));

}

SavetheprogramasKeystrokes.javathencompileandruntheprogram,typinginthetoptextfield

ThegetKeyCode()methodonlyreturnsthekeycodeifcalledfromwithinthekeyPressed()orkeyReleased()event-handlers–notfromthekeyTyped()event-handler.

Runthisprogramandpressanon-characterkey,suchasBackspace,toseeitskeytextname.

Respondingtomouseevents

SwingcomponentscanrecognizeusermouseactionswiththeMouseListenerinterface,whichwillpasstheMouseEventeventtothesefiveevent-handlermethods:


mousePressed(MouseEvent) Buttonispressed

mouseReleased(MouseEvent) Buttonisreleased

mouseClicked(MouseEvent) Buttonhasbeenreleased

mouseEntered(MouseEvent) Mousemoveson

mouseExited(MouseEvent) Mousemovesoff

MousemovementscanberecognizedbytheMouseMotionListenerinterface,whichpassesMouseEventeventstotwoevent-handlers:


mouseMoved(MouseEvent) Mouseismoved

mouseDragged(MouseEvent) Mouseisdragged

WhenaprogramimplementstheMouseListenerorMouseMotionListenerinterface,itmustdeclareallitsassociatedevent-handlermethods–evenifnotallareactuallyused.

TheMouseEventobjectpassedbytheMouseMotionListenerinterfacehasgetX()andgetY()methods,whichreturnthecurrentmousecoordinatesrelativetothecomponentgeneratingtheevent.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Mouse”.Then,addaninitialstatementtoimportthefunctionalityofthejava.awt.eventpackageimportjava.awt.event.*;

Mouse.java

EdittheclassdeclarationtoaddaMouseListenerinterfaceandMouseMotionListenerinterfacetotheprogramclassMouseextendsJFrameimplementsMouseListener,MouseMotionListener

BeforetheMouse()constructor,createaJTextAreacomponentandtwointegervariablestostorecoordinatesJTextAreatxtArea=newJTextArea(8,38);intx,y;

IntheMouse()constructor,insertstatementstoaddtheJTextAreacomponenttotheJPanelcontainerandtomakeitgenerateMouseEventeventspnl.add(txtArea);txtArea.addMouseMotionListener(this);txtArea.addMouseListener(this);

Aftertheconstructormethod,addthetwoevent-handlersfortheMouseMotionListenerinterfacepublicvoidmouseMoved(MouseEventevent){x=event.getX();y=event.getY();}publicvoidmouseDragged(MouseEventevent){}

Addfiveevent-handlersfortheMouseListenerinterfacepublicvoidmouseEntered(MouseEventevent){txtArea.setText(“\nMouseEntered”);}

publicvoidmousePressed(MouseEventevent){txtArea.append(“\nMousePressedatX:“+x+“Y:“+y);}

publicvoidmouseReleased(MouseEventevent){txtArea.append(“\nMouseReleased”);}

publicvoidmouseClicked(MouseEventevent){}publicvoidmouseExited(MouseEventevent){}

SavetheprogramasMouse.java,thencompileandruntheprogram,clickingontheJTextAreacomponent

RollovereffectscanbecreatedbyswappingimageswiththemouseEntered()andmouseExited()event-handlermethods.

Announcingmessages

TheSwingJOptionPaneclassisdesignedtocreateastandarddialogboxcenteredonitsparentwindow.ItsshowMessageDialog()methoddisplaysamessagetotheuserprovidinginformation,warning,orerrordescription.

TheshowMessageDialog()methodcantakefourarguments:• Parentobject–typicallyreferencedbythethiskeyword• MessageStringtobedisplayed• DialogtitleString

• OneoftheJOptionPaneconstants:

INFORMATION_MESSAGE

WARNING_MESSAGEorERROR_MESSAGE

ThedialogboxwilldisplayanappropriateiconaccordingtowhichJOptionPaneconstantisspecified.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Messages”

Messages.java

Addaninitialstatementtoimportthefunctionalityofthejava.awt.eventpackageimportjava.awt.event.*;

EdittheclassdeclarationtoaddanActionListenerinterfacetotheprogramclass

MessagesextendsJFrameimplementsActionListener

BeforetheMessages()constructor,createthreeJButtoncomponentsJButtonbtn1=newJButton(“ShowInformationMessage”);JButtonbtn2=newJButton(“ShowWarningMessage”);JButtonbtn3=newJButton(“ShowErrorMessage”);

