Introduction to Java Programming - Tutorial

8/26/2015 IntroductiontoJavaprogrammingTutorial

http://www.vogella.com/tutorials/JavaIntroduction/article.html 1/29

IntroductiontoJavaprogrammingTutorial

LarsVogel

Version2.6

Copyright2008,2009,2010,2011,2012,2013,2014,2015vogellaGmbH

02.04.2015

IntroductiontoJavaprogramming

ThistutorialexplainstheinstallationandusageoftheJavaprogramminglanguage.Italsocontainsexamplesforstandardprogrammingtasks.

TableofContents

1.IntroductiontoJava

1.1.History1.2.Javaandopensourcesoftware1.3.Javavirtualmachine1.4.JavaRuntimeEnvironmentvs.JavaDevelopmentKit1.5.CharacteristicsofJava1.6.DevelopmentProcesswithJava1.7.Garbagecollector1.8.Classpath

2.InstallationofJava

2.1.Checkinstallation2.2.InstallJavaonUbuntu2.3.InstallJavaonMSWindows2.4.Installationproblemsandotheroperatingsystems2.5.Validateinstallation2.6.Howcanyoutellyouareusinga32bitor64bitversionofJava?

3.Exercise:Write,compileandrunaJavaprogram

3.1.Writesourcecode3.2.CompileandrunyourJavaprogram3.3.Usingtheclasspath

4.Javalanguagestructure

4.1.Basics:Package,ClassandObject4.2.Package4.3.Class4.4.Object4.5.Inheritance4.6.Objectassuperclass

5.Javainterfaces

5.1.WhatisaninterfaceinJava?5.2.Abstract,defaultandstaticmethodsinInterfaces5.3.ImplementingInterfaces5.4.Evolvinginterfaces5.5.Multipleinheritanceofmethods5.6.Functionalinterfaces

6.Javabasicterms

6.1.Overridemethodsandthe@Overrideannotation

7.ThetypesystemofJava

7.1.Primitivesandreferences7.2.Primitives7.3.Referencetypes7.4.Autoboxingandwrappertypes

8.Variablesandmethods

8.1.Variable

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8.2.Instancevariable8.3.Localvariable8.4.Methods8.5.Mainmethod8.6.Constructor

9.Modifiers

9.1.Accessmodifiers9.2.Othermodifiers

10.Importstatements

10.1.Usageofimportstatements10.2.Staticimports

11.MoreJavalanguageconstructs

11.1.Classmethodsandclassvariables11.2.Abstractclassandmethods

12.CheatSheets

12.1.Workingwithclasses12.2.Workingwithlocalvariable

13.IntegratedDevelopmentEnvironment14.ExercisesCreatingJavaobjectsandmethods

14.1.CreateaPersonclassandinstantiateit14.2.Useconstructor14.3.Definegetterandsettermethods14.4.CreateanAddressobject

15.SolutionCreatingJavaobjectsandmethods

15.1.CreateaPersonclassandinstantiateit15.2.Useconstructor15.3.Definegetterandsettermethods15.4.SolutionCreateanAddressobject

16.Javastatements

16.1.ifthenandifthenelse16.2.Switch16.3.BooleanOperations

17.LoopsinJava

17.1.Theforloop17.2.Thewhileloop17.3.Thedowhileloop

18.Arrays

18.1.ArraysinJava18.2.EnhancedforloopforArraysandCollections

19.Strings

19.1.StringsinJava19.2.StringpoolinJava19.3.CompareStringsinJava19.4.WorkingwithStrings

20.Lambdas

20.1.Whatarelambdas?20.2.Differencebetweenalambdaexpressionandaclosure20.3.Purposeoflambdaexpressions20.4.Usingmethodreferences

21.Streams

21.1.WhatareStreamsinJava8?21.2.IntStream21.3.Reductionoperationswithstreamsandlambdas

22.TypeConversion

22.1.ConversiontoString22.2.ConversionfromStringtoNumber22.3.Doubletoint22.4.SQLDateconversions

23.Scheduletasks24.Aboutthiswebsite25.LinksandLiterature



1.IntroductiontoJava

1.1.History

JavaisaprogramminglanguagecreatedbyJamesGoslingfromSunMicrosystems(Sun)in1991.ThefirstpubliclyavailableversionofJava(Java1.0)wasreleasedin1995.

SunMicrosystemswasacquiredbytheOracleCorporationin2010.OraclehasnowthesteermanshipforJava.

OvertimenewenhancedversionsofJavahavebeenreleased.ThecurrentversionofJavaisJava1.7whichisalsoknownasJava7.

FromtheJavaprogramminglanguagetheJavaplatformevolved.TheJavaplatformallowssoftwaredeveloperstowriteprogramcodeinotherlanguagesthantheJavaprogramminglanguagewhichstillrunsontheJavavirtualmachine.TheJavaplatformisusuallyassociatedwiththeJavavirtualmachineandtheJavacorelibraries.

1.2.Javaandopensourcesoftware

In2006SunstartedtomakeJavaavailableundertheGNUGeneralPublicLicense(GPL).OraclecontinuesthisprojectcalledOpenJDK.

1.3.Javavirtualmachine

TheJavavirtualmachine(JVM)isasoftwareimplementationofacomputerthatexecutesprogramslikearealmachine.

TheJavavirtualmachineiswrittenspecificallyforaspecificoperatingsystem,e.g.,forLinuxaspecialimplementationisrequiredaswellasforWindows.

JavaprogramsarecompiledbytheJavacompilerintobytecode.TheJavavirtualmachineinterpretsthisbytecodeandexecutestheJavaprogram.

1.4.JavaRuntimeEnvironmentvs.JavaDevelopmentKit

AJavadistributiontypicallycomesintwoflavors,theJavaRuntimeEnvironment(JRE)andtheJavaDevelopmentKit(JDK).

TheJavaruntimeenvironment(JRE)consistsoftheJVMandtheJavaclasslibraries.ThosecontainthenecessaryfunctionalitytostartJavaprograms.

TheJDKadditionallycontainsthedevelopmenttoolsnecessarytocreateJavaprograms.TheJDKthereforeconsistsofaJavacompiler,theJavavirtualmachineandtheJavaclasslibraries.

1.5.CharacteristicsofJava

ThetargetofJavaistowriteaprogramonceandthenrunthisprogramonmultipleoperatingsystems.

Javahasthefollowingproperties:

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Platformindependent:JavaprogramsusetheJavavirtualmachineasabstractionanddonotaccesstheoperatingsystemdirectly.ThismakesJavaprogramshighlyportable.AJavaprogram(whichisstandardcompliantandfollowscertainrules)canrununmodifiedonallsupportedplatforms,e.g.,WindowsorLinux.

Objectorientatedprogramminglanguage:Excepttheprimitivedatatypes,allelementsinJavaareobjects.

Stronglytypedprogramminglanguage:Javaisstronglytyped,e.g.,thetypesoftheusedvariablesmustbepredefinedandconversiontootherobjectsisrelativelystrict,e.g.,mustbedoneinmostcasesbytheprogrammer.

