Lesson: A Closer Look at the "Hello World!" Application

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later ... Documentation TheJava™Tutorials HideTOC ACloserLookatthe"HelloWorld!"Application Trail:GettingStarted HomePage > GettingStarted « Previous • Trail • Next » TheJavaTutorialshavebeenwrittenforJDK8.Examplesandpracticesdescribedinthispagedon'ttakeadvantageofimprovementsintroducedinlaterreleasesandmightusetechnologynolongeravailable.SeeJavaLanguageChangesforasummaryofupdatedlanguagefeaturesinJavaSE9andsubsequentreleases.SeeJDKReleaseNotesforinformationaboutnewfeatures,enhancements,andremovedordeprecatedoptionsforallJDKreleases. Lesson:ACloserLookatthe"HelloWorld!"Application Nowthatyou'veseenthe"HelloWorld!"application(andperhapsevencompiledandrunit),youmightbewonderinghowitworks.Hereagainisitscode: classHelloWorldApp{ publicstaticvoidmain(String[]args){ System.out.println("HelloWorld!");//Displaythestring. } } The"HelloWorld!"applicationconsistsofthreeprimarycomponents:sourcecodecomments,theHelloWorldAppclassdefinition,andthemainmethod.Thefollowingexplanationwillprovideyouwithabasicunderstandingofthecode,butthedeeperimplicationswillonlybecomeapparentafteryou'vefinishedreadingtherestofthetutorial. SourceCodeComments Thefollowingboldtextdefinesthecommentsofthe"HelloWorld!"application: /** *TheHelloWorldAppclassimplementsanapplicationthat *simplyprints"HelloWorld!"tostandardoutput. */ classHelloWorldApp{ publicstaticvoidmain(String[]args){ System.out.println("HelloWorld!");//Displaythestring. } } Commentsareignoredbythecompilerbutareusefultootherprogrammers.TheJavaprogramminglanguagesupportsthreekindsofcomments: /*text*/ Thecompilerignoreseverythingfrom/*to*/. /**documentation*/ Thisindicatesadocumentationcomment(doccomment,forshort).Thecompilerignoresthiskindofcomment,justlikeitignorescommentsthatuse/*and*/.Thejavadoctoolusesdoccommentswhenpreparingautomaticallygenerateddocumentation.Formoreinformationonjavadoc,seethe Javadoc™tooldocumentation. //text Thecompilerignoreseverythingfrom//totheendoftheline. TheHelloWorldAppClassDefinition Thefollowingboldtextbeginstheclassdefinitionblockforthe"HelloWorld!"application: /** *TheHelloWorldAppclassimplementsanapplicationthat *simplydisplays"HelloWorld!"tothestandardoutput. */ classHelloWorldApp{ publicstaticvoidmain(String[]args){ System.out.println("HelloWorld!");//Displaythestring. } } Asshownabove,themostbasicformofaclassdefinitionis: classname{ ... } Thekeywordclassbeginstheclassdefinitionforaclassnamedname,andthecodeforeachclassappearsbetweentheopeningandclosingcurlybracesmarkedinboldabove.Chapter2providesanoverviewofclassesingeneral,andChapter4discussesclassesindetail.Fornowitisenoughtoknowthateveryapplicationbeginswithaclassdefinition. ThemainMethod Thefollowingboldtextbeginsthedefinitionofthemainmethod: /** *TheHelloWorldAppclassimplementsanapplicationthat *simplydisplays"HelloWorld!"tothestandardoutput. */ classHelloWorldApp{ publicstaticvoidmain(String[]args){ System.out.println("HelloWorld!");//Displaythestring. } } IntheJavaprogramminglanguage,everyapplicationmustcontainamainmethodwhosesignatureis: publicstaticvoidmain(String[]args) Themodifierspublicandstaticcanbewrittenineitherorder(publicstaticorstaticpublic),buttheconventionistousepublicstaticasshownabove.Youcannametheargumentanythingyouwant,butmostprogrammerschoose"args"or"argv". ThemainmethodissimilartothemainfunctioninCandC++;it'stheentrypointforyourapplicationandwillsubsequentlyinvokealltheothermethodsrequiredbyyourprogram. Themainmethodacceptsasingleargument:anarrayofelementsoftypeString. publicstaticvoidmain(String[]args) Thisarrayisthemechanismthroughwhichtheruntimesystempassesinformationtoyourapplication.Forexample: javaMyApparg1arg2 Eachstringinthearrayiscalledacommand-lineargument.Command-lineargumentsletusersaffecttheoperationoftheapplicationwithoutrecompilingit.Forexample,asortingprogrammightallowtheusertospecifythatthedatabesortedindescendingorderwiththiscommand-lineargument: -descending The"HelloWorld!"applicationignoresitscommand-linearguments,butyoushouldbeawareofthefactthatsuchargumentsdoexist. Finally,theline: System.out.println("HelloWorld!"); usestheSystemclassfromthecorelibrarytoprintthe"HelloWorld!"messagetostandardoutput.Portionsofthislibrary(alsoknownasthe"ApplicationProgrammingInterface",or"API")willbediscussedthroughouttheremainderofthetutorial. «Previous • Trail • Next» AboutOracle| ContactUs| LegalNotices| TermsofUse| YourPrivacyRights Copyright©1995,2022Oracleand/oritsaffiliates.Allrightsreserved. Previouspage:PreviousLesson Nextpage:QuestionsandExercises:GettingStarted