InsertstatementstoaddthebuttoncomponentstotheJPanelcontainerpnl.add(btn1);pnl.add(btn2);pnl.add(btn3);

IntheMessages()constructor,insertstatementstomakeeachbuttongenerateanActionEventeventbtn1.addActionListener(this);btn2.addActionListener(this);btn3.addActionListener(this);

Aftertheconstructormethod,addanevent-handlermethodfortheActionListenerinterfacepublicvoidactionPerformed(ActionEventevent){}

Betweenthecurlybracketsoftheevent-handler,insertifstatementstodisplayadialogwhenabuttongetsclickedif(event.getSource()==btn1)JOptionPane.showMessageDialog(this,“Information...”,“MessageDialog”,JOptionPane.INFORMATION_MESSAGE);

if(event.getSource()==btn2)JOptionPane.showMessageDialog(this,“Warning...”,“MessageDialog”,JOptionPane.WARNING_MESSAGE);

if(event.getSource()==btn3)JOptionPane.showMessageDialog(this,“Error...”,“MessageDialog”,JOptionPane.ERROR_MESSAGE);

SavetheprogramasMessages.javathencompileandruntheprogram,clickingoneachbutton

Youcanalsosimplyspecifytheparentandmessageastwoargumentstocreateadialogwiththedefaultinformationiconandthedefault“Message”title.

Requestinginput

TheSwingJOptionPaneclasscanrequestinformationfromtheuserbyopeningadialogboxwithitsshowConfirmationDialog()method,requestingadecision,orwithitsshowInputDialog()method,requestinguserinput.

Boththesemethodscantakefourarguments:• Parentobject–typicallyreferencedbythethiskeyword• RequestStringtobedisplayed• DialogtitleString

• OneoftheJOptionPaneconstantssuchasPLAIN_MESSAGEortospecifydialogdecisionbuttonsasYES_NO_CANCEL_OPTION

ThedialogboxwillreturntheinputStringfromaninputdialogoranintegerfromadecisionbutton–zeroforyes,1forno,or2forcancel.

EditacopyofWindow.javafromhere,changingtheclassnameinthedeclaration,theconstructor,andtheinstancestatementfrom“Window”to“Request”.Then,addaninitialstatementtoimportthefunctionalityofthejava.awt.eventpackageimportjava.awt.event.*;

Request.java

EdittheclassdeclarationtoaddanActionListenerinterfacetotheprogramclassRequestextendsJFrameimplementsActionListener

BeforetheRequest()constructor,createaJTextFieldandtwoJButtoncomponentsJTextFieldfield=newJTextField(38);JButtonbtn1=newJButton(“RequestDecision”);JButtonbtn2=newJButton(“RequestInput”);

AddeachcomponenttotheJPanelcontainerpnl.add(field);pnl.add(btn1);pnl.add(btn2);

IntheRequest()constructor,insertstatementstomakeeachbuttongenerateanActionEventeventbtn1.addActionListener(this);btn2.addActionListener(this);

Aftertheconstructormethod,addanevent-handlermethodfortheActionListenerinterfacepublicvoidactionPerformed(ActionEventevent){}

Betweenthecurlybracketsoftheevent-handler,insertanifstatementtorespondtoadecisionbuttonclickif(event.getSource()==btn1)

{

intn=JOptionPane.showConfirmDialog(this,“Doyouagree?”,“ConfirmationDialog”,JOptionPane.YES_NO_CANCEL_OPTION);

switch(n)

{

case0:field.setText(“Agreed”);break;case1:field.setText(“Disagreed”);break;case2:field.setText(“Canceled”);break;

}

}

Insertanifstatementtohandleuserinputif(event.getSource()==btn2)field.setText(JOptionPane.showInputDialog(this,

“Enteryourcomment”,“InputDialog”,JOptionPane.PLAIN_MESSAGE));

SavetheprogramasRequest.javathencompileandruntheprogram,clickingoneachbutton

TheOK_CANCELconstantprovidestwodecisionbuttons–OKreturnszeroandCANCELreturns2.Refertothedocumentationforthefullrangeofconstants.

Summary

• TheimplementskeywordcanbeusedinaclassdeclarationtoaddoneormoreEventListenerinterfaces.

• Acomponent’saddActionListener()methodtakesthethiskeywordasitsargument–tomakethatcomponentgenerateanActionEventeventwhenitisactivated.