Interpretedandcompiledlanguage:Javasourcecodeistransferredintothebytecodeformatwhichdoesnotdependonthetargetplatform.ThesebytecodeinstructionswillbeinterpretedbytheJavaVirtualmachine(JVM).TheJVMcontainsasocalledHotspotCompilerwhichtranslatesperformancecriticalbytecodeinstructionsintonativecodeinstructions.

Automaticmemorymanagement:Javamanagesthememoryallocationanddeallocationforcreatingnewobjects.Theprogramdoesnothavedirectaccesstothememory.Thesocalledgarbagecollectorautomaticallydeletesobjectstowhichnoactivepointerexists.

TheJavasyntaxissimilartoC++.Javaiscasesensitive,e.g.,variablescalled myValue andmyvalue aretreatedasdifferentvariables.

1.6.DevelopmentProcesswithJava

Javasourcefilesarewrittenasplaintextdocuments.TheprogrammertypicallywritesJavasourcecodeinanIntegratedDevelopmentEnvironment(IDE)forprogramming.AnIDEsupportstheprogrammerinthetaskofwritingcode,e.g.,itprovidesautoformatingofthesourcecode,highlightingoftheimportantkeywords,etc.

Atsomepointtheprogrammer(ortheIDE)callstheJavacompiler(javac).TheJavacompilercreatesthebytecodeinstructions.Theseinstructionsarestoredin.classfilesandcanbeexecutedbytheJavaVirtualMachine.

1.7.Garbagecollector

TheJVMautomaticallyrecollectsthememorywhichisnotreferredtobyotherobjects.TheJavagarbagecollectorchecksallobjectreferencesandfindstheobjectswhichcanbeautomaticallyreleased.

Whilethegarbagecollectorrelievestheprogrammerfromtheneedtoexplicitlymanagememory,theprogrammerstillneedtoensurethathedoesnotkeepunneededobjectreferences,otherwisethegarbagecollectorcannotreleasetheassociatedmemory.Keepingunneededobjectreferencesaretypicallycalledmemoryleaks.

1.8.Classpath

TheclasspathdefineswheretheJavacompilerandJavaruntimelookfor.classfilestoload.TheseinstructionscanbeusedintheJavaprogram.

Forexample,ifyouwanttouseanexternalJavalibraryyouhavetoaddthislibrarytoyourclasspathtouseitinyourprogram.

2.InstallationofJava

2.1.Checkinstallation

TorunJavaprogramsonyourcomputeryoumustatleasthavetheJavaruntimeenvironment(JRE)installed.Thismightalreadybethecaseonyourmachine.YoucantestistheJREisinstalledandinyourcurrentpathbyopeningaconsole(ifyouareusingWindows:Win+R,entercmdandpressEnter)andbytypinginthefollowingcommand:

javaversion

IftheJREisinstalledandwithinyourpath,thiscommandsprintinformationaboutyourJavainstallation.InthiscaseyoucanskiptheJavainstallationdescription.Youmaywanttonotedownifyouhavea32bitor64bitversionofJava,seeSection2.6,Howcanyoutellyouareusinga32bitor64bitversionofJava?.

Ifthecommandlinereturnstheinformationthattheprogramcouldnotbefound,youhavetoinstallJava.

2.2.InstallJavaonUbuntu

OnUbuntuyoucaninstallJava7viathefollowingcommandonthecommandline.



sudoaptgetinstallopenjdk7jdk

2.3.InstallJavaonMSWindows

ForMicrosoftsWindows,OracleprovidesanativeinstallerwhichcanbefoundontheOraclewebsite.ThecentralwebsiteforinstallingJavaislocatedunderthefollowingURLandalsocontainsinstructionshowtoinstallJavaforotherplatforms.

java.com

2.4.Installationproblemsandotheroperatingsystems

IfyouhaveproblemsinstallingJavaonyoursystem,searchviaGoogleforHowtoinstallJDKonYOUR_OS.Thisshouldresultinhelpfullinks.ReplaceYOUR_OSwithyouroperatingsystem,e.g.,Windows,Ubuntu,MacOSX,etc.

2.5.Validateinstallation

Switchagaintothecommandlineandrunthefollowingcommand.

javaversion

Theoutputshouldbesimilartothefollowingoutput.

javaversion"1.7.0_25"OpenJDKRuntimeEnvironment(IcedTea2.3.10)(7u252.3.101ubuntu0.13.04.2)OpenJDK64BitServerVM(build23.7b01,mixedmode)

2.6.Howcanyoutellyouareusinga32bitor64bitversionofJava?

Youcanruna32bitora64bitversionofJavaona64bitsystem.Ifyouuse javaversion andtheoutputcontainsthe"64Bit"stringyouareusingthe64bitversionofJavaotherwiseyourareusingthe32bitversion.Thefollowingistheoutputofa64bitversion.

javaversion"1.7.0_25"OpenJDKRuntimeEnvironment(IcedTea2.3.10)(7u252.3.101ubuntu0.13.04.2)OpenJDK64BitServerVM(build23.7b01,mixedmode)

3.Exercise:Write,compileandrunaJavaprogram

3.1.Writesourcecode

ThefollowingJavaprogramisdevelopedunderLinuxusingatexteditorandthecommandline.Theprocessonotheroperatingsystemshouldbesimilar,butisnotcoveredinthisdescription.

SelectorcreateanewdirectorywhichwillbeusedforyourJavadevelopment.Inthisdescriptionthepath\home\vogella\javastarterisused.OnMicrosoftWindowsyoumightwanttousec:\temp\javastarter.Thispathiscalledjavadirinthefollowingdescription.

Openatexteditorwhichsupportsplaintext,e.g.,geditunderLinuxorNotepadunderWindowsandwritethefollowingsourcecode.

//asmallJavaprogrampublicclassHelloWorld{publicstaticvoidmain(String[]args){System.out.println("HelloWorld");}}

Warning: DonotusearicheditorlikeMicrosoftWordorLibreOfficeforwritingJavacode.Ifindoubt,googlefor"Plaintexteditorfor[your_OS]".



SavethesourcecodeinyourjavadirdirectorywiththeHelloWorld.javafilename.ThenameofaJavasourcefilemustalwaysequaltheclassname(withinthesourcecode)andendwiththe.javaextension.InthisexamplethefilenamemustbeHelloWorld.java,becausetheclassiscalled HelloWorld .

3.2.CompileandrunyourJavaprogram

Openashellforcommandlineaccess.

Tip: Ifyoudon'tknowhowtodothis,googlefor"Howtoopenashellunder[your_OS]".

Switchtothejavadirdirectorywiththecommand cdjavadir ,forexample,intheaboveexampleviathe cd\home\vogella\javastarter command.Usethe ls command( dir underMicrosoftWindows)toverifythatthesourcefileisinthedirectory.

CompileyourJavasourcefileintoaclassfilewiththefollowingcommand.

javacHelloWorld.java

Afterwardslistagainthecontentofthedirectorywiththe ls or dir command.ThedirectorycontainsnowafileHelloWorld.class.Ifyouseethisfile,youhavesuccessfullycompiledyourfirstJavasourcecodeintobytecode.

