java語言程序設計 基礎篇 課件(第1章)英文

Chapter1IntroductiontoComputers,Programs,andJava1Chapter1IntroductiontoCompObjectivesToreviewcomputerbasics,programs,andoperatingsystems(§§1.2-1.4).ToexploretherelationshipbetweenJavaandtheWorldWideWeb(§1.5).TodistinguishthetermsAPI,IDE,andJDK(§1.6).TowriteasimpleJavaprogram(§1.7).Todisplayoutputontheconsole(§1.7).ToexplainthebasicsyntaxofaJavaprogram(§1.7).Tocreate,compile,andrunJavaprograms(§1.8).(GUI)TodisplayoutputusingtheJOptionPaneoutputdialogboxes(§1.9).2ObjectivesToreviewcomputerbWhatisaComputer?AcomputerconsistsofaCPU,memory,harddisk,floppydisk,monitor,printer,andcommunicationdevices.3WhatisaComputer?AcomputerCPUThecentralprocessingunit(CPU)isthebrainofacomputer.Itretrievesinstructionsfrommemoryandexecutesthem.TheCPUspeedismeasuredinmegahertz(MHz),with1megahertzequaling1millionpulsespersecond.ThespeedoftheCPUhasbeenimprovedcontinuously.IfyoubuyaPCnow,youcangetanIntelPentium4Processorat3gigahertz(1gigahertzis1000megahertz).4CPUThecentralprocessingunitMemoryMemoryistostoredataandprograminstructionsforCPUtoexecute.Amemoryunitisanorderedsequenceofbytes,eachholdseightbits.Aprogramanditsdatamustbebroughttomemorybeforetheycanbeexecuted.Amemorybyteisneverempty,butitsinitialcontentmaybemeaninglesstoyourprogram.Thecurrentcontentofamemorybyteislostwhenevernewinformationisplacedinit.5MemoryMemoryistostoredataHowDataisStored?Dataofvariouskinds,suchasnumbers,characters,andstrings,areencodedasaseriesofbits(zerosandones).Computersusezerosandonesbecausedigitaldeviceshavetwostablestates,whicharereferredtoaszeroandonebyconvention.Theprogrammersneednottobeconcernedabouttheencodinganddecodingofdata,whichisperformedautomaticallybythesystembasedontheencodingscheme.Theencodingschemevaries.Forexample,character‘J’isrepresentedby01001010inonebyte.Asmallnumbersuchasthreecanbestoredinasinglebyte.Ifcomputerneedstostorealargenumberthatcannotfitintoasinglebyte,itusesanumberofadjacentbytes.Notwodatacanshareorsplitasamebyte.Abyteistheminimumstorageunit.6HowDataisStored?DataofvarStorageDevicesMemoryisvolatile,becauseinformationislostwhenthepowerisoff.Programsanddataarepermanentlystoredonstoragedevicesandaremovedtomemorywhenthecomputeractuallyusesthem.Therearethreemaintypesofstoragedevices:Diskdrives(harddisksandfloppydisks),CDdrives(CD-RandCD-RW),andTapedrives.7StorageDevicesMemoryisvolatOutputDevices:MonitorThemonitordisplaysinformation(textandgraphics).Theresolutionanddotpitchdeterminethequalityofthedisplay.

8OutputDevices:MonitorThemonMonitorResolutionandDotPitchTheresolutionspecifiesthenumberofpixelspersquareinch.Pixels(shortfor“pictureelements”)aretinydotsthatformanimageonthescreen.Theresolutioncanbesetmanually.Thehighertheresolution,thesharperandclearertheimageis.However,theimagemaybeverysmallifyousethighresolutiononasmallscreenmonitor.PCmonitorsareusually15-inch,17-inch,19-inch,or21-inch.Fora15-inchmonitor,acomfortableresolutionsettingwouldbe640480(307,200pixels).resolutionThedotpitchistheamountofspacebetweenpixels.Thesmallerthedotpitch,thebetterthedisplay.

dotpitch9MonitorResolutionandDotPitCommunicationDevicesAregularmodemusesaphonelineandcantransferdatainaspeedupto56,000bps(bitspersecond).ADSL(digitalsubscriberline)alsousesaphonelineandcantransferdatainaspeed20timesfasterthanaregularmodem.AcablemodemusestheTVcablelinemaintainedbythecablecompany.AcablemodemisasfastasaDSL.Networkinterfacecard(NIC)isadevicetoconnectacomputertoalocalareanetwork(LAN).TheLANiscommonlyusedinbusiness,universities,andgovernmentorganizations.AtypicaltypeofNIC,called10BaseT,cantransferdataat10mbps(millionbitspersecond).10CommunicationDevicesAregularProgramsComputerprograms,knownassoftware,areinstructionstothecomputer.