• TheActionListenerinterfacepassesageneratedActionEventeventastheargumenttoitsactionPerformed()event-handler,whichcanrespondtoapushbuttonclickmadebytheuser.

• ThegetSource()methodofanActionEventeventcanbeusedtoidentifytheoriginatingcomponentthatgeneratedtheevent.

• AnItemListenerinterfacepassesageneratedItemEventeventastheargumenttoitsitemStateChanged()event-handler,whichcanrespondtoanitemselectionmadebytheuser.

• ThegetItemSelectable()methodofanItemEventeventcanbeusedtoidentifythecomponentthatgeneratedtheevent.

• AKeyListenerinterfacepassesageneratedKeyEventeventastheargumenttothreerequiredevent-handlermethods,whichcanrespondtoakeypressandrevealthatkey’scharacter.

• AMouseListenerinterfacepassesageneratedMouseEventeventastheargumenttofiverequiredevent-handlermethods,whichcanrespondtomouseactionsmadebytheuser.

• AMouseMotionListenerinterfacepassesageneratedMouseEventeventastheargumenttotworequiredevent-handlers,whichcanrespondtomousemovement.

• TheshowMessageDialog()methodoftheJOptionPaneclasscreatesadialogdisplayingamessagetotheuser,anditsshowInputDialog()andshowConfirmationDialog()methodscanbeusedtorequestuserinput.

• AudioresourcescanberepresentedbytheAudioClipclassofthejava.appletpackage,andplayedusingitsplay()method.

10

Deployingprograms

ThischapterdemonstrateshowtodeployJavaprograms–bothasJavaarchives(JAR)andAndroid

applicationpackages(APK).

ProducinganapplicationDistributingprograms

BuildingJavaarchivesDeployingapplicationsCreatingAndroidprojectsExploringprojectfiles

Addingresources&controlsInsertingJavacodeTestingtheapplicationDeployingAndroidapps

Summary

Producinganapplication

Javaapplicationsforbothdesktopandhandhelddevicescanbecreatedfromcommoncode–liketheLotto.javaprogrambelow:importjavax.swing.*;importjava.awt.event.*;

ComponentspublicclassLottoextendsJFrameimplementsActionListener

{

ClassLoaderldr=this.getClass().getClassLoader();java.net.URLiconURL=ldr.getResource(“Lotto.png”);ImageIconicon=newImageIcon(iconURL);JLabelimg=newJLabel(icon);JTextFieldtxt=newJTextField(“”,18);JButtonbtn=newJButton(“GetMyLuckyNumbers”);JPanelpnl=newJPanel();

ConstructorpublicLotto(){

super(“LottoApp”);setSize(260,210);setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);pnl.add(img);pnl.add(txt);pnl.add(btn);btn.addActionListener(this);add(pnl);setVisible(true);

}

Event-handlerpublicvoidactionPerformed(ActionEventevent)

{

if(event.getSource()==btn)

{

int[]nums=newint[60];Stringstr=“”;for(inti=1;i<60;i++){nums[i]=i;}for(inti=1;i<60;i++){intr=(int)(59*Math.random())+1;intt=nums[i];nums[i]=nums[r];nums[r]=t;

}

for(inti=1;i<7;i++){str+=“”+Integer.toString(nums[i])+“”;}txt.setText(str);

}

}

Entry-pointpublicstaticvoidmain(String[]args){Lottolotto=newLotto();}

}

}

Lotto.java

Thealgorithminthisevent-handlershufflesintegers1-59inanarray,thenassignsthoseintegersinthefirstsixelementstoastring.

TheLottoprogrambeginswithimportstatementstomakeSwingcomponentsandtheActionListenerinterfaceavailable.

Lotto.png–Checkeredareasaretransparent

Components

Theprogramcomprisesasinglepanelcomponentcontainingalabelcomponenttodisplayanimage,atextfieldcomponenttodisplayoutput,andabuttoncomponentforuserinteraction.

Constructor

TheLotto()constructorbuildsasimpleSwinginterfacethatloadsthepanelintoawindowframemeasuring260x210.

Event-handlerThebutton’sevent-handlermethodexecutesanalgorithmtoselectasequenceofsixuniquerandomnumbersintherangeof1-59fordisplayinthetextfieldcomponent.

Entry-pointThemain()methodcreatesaninstanceoftheapp,andcallsuponClassLoader()toseektheimagefileresourceLotto.pnginthesamedirectoryastheprogramfile.Thefilesmustbearrangedinthiswaybeforeattemptingtocompiletheprogram.