Tip: Bydefault,thecompilerputseachclassfileinthesamedirectoryasitssourcefile.Youcanspecifyaseparatedestinationdirectorywiththedcompilerflag.

YoucannowstartyourcompiledJavaprogram.EnsurethatyouarestillinthejardirdirectoryandenterthefollowingcommandtostartyourJavaprogram.

javaHelloWorld

Thesystemshouldwrite"HelloWorld"onthecommandline.

3.3.Usingtheclasspath

Youcanusetheclasspathtoruntheprogramfromanotherplaceinyourdirectory.

Switchtothecommandline,e.g.,underWindowsStartRun cmd .Switchtoanydirectoryyouwant.Type:

javaHelloWorld

Ifyouarenotinthedirectoryinwhichthecompiledclassisstored,thenthesystemwillshowan



errormessage:Exceptioninthread"main"java.lang.NoClassDefFoundError:test/TestClass

Tousetheclass,typethefollowingcommand.Replace"mydirectory"withthedirectorywhichcontainsthetestdirectory.Youshouldagainseethe"HelloWorld"output.

javaclasspath"mydirectory"HelloWorld

4.Javalanguagestructure

4.1.Basics:Package,ClassandObject

ItisimportanttounderstandthebaseterminologyofJavaintermsofpackages,classesandobjects.Thissectiongivesanoverviewoftheseterms.

4.2.Package

Javagroupsclassesintofunctionalpackages.

Packagesaretypicallyusedtogroupclassesintologicalunits.Forexample,allgraphicalviewsofanapplicationmightbeplacedinthesamepackagecalled com.vogella.webapplication.views .

Itiscommonpracticetousethereversedomainnameofthecompanyastoplevelpackage.Forexample,thecompanymightownthedomain,vogella.comandinthisexampletheJavapackagesofthiscompanystartswith com.vogella .

Othermainreasonfortheusageofpackagesistoavoidnamecollisionsofclasses.Anamecollisionoccursiftwoprogrammersgivethesamefullyqualifiednametoaclass.ThefullyqualifiednameofaclassinJavaconsistsofthepackagenamefollowedbyadot(.)andtheclassname.

Withoutpackages,aprogrammermaycreateaJavaclasscalled Test .Anotherprogrammermaycreateaclasswiththesamename.Withtheusageofpackagesyoucantellthesystemwhichclasstocall.Forexample,ifthefirstprogrammerputsthe Test classintopackage report andthesecondprogrammerputshisclassintopackage xmlreader youcandistinguishbetweentheseclassesbyusingthefullyqualifiedname,e.g, xmlreader.Test or report.Test .

4.3.Class

Def.:Templatethatdescribesthedataandbehaviorassociatedwithaninstanceofthatclass.

Note: Theclasscanbeseenastheblueprintofanobject.Itdescribeshowanobjectiscreated.

InJavasourcecodeaclassisdefinedbytheclasskeywordandmuststartwithacapitalletter.Thebodyofaclassissurroundedby{}.

packagetest;

classMyClass{}

Thedataassociatedwithaclassisstoredinvariablesthebehaviorassociatedtoaclassorobjectisimplementedwithmethods.

AclassiscontainedinaJavasourcefilewiththesamenameastheclassplusthe.javaextension.

4.4.Object

Def.:Anobjectisaninstanceofaclass.

Theobjectistherealelementwhichhasdataandcanperformactions.Eachobjectiscreatedbasedontheclassdefinition.

4.5.Inheritance

Aclasscanbederivedfromanotherclass.Inthiscasethisclassiscalledasubclass.Anothercommonphraseisthataclassextendsanotherclass.

Theclassfromwhichthesubclassisderivediscalledasuperclass.

Inheritanceallowsaclasstoinheritthebehavioranddatadefinitionsofanotherclass.

Thefollowingcodesdemonstrateshowaclasscanextendanotherclass.InJavaaclasscanonlyextendamaximumofoneclass.

packagecom.vogella.javaintro.base;



classMyBaseClass{@Overridepublicvoidhello(){System.out.println("HellofromMyBaseClass");}}


classMyExtensionClassextendsMyBaseClass{}

4.6.Objectassuperclass

EveryobjectinJavaimplicitlyextendsthe Object class.TheclassdefinesthefollowingmethodsforeveryJavaobject:

equals(o1) allowscheckingifthecurrentobjectisequaltoo1

getClass() returnstheclassoftheobject

hashCode() returnsanidentifierofthecurrentobject

toString() givesastringrepresentationofthecurrentobject

5.Javainterfaces

5.1.WhatisaninterfaceinJava?

Aninterfacesisatypesimilartoaclassandisdefinedviathe interface keyword.Likeaclassaninterfacedefinesmethods.Classescanimplementtheinterfacesandbythistheymustobeythecontractdefinedintheinterface,e.g.,theclastprovideimplementationforallabstractmethodsdefinedintheinterface.

5.2.Abstract,defaultandstaticmethodsinInterfaces

Aninterfacecanhaveabstractmethodsanddefaultmethods.Adefaultmethodisdefinedviathedefault keywordatthebeginningofthemethodsignature.Allothermethodsdefinedinaninterfacesarepublicandabstractexplicitdeclarationofthesemodifiersisoptional.

Interfacescanhaveconstantswhicharealwaysimplicitlypublic,staticandfinal.

Thefollowingcodeshowsanexampleimplementationofaninterface.

packagetesting;

publicinterfaceMyInterface{

//constantdefinitionStringURL="http://www.vogella.com";

//publicabstractmethodsvoidtest();voidwrite(Strings);//defaultmethoddefaultStringreserveString(Strings){returnnewStringBuilder(s).reverse().toString();}}

Thefollowingclassimplementsthisinterface,itsmustimplementtheabstractmethodandcanusethedefaultmethods.

5.3.ImplementingInterfaces

Aclasscanimplementaninterface.Inthiscaseitmustprovideconcreteimplementationsoftheabstractinterfacemethods.Ifyouimplementamethoddefinedbyaninterface,youcanuse@Override annotationtoindicatetotheJavacompilerthatyouactuallywanttoimplementamethoddefinedbythisinterface.Thiswaythecompilercangiveyouanerrorinyoumistypedthenameofthemethodorinthenumberofarguments.




publicclassMyClassImplimplementsMyInterface{

@Overridepublicvoidtest(){}

@Overridepublicvoidwrite(Strings){}publicstaticvoidmain(String[]args){MyClassImplimpl=newMyClassImpl();System.out.println(impl.reserveString("LarsVogel"));}

}

5.4.Evolvinginterfaces

BeforeJava8evolvinginterfaces,e.g.,addingnewmethodstoaninterface,wasnotpossiblebecausesuchachangewouldbreakexistingimplementations.Java8introduceddefaultmethods,nowyoucanextendaninterfacewithoutbreakingclientsbysimplysupplingadefaultimplementationwithit.Addingsuchadefaultmethodisasourceanbinarycompatiblechange.

Aclasscanalwaysoverrideadefaultmethodsosupplyabetterbehavior.

