外文翻译 - Java编程介绍

0外文原文IntroductiontoJavaApplicationsJavaprogramsconsistofpiecescalledclasses.YoucanprogrameachpieceyoumayneedtofromaJavaprogram.However,mostJavaprogrammerstakeadvantageofrichcollectionsofexistingclassesinjavalibraries.Thus,therearereallytwopiecestolearningthejava“world”,thefirstislearningthejavalanguageitselfsothatyoucanprogramyourownclasses;thesecondishowtousetheclassesintheextensivejavaclasslibraries.TheJavalanguagefacilitatesadisciplinedapproachtocomputerprogramdesign.WenowintroduceJavaprogrammingandpresentexamplesthatillustrateseveralimportantfeaturesofJava.Eachexampleisanalyzedonelineatatime.WepresentadetailedtreatmentofprogramdevelopmentandprogramcontrolinJava.Javausesnotationsthatmayappearstrangetononprogrammers.Webeginbyconsideringasimpleapplicationthatdisplayedalineoftext.Anapplicationisaprogramthatexecutesusingthejavainterpreter(discussedlaterinthissection).ThisprogramillustratesseveralimportantfeaturesoftheJavalanguage.Weconsidereachlineoftheprogramindetail.Eachprogramwepresentinthisbookhaslinenumbersincludedforthereadersconvenience;linenumbersarenotpartofactualJavaprograms.Line9doesthe“realword”oftheprogram,namelydisplayingthephraseWelcometoJavaProgramming!Butletusconsidereachlineinorder.Line1,Beginswith/,indicatingthattheremainderofthelineisacomment.Programmersinsertcommentstodocumentprogramsandimproveprogramreadability.Commentsalsohelpotherpeoplereadandunderstandaprogram.Commentsdonotcausethecomputertoperformanyactionwhentheprogramisrun.TheJavacompilerignorescomments.Webegineveryprogramwithacommentindicatingthefigurenumberandfilename(line1).1/Welcome1.java2/AfirstprograminJava.3publicclassWelcome14/mainmethodbeginsexecutionofJavaapplication15publicstaticvoidmain(Stringargs)67System.out.println(“WelcometoJavaProgramming!”);89/endmethodmain0knowallofthedetailsofafeatureinorderforyoutousethatfeatureinJava.Allprogrammersinitiallylearnhowtoprogrambymimickingwhatotherprogrammershavedonebeforethem.Foreachdetailweaskyoutomimic,weindicatewherethefulldiscussionwillbepresentedlaterinthetext.Whenyousaveyourpublicclassdefinitioninafile,thefilenamemustbetheclassnamefollowedbythe“.java”file-nameextension.ForourapplicationthefilenameisWelcome1.java.AllJavaclassdefinitionsarestoredinfilesendingwiththefile-nameextension”java.”Itisanerrorforapublicclassifthefilenameisnotidenticaltotheclassname(plusthe.javaextension)intermsofbothspellingandcapitalization.Therefore,itisalsoanerrorforafiletocontaintwoormorepublicclasses.Itisanerrornottoendafilenamewiththe.javaextensionforafilecontaininganapplicationsclassdefinition.Iftheextensionismissing,theJavacompilerwillnotbeabletocompiletheclassdefinition.Aleftbrace(attheendofline4),beginthebodyofeveryclassdefinition.Acorrespondingrightbrace(inline13inthisprogram),mustendeachclassdefinition.Noticethatlines6-11areindented.Thisindentationisoneofthespacingconventionsmentionedearlier.WedefineeachspacingconventionasagoodprogrammingPractice.Wheneveryoutypeanopeningleftbrace,inyourprogram,immediatelytypetheclosingrightbrace,thenrepositionthecursorbetweenthebracestobegintypingthebody.Thispracticehelpspreventerrorsduetomissingbraces.Indenttheentirebodyofeachclassdefinitionone“level”ofindentationbetweenthe2leftbrace,andtherightbrace,thatdefinethebodyoftheclass.Thisformatemphasizesthestructureoftheclassdefinitionandhelpsmaketheclassdefinitioneasiertoread.Setaconventionfortheindentsizeyouprefer,andthenuniformlyapplythatconvention.TheTabkeymaybeusedtocreateindents,buttabstopsmayvarybetweeneditors.Werecommendusingthreespacestoformalevelofindent.Ifbracesdonotoccurinmatchingpairs,thecompilerindicatesanerror.Line5isablankline,insertedforprogramreadability.Line6,/mainmethodbeginsexecutionofJavaapplicationLine7,Publicstaticvoidmain(Stringargs)Javaapplicationsbeginexecutingatmain.AftermainindicatethatmainisaprogrambuildingblockcalledamethodJavaclassdefinitionsnormallycontainoneormoremethods.ForaJavaapplicationclass,exactlyoneofthosemethodsmustbecalledmainandmustbedefinedasshownonline7;otherwise,thejavainterpreterwillnotexecutetheapplication.Methodsareabletoperformtasksandreturninformationwhentheycompletetheirtasks.Thevoidkeywordindicatesthatthismethodwillperformatask(displayingalineoftext,inthisprogram),butwillnotreturnanyinformationwhenitcompletesitstask.Later,wewillseethatmanymethodsreturninformationwhentheycompletetheirtask.Methodsareexplainedindetail.Fornow,simplymimicmainsfirstlineinyourJavaapplications.Indenttheentirebodyofeachmethoddefinitionone“level”ofindentationbetweentheleft,andtherightbrace,thatdefinethebodyofthemethod.Thisformatmakesthestructureofthemethodstandoutandhelpsmakethemethoddefinitioneasiertoread.Line9,System.out.println(“WelcometoJavaProgramming!”);Instructsthecomputertoperformanaction,namelytoprintthestringofcharacterscontainedbetweenthedoublequotationmarks.Astringissometimescalledacharacterstring,amessageorastringliteral.Werefertocharactersbetweendoublequotationmarksgenericallyasstrings.White-spacecharactersinstringsarenotignoredbythecompiler.3System.outisknownasthestandardoutputobject.System.outallowsJavaapplicationstodisplaystringsandothertypesofinformationinthecommandwindowfromwhichtheJavaapplicationexecutes.InMicrosoftWindows95/98/ME,thecommandwindowistheMS-DOSprompt.InMicrosoftWindowsNT/2000,thecommandwindowistheCOMMANDPROMPT(cmd.exe).Thecommandwindowisnormallycalledacommandwindow,acommandtool,ashelltoolorashell.Oncomputersrunninganoperatingsystemthatdoesnothaveacommandwindow(suchasaMacintosh),thejavainterpreternormallydisplaysawindowcontainingtheinformationtheprogramdisplays.MethodSystem.out.printlndisplays(orprints)lineoftextinthecommandwindow.WhenSystem.out.printlncompletesitstask,itautomaticallypositionstheoutputcursor(thelocationwherethenextcharacterwillbedisplayed)tothebeginningtheofthenextlineinthecommandwindow.(ThismoveofthecursorissimilartoyoupressingtheEnterkeywhentypinginatexteditorthecursorappearsatthebeginningofthenextlineinyourfile.)Theentireline,includingSystem.out.println,itsargumentintheparentheses(thestring)andthesemicolon(;),isastatement.Everystatementmustendwithasemicolon(alsoknownasthestatementterminator).ItdisplaysthemessageWelcometoJavaProgramming!inthecommandwindow.Omittingthesemicolonattheendofastatementisasyntaxerror.Erroroccurswhenthecompilercannotrecognizeastatement.Thecompilernormallyissuesanerrormessagetohelptheprogrammeridentifyandfixtheincorrectstatement.Syntaxisviolationsofthelanguagerules.Syntaxerrorsarealsocalledcompileerrors,compile-timeerrororcompilationerrors,becausethecompilerdetectsthemduringthecompilationphase.Youwillbeunabletoexecuteyourprogramuntilyoucorrectallofthesyntaxerrorsinit.Whenthecompilerreportsasyntaxerror,theerrormaynotbeonthelinenumberindicatedbytheerrormessage.First,checkthelineforwhichtheerrorwasreported.Ifthatlinedoesnotcontainsyntaxerrors,checktheprecedingseverallinesintheprogram.Someprogrammersfinditdifficultwhenreadingand/orwritingaprogramtomatch4theleftandrightbraces(and)thatdelimitthebodyofaclassdefinitionoramethoddefinition.Forthisreason,someprogrammersprefertoincludeasingle-linecommentafteraclosingrightbrace()0thatendsamethoddefinitionandafteraclosingrightbracethatendsaclassdefinition.Forexample,line11,/endmethodmainSpecifiestheclosingrightbrace()ofmethodmain,andline13,/endclassWelcome1Specifiestheclosingrightbrace()ofclassWelcome1.Eachcommentindicatesthemethodorclassthattherightbraceterminates.Weusesuchcommentstohelpbeginningprogrammersdeterminewhereeachprogramcomponentterminates.Weusesuchcommentswhenpairsofbracescontainmanystatements,whichmaketheclosingbracesdifficulttoidentify.Someprogrammersprefertofollowtheclosingrightbrace()ofamethodbodyorclassdefinitionwithasingle-linecommentindicatingthemethodorclassdefinitiontowhichthebracebelongs.Thiscommentimprovesprogramreadability.Wearenowreadytocompileandexecuteourprogram.Tocompiletheprogram,weopenacommandwindow,changetothedirectorywheretheprogramisstoredandtypeJavacWelcome1.javaIftheprogramcontainsnosyntaxerrors,theprecedingcommandcreatesanewfilecalledWelcome1.classcontainingtheJavabytecodesthatrepresentourapplication.Thesewillbeinterpretedbythejavainterpreterwhenwetellittoexecutetheprogram,asshownintheMicrosoftWindows2000CommandPrompt.JavaWelcome1Tolaunchthejavainterpreterandindicatethatitshouldloadthe“.class”fileforclassWelcome1.Notethatthe“.class”file-nameextensionisomittedfromtheprecedingcommand;otherwisetheinterpreterwillnotexecutetheprogram.Theinterpreterautomaticallycallsmethodmain.Next,thestatementonline7ofmaindisplays“WelcometoJavaProgramming!”5TheJavacompilergeneratessyntaxerrormessageswhenthesyntaxofaprogramisincorrect.Whenyouarelearninghowtoprogram,sometimesitishelpfulto“break”awordingprogramsoyoucanseetheerrormessagesproducedbythecompiler.Then,whenyouencounterthaterrormessageagain,youwillhaveanideaoftheerrorscause.Tryremovingasemicolonorcurlybracefromtheprogram,thenrecompiletheprogramtoseetheerrormessagesgeneratedbytheomission.1/Displayingmultiplestrings2importjava.applet.Applet;/importAppletclass3importjava.awt.Graphics;/importGraphicsclass4publicclassWelcomeextendsApplet5publicvoidpaint（Graphicsg）67g.drawString（“Welcometo”,25,25）;8G.drawString（“JavaProgramming”,25,40）;9106中文翻译Java编程介绍Java程序由类和方法构成，程序员可以自己编写Java程序的每一部分。但是，大部分Java程序员利用了Java类库中的类和方法。在Java世界中有两部分需要学习：第一个部分就是Java语言本身，可以用它来编写自己的类和方法；第二个部分是如何使用扩充的Java类库。这个程序表述了Java语言的几种重要特性，我们将详细解释程序的每一行。为了阅读方便，程序中都加上了行号，但这些行号不是Java程序的原有部分。第行执行了程序的“真正工作”，在屏幕上显示“”WelcometoJavaProgramming这一短句。现在，让我们按顺序考虑程序的每一行。第一行：AfirstprograminJava这一行以“”开始，表明本行是一个注释。程序员在程序中加入注释来使程序文档化以及增加程序的可读性。注释还可以帮助其他人阅读和理解你的程序。注释在程序执行时不会导致计算机执行任何动作。Java编译器将忽略注释，并且不会产生任何字节码。“AfirstprograminJava”注释简单地描述了程序的目的。一个以“”开始的注释称为单行注释，因为该注释的作用范围在行尾结束。稍后的中间开始，它能够使这一行的剩余部分（如第行和第行）成为注释。1/AfirstprograminJava2importjava.applet.Applet;/importAppletclass3importjava.awt.Graphics;/importGraphicsclass45publicclassWelcomeextendsApplet6publicvoidpaint（Graphicsg）78g.drawString(“WelcometoJavaProgramming!”,25,25);910每一个程序由一个表明本程序用途的注释开始。Java包括许多预先定义的类，并且在磁盘上将相关的类放入一个目录中，我们称之为软件包。这些软件包称为Java类库或者java应用程序编程接口（API）。下面这两行：importjava.applet.Applet;/importAppletclassimportjava.awt.Graphics;/importGraphicsclass是import语句，用来载入编译一个Java程序所需要的类。这些特定的行告诉编译器从java.applet软件包中载入Applet类，并且从java.awt软件包中载入Graphics累。当用户在Java中创建一个applet时，必须载入Applet类。载入Graphics类的目的是程序可以在屏幕上显示信息。Java的最强大的功能是JavaAPI的各个软件包中含有树木庞大的类，程序员可以重用它们而不必重新开发。我们运用了这些类的大量成员，并且将详细讨论类和软件7包。从类中继承一个JavaAPI的子类之前，请仔细阅读它的说明文档以了叫了解这些类的功能。类用来在程序执行时在内存中实例化（或创建）一个对象是内存中的一个区域。其中存储程序使用的信息。类库主要由编译器的供应商提供，但是许多是由独立的软件供应商提供的。我们的Welcome类用来创建一个执行这个Javaapplet的对象，其中运行这个applet的浏览器创建了Welcome类的一个事例。关键字public使得浏览器能够创建并执行我们的applet在Java中，主要的Applet类（即从Applet中继承的）总是一个public（公有）类型的类。当把applet存入文件时，这个applet的类名将作为文件名的一部分。在本例中，文件名为Welcome.java。如果文件名在拼写