TheLottoprogramisusedthroughoutthischaptertocreateappsfordesktopandhandhelddevices.

Observingtherequiredfilearrangement,thejavaccompilercanbeemployedintheusualwaytocreateaLotto.classfile,thenthejavainterpretercanbeemployedtoexecutetheprogram:

Distributingprograms

TheLottoprogramopensanewwindowofthespecifiedsizecontainingtheSwinginterfacecomponents.Eachtimetheuserclicksthepushbutton,itsevent-handlerdisplayssixnewrandomnumbersintherange1-59withinthetextfield

component:

Aswithallotherexamplesinthisbook,theexampleLottoprogramhasbeencompiledhereforJava9andcanbedistributedforexecutiononothercomputerswheretheJava9RuntimeEnvironmentispresent–regardlessoftheiroperatingsystem.Forexample,inthescreenshotsbelow,Lotto.classandLotto.pngfileshavebeencopiedtothedesktopofacomputerrunningtheLinuxoperatingsystemwiththeJava9runtimeinstalled.TheLottoprogramcan,therefore,beexecutedbythejavainterpreterinthesamewayasontheoriginatingWindowssystem.

The.javasourcecodefileneednotbeincludedwhendistributingaprogram–only.classandresourcefilesareneeded.

Thereis,however,adangerindistributingJavaprogramsthiswayastheprogramwillfailtoexecuteifresourcefilesbecomeunavailable–inthiscase,

removingLotto.pngproducesthiserror:

TheJDKcontainsajarutilitytoolthatallowsprogramclassandresourcefilestobebundledintoasingleJavaARchive(JAR)file.Thiscompressesallprogramfiles,usingthepopularZIPformat,intoasinglefilewitha.jarfileextension.AJARfilestorestheprogramefficientlyandhelpsensurethatresourcefilesdonotbecomeaccidentallyisolated.ThejavainterpreterfullysupportsJARfilessoJavaapplicationscanbeexecutedwithoutextractingtheindividualfiles.Likethejavainterpreterandjavaccompiler,thejartoolislocatedinJava’sbindirectoryandrunsfromthecommandlinetoperformthesecommonjaroperations:

Commandsyntax: Operation:

jarcfjar-fileinput-file/s CreateaJARfile

jarcfejar-fileentry-pointinput-file/s CreateaJARfilewithaspecifiedentrypointinastand-aloneapplication

jartfjar-file ViewcontentsofaJARfile

jarufjar-file UpdatecontentsofJARfile

jarufmjar-fileattribute-file UpdatecontentsofJARfilemanifest,addingattribute/s

jarxfjar-file ExtractallcontentsofJAR

jarxfjar-filearchived-file/s ExtractspecificfilesfromJAR

Largerprogramsmayusemanyresourcefileswhoselocationcaneasilybedisruptedbyauser–thesolutionistopackagetheprogramandallits

resourcesintoasingleexecutablearchivefile.

Forlargerprograms,the*wildcardcharactercanbeusedtoarchivemultiplefileswithinthedirectory–forinstance,jarcfProgram.jar*.classarchivesallclassfilesinthecurrentdirectory.

BuildingJavaarchives

FollowthesestepstocreateaJARfilefortheLottoprogramdescribedatthestartofthischapter:

Openacommand-line/terminalwindow,thennavigatetothedirectorywheretheLottoprogramfilesarelocated–Lotto.classandLotto.png

Lotto.jar

EnterjarcfeLotto.jarLottoLotto.classLotto.png,thenhittheEnterkeytocreateaLotto.jararchiveNext,enterjartfLotto.jartoseeallcontentsoftheJAR

NoticethatthejartoolautomaticallycreatesaMETA-INFdirectoryalongsidethearchivedfiles.Thiscontainsatext-basedmanifestmetafilenamedMANIFEST.MFthatyoucanexamine:

Now,enterjarxfLotto.jarMETA-INFtoextractacopyoftheMETA-INFdirectoryFinally,entertypeMETA-INF\MANIFEST.MFtoseethetextcontainedwithinthearchivemanifest

TheJARmanifestcanbemodifiedforadvancedpurposes,suchastheadditionofpermissionstousesystemresources.