5.5.Multipleinheritanceofmethods

Ifaclassimplementstwointerfacesandiftheseinterfacesprovidethesamedefaultmethod,Javaresolvesthecorrectmethodfortheclassbythefollowingrules:

1. SuperclasswinsalwaysagainstthesuperinterfaceIfaclasscaninheritamethodfromasuperclassandasuperinterface,theclassinhertsthesuperclassmethod.Thisistrueforconcreteandabstractsuperclassmethods.Thisruleimpliesthatdefaultmethodsarenotusedifthismethodisalsodeclaredinthesuperclasschain.

2. SubtypeswinoverSupertypesIfaclasscaninheritamethodfromtwointerfaces,andoneisasubtypeoftheother,theclassinhertsthemethodfromthesubtype

3. Inallothercasestheclassneedstoimplementthedefaultmethod

Thefollowinglistingdemonstrateslistingnumber3.

publicinterfaceA{defaultvoidm(){}}

publicinterfaceB{defaultvoidm(){}}

publicclassCimplementsA,B{@Overridepublicvoidm(){}}

Inyourimplementationyoucanalsocallthesupermethodyouprefer.

publicclassCimplementsA,B{@Overridepublicvoidm(){A.super.m();}}

5.6.Functionalinterfaces

Allinterfacesthathaveonlyonemethodarecalledfunctionalinterfaces.Functionalinterfaceshavetheadvantagethattheycanbeusedtogetherwithlambdaexpressions.SeeSection20.1,Whatarelambdas?tolearnmoreaboutlambdas,e.g.,thetypeoflambdasisafunctionalinterface.

TheJavacompilerautomaticallyidentifiesfunctionalinterfaces.Theonlyrequirementisthattheyhaveonlyoneabstractmethod.However,ispossibletocapturethedesignintentwitha@FunctionalInterface annotation.

SeveraldefaultJavainterfacesarefunctionalinterfaces:



java.lang.Runnable

java.util.concurrent.Callable

java.io.FileFilter

java.util.Comparator

java.beans.PropertyChangeListener

Javaalsocontainsthe java.util.function packagewhichcontainsfunctionalinterfaceswhicharefrequentlyusedsuchas:

Predicateabooleanvaluedpropertyofanobject

Consumeranactiontobeperformedonanobject

FunctionafunctiontransformingaTtoaR

SupplierprovidesaninstanceofT(suchasafactory)

UnaryOperatorafunctionfromTtoT

BinaryOperatorafunctionfrom(T,T)toT

6.Javabasicterms

6.1.Overridemethodsandthe@Overrideannotation

Ifaclassextendsanotherclass,itinheritsthemethodsfromitssuperclass.Ifitwantstochangethesemethods,itcanoverridethesemethods.Tooverrideamethod,youusethesamemethodsignatureinthesourcecodeofthesubclass.

ToindicatetothereaderofthesourcecodeandtheJavacompilerthatyouhavetheintentiontooverrideamethod,youcanusethe @Override annotation.

Thefollowingcodedemonstrateshowyoucanoverrideamethodfromasuperclass.


classMyBaseClass{@Overridepublicvoidhello(){System.out.println("HellofromMyBaseClass");}}


classMyExtensionClass2extendsMyBaseClass{publicvoidhello(){System.out.println("HellofromMyExtensionClass2");}}

Tip: Itisgoodpracticetoalwaysusethe @Override annotation.ThiswaytheJavacompilervalidatesifyoudidoverrideallmethodsasintendedandpreventserrors.

7.ThetypesystemofJava

7.1.Primitivesandreferences

Javadistinguishesbetweenprimitivetypes(suchasboolean,short,int,double,float,charandbyte)andreferencetypes(suchasObjectandString).

7.2.Primitives

Primitivetypesvariablesrepresentsthenumber,true/falseorcharacter.Youcannotcallmethodsonvariableswhichhaveaprimitivetypes.

Operatorssuchas*,,+or/areonlysupportedforprimitivetypes.Only+canalsousedforthereferencetypeStringtoconcatenatestrings.

7.3.Referencetypes

Referencetypesvariablesrepresentsareferencetoanobject,e.g.,avariableisapointertothe



realobject.Ifyouchangethevalueofareferencetypevariable,thevariablepointstoadifferentobjectorto null whichrepresentsthenonexistingobjectreference.Changingthevalueofareferencetypevariabledoesnotchangetheattributesoftheobjectiswaspointingto.Alsochangingthecontentsofanobjectdoesnotaffectthevalueofavariablereferringtothatobject.

7.4.Autoboxingandwrappertypes

EveryprimitivetypehasinJavaafittingreferencetype.Thisreferencetypeallowstostorethevalueoftheprimitivetypeinanobject.Forexampleyouhave java.lang.Integer forint.

Convertingaprimitivevalueintoaninstanceofawrappertypeandviceversaiscalledboxing/unboxing.Javaperformstheseoperationsautomaticallyofnecessary.Thisallowsyoutouseaprimitiveasparameterinamethodwhichexpectsanobjectofthewrappertype.Thisautomaticboxingandunboxingisknownasautoboxing.

8.Variablesandmethods

8.1.Variable

VariablesallowtheJavaprogramtostorevaluesduringtheruntimeoftheprogram.

Avariablecaneitherbeaprimitivevariableorareferencevariable.Aprimitivevariablecontainsthevaluewhilethereferencevariablecontainsareference(pointer)totheobject.Hence,ifyoucomparetworeferencevariables,youcompareifbothpointtothesameobject.Tocompareobjects,usethe object1.equals(object2) methodcall.

8.2.Instancevariable

Instancevariableisassociatedwithaninstanceoftheclass(alsocalledobject).Accessworksovertheseobjects.

Instancevariablescanhaveanyaccesscontrolandcanbemarked final or transient .Instancevariablesmarkedasfinalcannotbechangedafteravaluehasbeenassignedtothem.

8.3.Localvariable

Local(stack)variabledeclarationscannothaveaccessmodifiers.

finalistheonlymodifieravailabletolocalvariables.Thismodifierdefinesthatthevariablecannotbechangedafterthefirstassignment.

Localvariablesdonotgetdefaultvalues,sotheymustbeinitializedbeforeuse.

8.4.Methods

Amethodisablockofcodewithparametersandareturnvalue.Itcanbecalledontheobject.


publicclassMyMethodExample{voidtester(Strings){System.out.println("HelloWorld");}}

Methodscanbedeclaredwithvarargs.Inthiscasethemethoddeclaresaparameterwhichacceptseverythingfromzerotomanyarguments(syntax:type...name)Amethodcanonlyhaveonevarargsparameterandthismustbethelastparameterinthemethod.

Overwriteofasuperclassmethod:Amethodmustbeoftheexactsamereturnparameterandthesamearguments.Alsothereturnparametermustbethesame.Overloadmethods:Anoverloadedmethodisamethodwiththesamename,butdifferentarguments.Thereturntypecannotbeusedtooverloadamethod.

8.5.Mainmethod

ApublicstaticmethodwiththefollowingsignaturecanbeusedtostartaJavaapplication.Suchamethodistypicallycalled main method.

publicstaticvoidmain(String[]args){

}

8.6.Constructor

Aclasscontainsconstructorsthatareinvokedtocreateobjectsbasedontheclassdefinition.