Deployingapplications

JavaJARfilesareexecutableonanysystemonwhichtheappropriateversionoftheJavaRuntimeisinstalled:

Atthecommandline,navigatetothedirectorywheretheLotto.jarfileislocated,thentypejava-jarLotto.jarandhittheEnterkeytoruntheLottoapplication

Alternatively,double-clickorright-clicktheLotto.jarfileicon,andchooseto“OpenWith”theJavaRuntime

The.jarfileextensionisrequiredwhenexecutingJARfilesfromaprompt.

SettheJREasthedefaultJARfilehandleronyoursystemforpermanentdouble-clickexecution.

CreatingAndroidprojects

TheAndroidoperatingsystem,prevalentonhandhelddevices,includesasetofcorelibrariesthatprovidemostofthefunctionalityofthoseintheJavaprogramminglanguage.ThismeansthatJavaprogramscanbereadilyconvertedforAndroid.

AndroidStudioisavailablefreefromdeveloper.android.com/studioThisexampledescribesversion2.3.3–instructionsmayvaryforotherversions.AndroidStudioisasizeabledownloadofaround1.9GB,andmayrequireadditionaldownloadstocompletesetup.CheckthesystemrequirementstoensureyourcomputercanrunAndroidStudiobeforedownloading.

AndroidappdevelopmentisbestundertakenusingtheofficialAndroidStudioIntegratedDevelopmentEnvironment(IDE).ThisprovidesaunifiedenvironmentwhereyoucandevelopappsforallAndroiddevices,andprovides

https://developer.android.com/studio/index.html

extensivetestingtools.CompletedappsaredistributedasanAndroidApplicationPackage(APK)archivefile,whichissimilartoaJavaarchive(JAR)file.Thiscompressesallprogramfiles,usingthepopularZIPformat,intoasinglefilewitha.apkfileextension.EachappisfirstcreatedinAndroidStudioasa“project”,towhichthedeveloperaddscodeandresources:

LaunchAndroidStudio,thenchoosetoStartanewAndroidStudioprojectinthe“Welcome”dialogoptions–toopenthe“NewProject”dialogEnteranApplicationname(forexample,“Lotto”)andaCompanyDomain,thenchooseyourpreferredProjectlocationatwhichtosavetheprojectonyourcomputer

ClickNexttoopenthe“TargetAndroidDevices”dialogSelectthedevicetypeandplatformlevel(forexample,“PhoneandTablet”running“IceCreamSandwich”)

ThePackagenameisanautomaticallyassigneduniqueidentifierfortheapp,comprisedofcom.domain.appname

ClickNexttoopenthe“AddanActivitytoMobile”dialog,andsimplyselecttheEmptyActivityoptionClickNexttoopenthe“CustomizetheActivity”dialog

ClickFinishtoacceptthesuggestedconfiguration–AndroidStudiowillnowgenerateseveralfilesandfoldersforthenewproject(thiscantakeawhile)theneventuallyopentheIDEinterface

ChooseAPI15ifyouwouldliketheapptorunon100%ofdevicesactiveontheGooglePlayStore.

Youcanchangethesuggestednameshere,butbesuretoleavetheGenerateLayoutFileandBackwardsCompatibilityboxeschecked.

Exploringprojectfiles

TheAndroidStudioIDEprovidesaProjectwindowthatdisplaysanexpandabletreeviewofallfilesandfolderswithintheproject.YoucanclickanyarrowintheProjectwindowtoexpandafolder,anddouble-clickonanyfileintheProjectwindowtoopenitinthetabbedEditorwindow.

Despiteitsinitialappearanceofcomplexity,onlytwofilesneedmodificationbythedevelopertocreateacustomizedapplication:• MainActivity.java–theJavafilethatloadscontrolsintotheappinterfaceandcancontainevent-handlercodetorespondtouseractionswithintheappinterface.

• activity_main.xml–theXMLfilethatdefineseachcontroltoappearintheappinterfaceandtheirlayout.

IftheProjectwindowisnotimmediatelyvisibleclicktheProjectbuttonintheleftsidebar,oropenitusingtheshortcutkeysAlt+F1.

Selectingtheactivity_main.xmlfileintheProjectwindowpresentsitintheEditorwindowinoneoftwopossibleviews–visuallyinDesignvieworprogrammaticallyinTextview.Therearetabsatthebottomofthewindowtoswitchbetweenviews.

DesignviewprovidesaPaletteofcontrolsthatcanbedraggedontoagraphicalrepresentationoftheapp,aComponentTreetoselectanyaddedcontrol,andaPropertieswindowinwhichtomodifytheappearanceofaselectedcontrol.