Constructordeclarationslooklikemethoddeclarationsexceptthattheyusethenameoftheclassandhavenoreturntype.



Aclasscanhaveseveralconstructorswithdifferentparameters.Eachclassmustdefineatleastoneconstructor.

Inthefollowingexampletheconstructoroftheclassexpectsaparameter.


publicclassMyConstructorExample2{

Strings;

publicMyConstructorExample2(Strings){this.s=s;}}

Ifnoexplicitconstructorisdefined,thecompilerimplicitlyaddsaconstructor.Iftheclassissubclassed,thentheconstructorofthesuperclassisalwayscalledimplicitlyinthiscase.

Inthefollowingexamplethedefinitionoftheconstructorwithoutparameters(alsoknownastheemptyconstructor)isunnecessary.Ifnotspecified,thecompilerwouldcreateone.


publicclassMyConstructorExample{

//unnecessary:wouldbecreatedbythecompilerifleftoutpublicMyConstructorExample(){}}

Thenamingconventionforcreatingaconstructoristhefollowing:classname(Parameterp1,...){} .

Everyobjectiscreatedbasedonaconstructor.Thisconstructormethodisthefirststatementcalledbeforeanythingelsecanbedonewiththeobject.

9.Modifiers

9.1.Accessmodifiers

TherearethreeaccessmodifierskeywordsavailableinJava:public,protectedandprivate.

Therearefouraccesslevels:public,protected,defaultandprivate.Theydefinehowthecorrespondingelementisvisibletoothercomponents.

Ifsomethingisdeclaredpublic,e.g.,classesormethodscanbefreelycreatedorcalledbyotherJavaobjects.Ifsomethingisdeclaredprivate,e.g.,amethod,itcanonlybeaccessedwithintheclassinwhichitisdeclared.

Theaccesslevelsprotectedanddefaultaresimilar.Aprotectedclasscanbeaccessedfromthepackageandsubclassesoutsidethepackage,whileadefaultclasscangetaccessedonlyviathesamepackage.

Thefollowingtabledescribesthevisibility:

Table1.AccessLevel

Modifier Class Package Subclass World

public Y Y Y Y

protected Y Y Y N

no modifier Y Y N N

private Y N N N

9.2.Othermodifiers

finalmethods:cannotbeoverwritteninasubclass

abstractmethod:nomethodbody

synchronizedmethod:threadsafe,canbefinalandhaveanyaccesscontrol

nativemethods:platformdependentcode,applyonlytomethods



strictfp:classormethod

10.Importstatements

10.1.Usageofimportstatements

InJavayouhavetoaccessaclassalwaysviaitsfullqualifiedname,i.e.,thepackagenameandtheclassname.

Youcanadd import statementsforclassesorpackagesintoyourclassfile,whichallowyoutousetherelatedclassesinyourcodewithoutthepackagequalifier.

10.2.Staticimports

Staticimportisafeaturethatallowsmembers(fieldsandmethods)whicharedefinedinaclasswiththe publicstatic accessmodifiertobeusedinJavacodewithoutspecifyingtheclassinwhichthememberisdefined.

Thefeatureprovidesatypesafemechanismtoincludeconstantsintocodewithouthavingtoreferencetheclassthatoriginallydefinedthefield.

11.MoreJavalanguageconstructs

11.1.Classmethodsandclassvariables

Classmethodsandclassvariablesareassociatedwiththeclassandnotaninstanceoftheclass,i.e.,objects.Torefertotheseelements,youcanusetheclassnameandadot(".")followedbytheclassmethodorclassvariablename.

Classmethodsandclassvariablesaredeclaredwiththe static keyword.Classmethodsarealsocalledstaticmethodsandclassvariablesarealsocalledstaticvariablesorstaticfields.

Anexamplefortheusageofastaticfieldis println ofthefollowingstatement:System.out.println("HelloWorld") .Hereby out isastaticfield,anobjectoftypePrintStream andyoucallthe println() methodonthisobject.

Ifyoudefineastaticvariable,theJavaruntimeenvironmentassociatesoneclassvariableforaclassnomatterhowmanyinstances(objects)exist.Thestaticvariablecanthereforebeseenasaglobalvariable.

Thefollowingcodedemonstratestheusageof static fields.


publicclassMyStaticExample{

staticStringPLACEHOLDER="TEST";

staticvoidtest(){System.out.println("Hello");}}


publicclassTester{

publicstaticvoidmain(String[]args){System.out.println(MyStaticExample.PLACEHOLDER);MyStaticExample.test();}

}

Ifavariableshouldbedefinedasconstant,youdeclareitwiththe static andthe finalkeyword.

Thestaticmethodrunswithoutanyinstanceoftheclass,itcannotdirectlyaccessnonstaticvariablesormethods.

11.2.Abstractclassandmethods

Aclassandmethodcanbedeclaredas abstract .An abstract classcannotbedirectly



instantiated.

Ifaclasshasatleastonemethod,whichonlycontainsthedeclarationofthemethod,butnottheimplementation,thenthisclassis abstract andcannotbeinstantiated.Subclassesneedthentodefinethemethodsexceptiftheyarealsodeclaredasabstract.

Ifaclasscontainsanabstractmethod,italsoneedstogetdefinedwiththekeyword abstract .

Thefollowingexampleshowsanabstractclass.


publicabstractclassMyAbstractClass{abstractdoublereturnDouble();}

12.CheatSheets

Thefollowingcanbeusedasareferenceforcertaintaskwhichyouhavetodo.

12.1.Workingwithclasses

WhileprogrammingJavayouhavetocreateseveralclasses,methods,instancevariables.Thefollowingusesthepackagetest.

Table2.

What to do How to do it

Create a newclass calledMyNewClass.

packagetest;

publicclassMyNewClass{

}

Create a newattribute(instancevariable) calledvar1 of typeString in theMyNewClass

class

packagetest;

publicclassMyNewClass{privateStringvar1;}

Create aConstructorfor yourMyNewClass

class whichhas a Stringparameter andassigns thevalue of it tothe var1instancevariable.

packagetest;

publicclassMyNewClass{privateStringvar1;

publicMyNewClass(Stringpara1){var1=para1;//orthis.var1=para1;}}

Create a newmethod calleddoSomeThingin your classwhich doesnot return avalue and hasnoparameters.

packagetest;


publicMyNewClass(Stringpara1){var1=para1;//orthis.var1=para1;}

publicvoiddoSomeThing(){

}

}

Create a newmethod calleddoSomeThing2in your classwhich doesnot return a

packagetest;




value and hastwoparameters,an int and aPerson



}

publicvoiddoSomeThing2(inta,Personperson){

}

}

Create a newmethod calleddoSomeThing2in your classwhich returnsan int valueand has threeparameters,two Stringsand aPerson .

packagetest;




}

publicvoiddoSomeThing2(inta,Personperson){

}

publicintdoSomeThing3(Stringa,Stringb,Personperson){return5;//anyvaluewilldoforthisexample}

}

Create a classcalledMyOtherClasswith twoinstancevariables. Onewill store aString , the

other will storea Dog . Creategetter andsetter forthesevariables.

packagetest;

publicclassMyOtherClass{Stringmyvalue;Dogdog;

publicStringgetMyvalue(){returnmyvalue;}

publicvoidsetMyvalue(Stringmyvalue){this.myvalue=myvalue;}

publicDoggetDog(){returndog;}

publicvoidsetDog(Dogdog){this.dog=dog;}}

12.2.Workingwithlocalvariable

Alocalvariablemustalwaysbedeclaredinamethod.

Table3.

What to do How to do it

Declare a (local) variable of type String . Stringvariable1;

Declare a (local) variable of type String and assign"Test" to it.

Stringvariable2="Test";

Declare a (local) variable of type Person Personperson;

Declare a (local) variable of type Person , create a new Personperson=newPerson();



Object and assign the variable to this object.

Declare an array of type String Stringarray[];

Declare an array of type Person and create an array forthis variable which can hold 5 persons.

Personarray[]=newPerson[5];

Assign 5 to the int variable var1 (which was alreadydeclared);

var1=5;

Assign the existing variable pers2 to the exiting variablepers1 ;

pers1=pers2;

Declare an ArrayList variable which can hold objects oftype Person

ArrayList persons;

Create a new ArrayList with objects of type Person andassign it to the existing variable persons .

persons = new ArrayList();

Declare an ArrayList variable which can hold objects oftype Person and create a new Object for it.

ArrayList persons = newArrayList();

13.IntegratedDevelopmentEnvironment

ThepreviouschapterexplainedhowtocreateandcompileaJavaprogramonthecommandline.AJavaIntegratedDevelopmentEnvironment(IDE)provideslotsofeaseofusefunctionalityforcreatingJavaprograms.ThereareotherverypowerfulIDEsavailable,forexample,theEclipseIDE.

ForanintroductiononhowtousetheEclipseIDEpleaseseeEclipseIDETutorial.

Theremainingdescriptionusesthephrase:"CreateaJavaprojectcalled...".ThisreferstocreatingaJavaprojectinEclipse.IfyouareusingadifferentIDE,pleasefollowtherequiredstepsinthatIDE.

14.ExercisesCreatingJavaobjectsandmethods

14.1.CreateaPersonclassandinstantiateit

CreateanewJavaprojectcalled com.vogella.javastarter.exercises1 andapackagewiththesamename.

CreateaclasscalledPerson.

Addthreeinstancevariablestoit,oneforstoringthefirstnameoftheperson,oneforstoringthelastnameandoneforstoringtheageofthePerson.

Usetheconstructorofthe Person objecttosetthevaluestosomedefaultvalue.

Adda toString methodasdescribedbythefollowingcodeingandsolvetheTODO.ThismethodisusedtoconverttheobjecttoaStringrepresentation.

@OverridepublicStringtoString(){//TODOreplace""withthefollowing://firstName+""+lastNamereturn"";}

CreateanewclasscalledMainwitha publicstaticvoidmain(String[]args) .Inthismethodcreateaninstanceofthe Person class.

14.2.Useconstructor

Addaconstructortoyour Person classwhichtakesfirstname,lastnameandageasparameter.Assignthevaluestoyourinstancevariables.

Inyourmainmethodcreateatleastoneobjectoftype Person anduse System.out.println()withtheobjectasparameter.

14.3.Definegetterandsettermethods

Definemethodswhichallowyoutoreadthevaluesoftheinstancevariablesandtosetthem.Thesemethodsarecalledsetterandgetter.

Gettersshouldstartwith get followedbythevariablenamewherebythefirstletterofthevariableiscapitalized.



Settershouldstartwith set followedbythevariablenamewherebythefirstletterofthevariableiscapitalized.

Forexample,thevariablecalledfirstNamewouldhavethe getFirstName() gettermethodandthe setFirstName(Strings) settermethod.

Changeyour main methodsothatyoucreateone Person objectandusethesettermethodtochangethelastname.

14.4.CreateanAddressobject

CreateanewobjectcalledAddress.The Address shouldallowyoutostoretheaddressofaperson.

Addanewinstancevariableofthistypeinthe Person object.Also,createagetterandsetterforthe Address objectinthe Person object.

15.SolutionCreatingJavaobjectsandmethods

15.1.CreateaPersonclassandinstantiateit

ThefollowingistheexpectedresultafterSection14.1,CreateaPersonclassandinstantiateit.

packageexercises.exercise04;

classPerson{Stringfirstname="Jim";Stringlastname="Knopf";intage=12;

@OverridepublicStringtoString(){returnfirstName+""+lastName;}

}

packageexercises.exercise04;

publicclassMain{publicstaticvoidmain(String[]args){Personperson=newPerson();//thiscallsthetoStringmethodonthepersobjectSystem.out.println(pers);}}

15.2.Useconstructor

ThefollowingistheexpectedresultafterSection14.2,Useconstructor.

packagecom.vogella.javastarter.exercises1;

classPerson{StringfirstName;StringlastName;intage;

publicPerson(Stringa,Stringb,intvalue){firstName=a;lastName=b;age=value;}@OverridepublicStringtoString(){returnfirstName+""+lastName;}

}


publicclassMain{



publicstaticvoidmain(String[]args){Personp1=newPerson("Jim","Knopf",12);System.out.println(p1);

//reusethesamevariableandassignanewobjecttoitPersonp2=newPerson("Henry","Ford",104);System.out.println(p2);}}

15.3.Definegetterandsettermethods

ThefollowingistheexpectedresultafterSection14.3,Definegetterandsettermethods.


classPerson{StringfirstName;StringlastName;intage;

publicPerson(Stringa,Stringb,intvalue){firstName=a;lastName=b;age=value;}

publicStringgetFirstName(){returnfirstName;}

publicvoidsetFirstName(StringfirstName){this.firstName=firstName;}

publicStringgetLastName(){returnlastName;}

publicvoidsetLastName(StringlastName){this.lastName=lastName;}

publicintgetAge(){returnage;}

publicvoidsetAge(intage){this.age=age;}@OverridepublicStringtoString(){returnfirstName+""+lastName;}

}


publicclassMain{publicstaticvoidmain(String[]args){Personperson=newPerson("Jim","Knopf",21);Personp2=newPerson("Jill","Sanders",20);//JillgetsmarriedtoJim//andtakeshisnamep2.setLastName("Knopf");System.out.println(p2);}}

15.4.SolutionCreateanAddressobject

ThefollowingistheexpectedresultafterSection14.4,CreateanAddressobject.