Textviewprovidesa“code-behind”versionofthelayoutthatdescribeseachaspectofaddedcontrolsusingXMLattributes.

ControlscanbeaddedtotheappinterfaceandmodifiedeithervisuallyinDesignvieworprogrammaticallyinTextview.

Addingresources&controlsImageResources

TobegincustomizingthedefaultAndroidStudioemptyappfortheLottoprogram,theimagecanfirstbeaddedasa“resource”:

Right-clickontheLotto.pngimage,andchooseCopyNext,right-clickontheapp>res>drawablefolderandchoosePaste–toseea“Copy”dialogappearRenamethefiletolowercaseLotto.png

ClickOKtoseethefilenowappearinthedrawablefolder

Androidonlysupportslowercasefilenamesforresourceitems.

InterfaceControls

TheLottoappwillrequirethreeinterfacecontrolsalignedoneabovetheotherinaverticallayout.AnImageViewcontrolisrequiredforthelogo,aTextView

averticallayout.AnImageViewcontrolisrequiredforthelogo,aTextViewcontrolisrequiredfortheoutput,andaButtoncontrolisrequiredforuserinteraction:

Opentheactivity_main.xmlfileintheEditor’sTextview

activity_main.xml

InsertthisImageViewcontrolelementimmediatelybeforetheexistingdefaultTextViewelement

<ImageViewandroid:layout_width=”match_parent”android:layout_height=”wrap_content”app:srcCompat=”@drawable/lotto”android:id=”@+id/imageView”app:layout_constraintTop_toTopOf=”parent”/>

Theapp:srcCompatattributereferencestheimageresourceaddedtothedrawablefolder.

Theattributesinthiselementfitthecontroltothewidthofthelayoutcontainer,andpositionthecontrolatthetopofthecontainer.Theimageresourceisdefinedasthecontentsource,andtheelementisgivenanidforreferencebyotherelements.

EdittheexistingTextViewelementtolooklikethis<TextViewandroid:layout_width=”match_parent”android:layout_height=”wrap_content”android:id=”@+id/textViewandroid:height=”60dp”android:textSize=”36sp”android:gravity=”center_horizontal”app:layout_constraintTop_toBottomOf=”@+id/imageView”/>

InsertthiselementrightaftertheTextViewelement<Button

android:layout_width=”match_parent”android:layout_height=”wrap_content”android:id=”@+id/button”android:textSize=”24sp”android:onClick=”lotto”android:text=”GETMYLUCKYNUMBERS”app:layout_constraintTop_toBottomOf=”@+id/textView”/>

Theandroid:idattributeineachelementspecifiesauniquenamebywhichtheelementcanbereferencedinJavacode.

Theandroid:onClickattributespecifiesthenameofanevent-handlertobecalled.

StringResources

Thestringsassignedtodescribetheimagecontentandtospecifythebuttontextshouldeachbeconvertedtoastringresource:

Clickwithinthe“LottoLogo”stringassignmenttogiveitfocus,thenpressAlt+EntertoseeaQuickFixdialog

ChooseExtractstringresource,thenprovidearesourcenameinthe“ExtractResource”dialogthatnowappears

ClickOKtoclosethedialog,andseetheassignedstringgetreplacedbyareferencetothenewstringresource

Repeatthisforthe“GETMYLUCKYNUMBERS”string

InsertingJavacode

Afteraddingresourcesandcontrolshere,theappinterfaceshouldnowresemble

thatoftheLotto.jarapplication:

Ifthecontrolsdonotlooklikethisscreenshot,youshouldreturntohere

andcarefullycheckeachelement’sattributes.

Functionalitycannextbeaddedtotheappbyinsertinganevent-handlerintotheMainActivity.javafilethatwillrespondtothebutton’sonClickeventwhentheuserpushesthebutton.Thiseventautomaticallypassesoneargumenttotheevent-handler,whichisanidentifyingreferencetothecontrolthathasbeenclicked.Consequently,theevent-handlersignaturemustaccommodatetheargumentbyincludingaparameterfortheViewclass–thebaseclassofallwidgets.Themethodmustalsohaveavoidreturntype:

Insertthisevent-handlersignatureintotheMainActivityclass,immediatelyafteritsonCreate()methodblockpublicvoidlotto(Viewvue){}

MainActivity.javaInterfacecontrolscanbereferencedincodebyspecifyingtheiridentityastheargumenttoafindViewById()method.Thisisthenameassignedtotheirandroid:idattributeinactivity_main.xmlprefixedbyR.id.:

Insidetheevent-handlerblock,assignareferencetothe<TextView>controlelementtoavariableTextViewtxt=(TextView)findViewById(R.id.textView);

Finally,insidetheevent-handlerblock,copythecodefromtheevent-handlerintheLotto.javaprogram(listedhere)thatoutputssixuniquerandomnumbersint[]nums=newint[60];Stringstr=“”;for(inti=1;i<60;i++){nums[i]=i;}for(inti=1;i<60;i++)

{

intr=(int)(59*Math.random())+1;intt=nums[i];nums[i]=nums[r];nums[r]=t;

}

for(inti=1;i<7;i++){str+=“”+Integer.toString(nums[i])+“”;}txt.setText(str);

Inthiscase,thereisnoneedforthecodetocheckthesourceofthecallastheevent-handlerisexplicitlyassignedtothebuttonbytheandroid:onClickattributeinactivity_main.xml

NowthattheLottoapphasallresources,controls,andfunctionalcodeinplace,anattemptcanbemadetobuildtheproject:

OntheAndroidStudiomaintoolbar,clickView,ToolWindows,Messages–toopenthe“Messages”windowThen,clickBuild,MakeProject(orpresstheCtrl+F9shortcut)tostartbuildingThe“Messages”windowwillsoondisplayaconfirmationofsuccess,orreportanyerrorsthatneedfixing

Fixanyreportederrorsifnecessary,thenbuildagainuntilyouseeaconfirmationofsuccess

Ifthebuildattemptfails,lookforredlightbulbiconsinthecode–AndroidStudioaddsthesesoyoucaneasilyfindandcorrecterrors.

Testingtheapplication

Onceanapplicationprojecthasbuiltsuccessfully,itisreadytobetested.

Onceanapplicationprojecthasbuiltsuccessfully,itisreadytobetested.TestingcanbeperformedonarealAndroiddevice,connectedtoyourcomputerviaaUSBsocket,oronanAndroidVirtualDevice(AVD)emulator.AVDsallowyoutotesthowtheappwillperformonarangeofdevices,buttheydousemoresystemresourcesandcanbepainfullyslow.Applicationsshouldbetestedonatleastonephonedeviceandonetabletdevice:

ClickRun,Run‘app’(orpresstheShift+F10shortcut)toopenthe“SelectDeploymentTarget”dialog

SelectanAVDphoneemulatortoconnect,thenclickOKtoinstalltheLottoappontheemulator

PushtheappbuttontoseetheTextViewcontentunhappilywraponthissmalldevicescreen

Editactivity_main.xmltoreducethetextsizeandroid:textSize=”32sp”

ClickBuild,RebuildProjecttoapplythechange,thenruntheappintheemulatoroncemoretoseethesolution

CreationofeachAVDemulatormayrequirealargesystemimagefiletobedownloaded.YoumayprefertotestonarealAndroiddevice.AlsonotethatacceleratedemulatorswillonlyrunifyourcomputerhasaCPUthatsupportshardwarevirtualization(IntelVT-xorAMDSVM).

AndroidStudiohasanInstantRunfeaturethatautomaticallyupdateschangestotheappsotheyappearinthetestdevicemorequickly.

Itisbettertotestonrealdevicesratherthantheemulators.Ideally,youshouldtestonasmanydifferentdevicesaspossiblebeforefinaldeploymentofapps.

TakeanAndroidtabletandenable“USBDebugging”ontheSettings,DeveloperOptionsmenuConnectthetablettoyourcomputerviaaUSBporttoseethisnotification

ClickRun,Run‘app’(orpresstheShift+F10shortcut)toopenthe“SelectDeploymentTarget”dialog

Selecttheconnectedtabletdevice,thenclickOKtoinstalltheLottoapponthetabletPushtheappbuttontoseetheLottoapphappilyperformasexpected–asuccessfultest

Thetestprocessesprovideanapplicationlaunchersothetabletcanbedisconnectedandtheapplaunchedbytappingthelaunchericon–butremember,thisisaDebugbuildoftheapp.

IfyoucannotseeDeveloperOptionsontheAndroidSettingsyoucanenableitbytappingontheBuildNumberseventimes–typicallyfoundatSettings,About,SoftwareInformation,BuildNumber.