publicclassAddress{

privateStringstreet;privateStringnumber;privateStringpostalCode;privateStringcity;privateStringcountry;

publicStringgetStreet(){returnstreet;}

publicvoidsetStreet(Stringstreet){this.street=street;}

publicStringgetNumber(){returnnumber;}

publicvoidsetNumber(Stringnumber){this.number=number;}

publicStringgetPostalCode(){returnpostalCode;}

publicvoidsetPostalCode(StringpostalCode){this.postalCode=postalCode;}

publicStringgetCity(){returncity;}

publicvoidsetCity(Stringcity){this.city=city;}

publicStringgetCountry(){returncountry;}

publicvoidsetCountry(Stringcountry){this.country=country;}

publicStringtoString(){returnstreet+""+number+""+postalCode+""+city+""+country;}

}


classPerson{StringfirstName;StringlastName;intage;privateAddressaddress;

publicPerson(Stringa,Stringb,intvalue){firstName=a;lastName=b;age=value;}

publicStringgetFirstName(){returnfirstName;}

publicvoidsetFirstName(StringfirstName){this.firstName=firstName;}

publicStringgetLastName(){returnlastName;}



publicvoidsetLastName(StringlastName){this.lastName=lastName;}

publicintgetAge(){returnage;}

publicvoidsetAge(intage){this.age=age;}

publicAddressgetAddress(){returnaddress;}

publicvoidsetAddress(Addressaddress){this.address=address;}

@OverridepublicStringtoString(){returnfirstName+""+lastName;}

}


publicclassMain{publicstaticvoidmain(String[]args){//IcreateapersonPersonpers=newPerson("Jim","Knopf",31);//settheageofthepersonto32pers.setAge(32);

//justfortestingIwritethistotheconsoleSystem.out.println(pers);/**actuallySystem.out.printlnalwayscallstoString,ifyoudonot*specifyitsoyoucouldalsohavewrittenSystem.out.println(pers);*///createanaddressAddressaddress=newAddress();//setthevaluesfortheaddressaddress.setCity("Heidelberg");address.setCountry("Germany");address.setNumber("104");address.setPostalCode("69214");address.setStreet("Musterstr.");

//assigntheaddresstothepersonpers.setAddress(address);

//disposereferencetoaddressobjectaddress=null;

//personismovingtothenexthouseinthesamestreetpers.getAddress().setNumber("105");

}

}

16.Javastatements

TheJavalanguagedefinescertainstatementswithapredefinedmeaning.Thefollowingdescriptionlistssomeofthem.

16.1.ifthenandifthenelse

The ifthen statementisacontrolflowstatement.Ablockofcodeisonlyexecutedwhenthetestspecifiedbythe if partevaluatesto true .Theoptional else blockisexecutedwhenthe ifpartevaluatesto false .

Thefollowingexamplecodeshowsaclasswithtwomethods.Thefirstmethoddemonstratestheusageof ifthen andthesecondmethoddemonstratestheusageof ifthenelse .



16.2.Switch

Theswitchstatementcanbeusedtohandleseveralalternativesiftheyarebasedonthesameconstantvalue.

switch(expression){caseconstant1:command;break;//willpreventthattheothercasesoralsoexecutedcaseconstant2:command;break;...default:}

//Example:

switch(cat.getLevel()){case0:returntrue;case1:if(cat.getLevel()==1){if(cat.getName().equalsIgnoreCase(req.getCategory())){returntrue;}}case2:if(cat.getName().equalsIgnoreCase(req.getSubCategory())){returntrue;}}

16.3.BooleanOperations

Use == tocomparetwoprimitivesortoseeiftworeferencesrefertothesameobject.Usetheequals() methodtoseeiftwodifferentobjectsareequal.

&&and||arebothShortCircuitMethodswhichmeansthattheyterminateoncetheresultofanevaluationisalreadyclear. Example(true||...) isalways true while (false&&...)alwaysisalwaysinterpretedas false .Usage:

( var!=null&&var.method1()... )ensuresthat var isnot null beforedoingtherealcheck.

Table4.Boolean

Operations Description

== Is equal, in case of objects the system checks if the referencevariable point to the same object. It will not compare thecontent of the objects!

&& And

!= is not equal, similar to == .

a.equals(b) Checks if string a equals b.

a.equalsIgnoreCase(b) Checks if string a equals b while ignoring lower cases.

If(value?false:true){} Negotiation: return true if value is not true.

17.LoopsinJava

17.1.Theforloop

Aforloopisarepetitioncontrolstructurethatallowsyoutowriteablockofcodewhichisexecutedaspecificnumberoftimes.Thesyntaxisthefollowing.

for(initialization;expression;update_statement){//blockofcodetorun}

Thefollowingshowsanexampleforaforloop.



publicclassForTest{

publicstaticvoidmain(Stringargs[]){

for(inti=1;i



int[]array;

//allocatesmemoryfor10integersarray=newint[10];

//initializevaluesarray[0]=10;//initializesecondelementarray[1]=20;array[2]=30;array[3]=40;array[4]=50;array[5]=60;array[6]=70;array[7]=80;array[8]=90;array[9]=100;

}}

18.2.EnhancedforloopforArraysandCollections

Arraysandcollectionscanbeprocessedwithasimplerforloop.

for(declaration:expression){//bodyofcodetobeexecuted}

Thefollowingcodedemonstratesitsusage.

packagecom.vogella.javaintro.array;

publicclassTestMain{publicstaticvoidmain(String[]args){//declaresanarrayofintegersint[]array;

//allocatesmemoryfor10integersarray=newint[10];

//initializevaluesarray[0]=10;//initializesecondelementarray[1]=20;array[2]=30;array[3]=40;array[4]=50;array[5]=60;array[6]=70;array[7]=80;array[8]=90;array[9]=100;

for(inti:array){System.out.println("Elementatindex"+i+":"+array[i]);}}}

19.Strings

19.1.StringsinJava

The String classrepresentscharacterstrings.Allstringliterals,forexample,"hello",areimplementedasinstancesofthisclass.Aninstanceofthisclassisanobject.Stringsareimmutable,e.g.,anassignmentofanewvaluetoa String objectcreatesanewobject.

19.2.StringpoolinJava

FormemoryefficiencyJavausesa String pool.Thestringpoolallowsstringliteralstobereused.ThisispossiblebecausestringsinJavaareimmutable.

IfthesamestringliteralisusedinseveralplacesintheJavacode,onlyonecopyofthatstringiscreated.

Whenevera String objectiscreatedandgetsastringliteralassigned,e.g.,asin



Strings="constant" ,thestringpoolisused.However,the new operatorforcesanewString copytobeallocated,forexample,in Strings=newString("constant"); .

19.3.CompareStringsinJava

Tocomparethe String objects s1 and s2 ,usethe s1.equals(s2) method.

A String comparisonwith == isincorrect,as == checksforobjectreferenceequality. ==sometimesgivesthecorrectresult,asJavausesa String pool.Thefollowingexamplewouldworkwith == .

Thiswouldworkasexpected.

Stringa="Hello";Stringb="Hello";if(a==b){//ifstatementistrue//becauseStringpoolisusedand//aandbpointtothesameconstant}

Thiscomparisonwouldfail.

Stringa="Hello";Stringb=newString("Hello");if(a==b){}else{//ifstatementisfalse//becauseStringpoolisusedand//aandbpointtothesameconstant}

Warning: Therefore,youshouldalwaysusethe equals() methodwhenyoucomparestrings.

19.4.WorkingwithStrings

Thefollowingliststhemostcommonstringoperations.

Table5.

Command Description

"Testing".equals(text1); Return true if text1 is equal to "Testing" .The check is case-sensitive.

"Testing".equalsIgnoreCase(text1); Return true if text1 is equal to "Testing" .The check is not case-sensitive. For example, itwould also be true for "testing" .

StringBufferstr1=newStringBuffer(); Define a new String with a variable length.

str.charat(1); Return the character at position 1. (Note:strings are arrays of chars starting with 0)

str.substring(1); Removes the first characters.

str.substring(1,5); Gets the substring from the second to the fifthcharacter.

str.indexOf("Test") Look for the String "Test" in String str .Returns the index of the first occurrence of thespecified string.

str.lastIndexOf("ing") Returns the index of the last occurrence of thespecified String "ing" in the String str.StringBuffer does not support this method.

Hence first convert the StringBuffer toString via method toString .

str.endsWith("ing") Returns true if str ends with String"ing"

str.startsWith("Test") Returns true if String str starts withString "Test" .

str.trim() Removes leading and trailing spaces.



str.replace(str1,str2) Replaces all occurrences of str1 by str2

str2.concat(str1); Concatenates str1 at the end of str2 .

str.toLowerCase() / str.toUpperCase() Converts the string to lower- or uppercase

str1+str2 Concatenate str1 and str2

String[]array=myString.split("");String[]array2=myString.split("\\.");

Splits the character separated myString intoan array of strings. Attention: the split stringis a regular expression, so if you usingspecial characters which have a meaning inregular expressions, you need to quotethem. In the second example the . is usedand must be quoted by two backslashes.

20.Lambdas

20.1.Whatarelambdas?

TheJavaprogramminglanguagesupportslambdasasofJava8.Alambdaexpressionisablockofcodewithparameters.Lambdasallowstospecifyablockofcodewhichshouldbeexecutedlater.Ifamethodexpectsafunctionalinterfaceasparameteritispossibletopassinthelambdaexpressioninstead.

ThetypeofalambdaexpressioninJavaisafunctionalinterface.

20.2.Differencebetweenalambdaexpressionandaclosure

Alambdaisananonymousfunction,e.g.,itcanbedefinedasparameter.Aclosureisanyfunctionwhichclosesovertheenvironmentinwhichitwasdefined.Thismeansthatitcanaccessvariablesnotinitsparameterlistandassignedtoavariable.

Javasupportslambdasbutnotclosures.

20.3.Purposeoflambdaexpressions

UsinglambdasallowstouseacondensedsyntaxcomparedtootherJavaprogrammingconstructs.Forexamplethe Collections inJava8havea forEach methodwhichacceptsalambdaexpression.

Listlist=newArrayList();list.add("vogella.com");list.add("google.com");list.add("heise.de");list.forEach(System.out::println);

20.4.Usingmethodreferences

Youcanusemethodreferencesinalambdaexpression.Methodreferencedefinethemethodtobecalledvia CalledFrom::method .CalledFromcanbe

instance::instanceMethod

SomeClass::staticMethod

SomeClass::instanceMethod

Listlist=newArrayList();list.add("vogella.com");list.add("google.com");list.add("heise.de");list.forEach(s>System.out.println(s));

21.Streams

21.1.WhatareStreamsinJava8?

Astreamfromthe java.util.stream packageisasequenceofelementsfromasourcethatsupportsaggregateoperations.

21.2.IntStream

Allowtocreateastreamofsequenceofprimitiveintvaluedelementssupportingsequentialand



parallelaggregateoperations.

packagecom.vogella.java.streams;

importjava.util.ArrayList;importjava.util.List;importjava.util.stream.IntStream;

publicclassIntStreamExample{

publicstaticvoidmain(String[]args){//printoutthenumbersfrom1to100IntStream.range(1,101).forEach(s>System.out.println(s));//createalistofintegersfor1to100Listlist=newArrayList();IntStream.range(1,101).forEach(it>list.add(it));System.out.println("Size"+list.size());}

}

21.3.Reductionoperationswithstreamsandlambdas

Allowtocreateastreamofsequenceofprimitiveintvaluedelementssupportingsequentialandparallelaggregateoperations.


publicclassTask{privateStringsummary;privateintduration;publicTask(Stringsummary,intduration){this.summary=summary;this.duration=duration;}publicStringgetSummary(){returnsummary;}publicvoidsetSummary(Stringsummary){this.summary=summary;}publicintgetDuration(){returnduration;}publicvoidsetDuration(intduration){this.duration=duration;}}


importjava.util.ArrayList;importjava.util.List;importjava.util.Random;importjava.util.stream.Collectors;importjava.util.stream.IntStream;

publicclassStreamTester{

publicstaticvoidmain(String[]args){Randomrandom=newRandom();//GeneratealistofrandomtaskListvalues=newArrayList();IntStream.range(1,20).forEach(i>values.add(newTask("Task"+random.nextInt(10),random.nextInt(10))));

//getalistofthedistincttasksummaryfieldListresultList=values.stream().filter(t>t.getDuration()>5).map(t>t.getSummary()).distinct().collect(Collectors.toList());System.out.println(resultList);//getaconcatenatedstringofTaskwithadurationlongerthan5hoursStringcollect=values.stream().filter(t>t.getDuration()>5).map(t>t.getSummary()).distinct().collect(Collectors.joining(""));



System.out.println(collect);}

}

22.TypeConversion

IfyouusevariablesofdifferenttypesJavarequiresforcertaintypesanexplicitconversion.Thefollowinggivesexamplesforthisconversion.

22.1.ConversiontoString

UsethefollowingtoconvertfromothertypestoStrings

//ConvertfrominttoStringStrings1=String.valueOf(10);//"10"//ConvertfromdoubletoStringStrings2=String.valueOf(Math.PI);//"3.141592653589793"//ConvertfrombooleantoStringStrings3=String.valueOf(1



privatevoidconvertDateToSQL(){SimpleDateFormattemplate=newSimpleDateFormat("yyyyMMdd");java.util.Dateenddate=newjava.util.Date("10/31/99");java.sql.DatesqlDate=java.sql.Date.valueOf(template.format(enddate));}publicstaticvoidmain(String[]args){ConvertDateToSQLDatedate=newConvertDateToSQLDate();date.convertDateToSQL();}

}

23.Scheduletasks

Javaallowsyoutoscheduletasks.Ascheduledtaskscanperformonceorseveraltimes.

java.util.Timer and java.util.TimerTask canbeusedtoscheduletasks.Theobjectwhichimplements TimeTask willthenbeperformedbythe Timer basedonthegiveninterval.

packageschedule;

importjava.util.TimerTask;

publicclassMyTaskextendsTimerTask{privatefinalStringstring;privateintcount=0;

publicMyTask(Stringstring){this.string=string;}

@Overridepublicvoidrun(){count++;System.out.println(string+"called"+count);}

}

packageschedule;

importjava.util.Timer;

publicclassScheduleTest{

publicstaticvoidmain(String[]args){Timertimer=newTimer();

//wait2seconds(2000millisecs)andthenstarttimer.schedule(newMyTask("Task1"),2000);

for(inti=0;i



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