AndroidStudioprovidesadefaultimagelaunchericon,butyoucanuse

yourownimage.IntheProjectwindow,right-clickontheresfolderthenchooseNew,ImageAssettoopentheAssetStudio,thenselectImageandbrowsetoanimageyouwishtoimportintotheproject.

DeployingAndroidapps

Aftersuccessfultesting,thedevelopmentDebugversionofanappshouldbechangedtoaReleaseversionbeforedeploymentasanAndroidApplicationPackage(APK).AndroidrequiresthatallAPKsmustbedigitallysignedwithacertificate,andAndroidStudioallowsyoutoeasilygenerateasignedAPK:

ClickBuild,SelectBuildVariant,thenchoosethereleaseversionoption

Next,clickBuild,GenerateSignedAPK

EnterexistingkeystoredetailsorclickCreatenew...

Choosealocationatwhichtosavethekeystore,thenenterpasswords

Selectyourpreferredvalidityperiod,thenentercertificateinformation

ClickOKtocreatethenewkeystore

ClickNexttousethenewkeystoretogenerateasignedAPKfortheapp

AdditionalstepsarerequiredifyouwishtodistributeyourappsviatheGooglePlayStore.Youcandiscoverwhatisneededonlineatdeveloper.android.com/distribute/tools/launch-checklist.html

Akeystoreholdsoneormorecorrespondingpublic/privatekeypairs.You,astheownerofthecertificate,retaintheprivatekeywhiletheAndroidStudiosigningtoolattachesthepublickeycertificatetotheAPK.ThisuniquelyassociatestheAPKtoyouandyourcorrespondingprivatekeytoensurethatanyfutureupdatestotheAPKcomefromtheoriginaldeveloper.

ChoosealocationatwhichtosavetheAPKandbesuretheBuildTypeis“release”,thenclickFinishtogenerateasignedAPKnamedapp-release.apk

https://developer.android.com/distribute/best-practices/launch/launch-checklist.html

Uponsuccess,changetheAPKnametoamoremeaningfulLotto.apk

Aproductflavorcanbespecifiedtodefinedifferentcustomizedbuildsoftheapp.Forexample,thebuttontextonaSpanishlanguageflavoras“Consiguemisnúmerosdelasuerte”.

TheAndroidappAPKcannowbedeployedinseveralways:• AppMarketplace–publishonGooglePlayStore• Email–senddirectasanattachment• Website–hostonlinefordownloadInstallationofAPKsfromsourcesotherthanGooglePlayisblockedunlesstheuseroptstoallowthem:

OnanAndroiddevice,gotoSettings,Securityandoptto“Allowinstallationofappsfromunknownsources”

Next,downloadLotto.apktotheAndroiddeviceNavigatetothedownloadfolderandclickInstall,thentapthe launchericontoruntheapp

Summary

• Javaprogramscanbedeployedasstand-alonedesktopapplicationsrunningonanappropriateversionoftheJRE.

• Applicationfilescanbedistributedforexecutiononotheroperatingsystemsusingtheappropriatejavainterpreter.

• BundlingallprogramfilesintoasingleJARarchivefilehelpsensureresourcefilesdonotbecomeaccidentallyisolated.

• ExecutableJARapplicationscanbeexecutedfromapromptwiththejava-jarcommandorbyclickingontheirfileicon.

• JavaprogramscanbereadilyconvertedfortheAndroidoperatingsystemasitincludessimilarcorelibraries.

• AndroidStudioistheofficialIDEforthedevelopmentofAndroidapplications.

• APKandJARarchivefilesarebothcompressedinZIPformat.

• EachAndroidappisfirstcreatedasaprojecttowhichthedeveloperaddscodeandresources.

• ThemostusedwindowsintheAndroidStudiointerfacearetheProjectwindowandtheEditorwindow.

• FunctionalcodecanbeaddedtotheMainActivity.javafileandinterfacecomponentsaddedtotheactivity_main.xmlfile.

• AnAndroidappcanstoreimagesandstringsasresources.

• Thesignatureofabutton’sonClickevent-handlermustincludeaparameterforaViewclassobject.

• InterfacecomponentscanbereferencedincodebyspecifyingtheiridentityastheargumenttofindViewById().

• AndroidStudioprovidesAVDemulatorsfortestingandalsoallowstestingtobeperformedonrealconnecteddevices.

• TheReleaseversionofanAndroidappmustbedigitallysignedwithapublickeycertificate.

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