Youtellacomputerwhattodothroughprograms.Withoutprograms,acomputerisanemptymachine.Computersdonotunderstandhumanlanguages,soyouneedtousecomputerlanguagestocommunicatewiththem.Programsarewrittenusingprogramminglanguages.11ProgramsComputerprograms,knoProgrammingLanguagesMachineLanguageAssemblyLanguageHigh-LevelLanguageMachinelanguageisasetofprimitiveinstructionsbuiltintoeverycomputer.Theinstructionsareintheformofbinarycode,soyouhavetoenterbinarycodesforvariousinstructions.Programwithnativemachinelanguageisatediousprocess.Moreovertheprogramsarehighlydifficulttoreadandmodify.Forexample,toaddtwonumbers,youmightwriteaninstructioninbinarylikethis:

110110101001101012ProgrammingLanguagesMachineLProgrammingLanguagesMachineLanguageAssemblyLanguageHigh-LevelLanguageAssemblylanguagesweredevelopedtomakeprogrammingeasy.Sincethecomputercannotunderstandassemblylanguage,however,aprogramcalledassemblerisusedtoconvertassemblylanguageprogramsintomachinecode.Forexample,toaddtwonumbers,youmightwriteaninstructioninassemblycodelikethis:ADDF3R1,R2,R313ProgrammingLanguagesMachineLProgrammingLanguagesMachineLanguageAssemblyLanguageHigh-LevelLanguageThehigh-levellanguagesareEnglish-likeandeasytolearnandprogram.Forexample,thefollowingisahigh-levellanguagestatementthatcomputestheareaofacirclewithradius5:area=5*5*3.1415;

14ProgrammingLanguagesMachineLPopularHigh-LevelLanguagesCOBOL(COmmonBusinessOrientedLanguage)FORTRAN(FORmulaTRANslation)BASIC(BeginnerAll-purposeSymbolicInstructionalCode)Pascal(namedforBlaisePascal)Ada(namedforAdaLovelace)C(whosedeveloperdesignedBfirst)

VisualBasic(Basic-likevisuallanguagedevelopedbyMicrosoft)Delphi(Pascal-likevisuallanguagedevelopedbyBorland)C++(anobject-orientedlanguage,basedonC)C#(aJava-likelanguagedevelopedbyMicrosoft)Java(Weuseitinthebook)15PopularHigh-LevelLanguagesCOCompilingSourceCodeAprogramwritteninahigh-levellanguageiscalledasourceprogram.Sinceacomputercannotunderstandasourceprogram.Programcalledacompilerisusedtotranslatethesourceprogramintoamachinelanguageprogramcalledanobjectprogram.Theobjectprogramisoftenthenlinkedwithothersupportinglibrarycodebeforetheobjectcanbeexecutedonthemachine.16CompilingSourceCodeAprogramOperatingSystemsTheoperatingsystem(OS)isaprogramthatmanagesandcontrolsacomputer’sactivities.YouareprobablyusingWindows98,NT,2000,XP,orME.WindowsiscurrentlythemostpopularPCoperatingsystem.ApplicationprogramssuchasanInternetbrowserandawordprocessorcannotrunwithoutanoperatingsystem.17OperatingSystemsTheoperatingWhyJava?TheansweristhatJavaenablesuserstodevelopanddeployapplicationsontheInternetforservers,desktopcomputers,andsmallhand-helddevices.ThefutureofcomputingisbeingprofoundlyinfluencedbytheInternet,andJavapromisestoremainabigpartofthatfuture.JavaistheInternetprogramminglanguage.Javaisageneralpurposeprogramminglanguage.JavaistheInternetprogramminglanguage.18WhyJava?TheansweristhatJaJava,Web,andBeyondJavacanbeusedtodevelopWebapplications.JavaAppletsJavaWebApplicationsJavacanalsobeusedtodevelopapplicationsforhand-helddevicessuchasPalmandcellphones19Java,Web,andBeyondJavacanExamplesofJava’sVersatility(Applets)20ExamplesofJava’sVersatilityPDAandCellPhone21PDAandCellPhone21Java’sHistoryJamesGoslingandSunMicrosystemsOakJava,May20,1995,SunWorldHotJavaThefirstJava-enabledWebbrowserEarlyHistoryWebsite:/features/1998/05/birthday.html22Java’sHistoryJamesGoslinganCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicCompanionWebsite/liang/intro8e/JavaCharacteristics.pdf23CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimple

JavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicJavaispartiallymodeledonC++,butgreatlysimplifiedandimproved.SomepeoplerefertoJavaas"C++--"becauseitislikeC++butwithmorefunctionalityandfewernegativeaspects.CompanionWebsite24CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-Oriented

JavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicJavaisinherentlyobject-oriented.Althoughmanyobject-orientedlanguagesbeganstrictlyasprocedurallanguages,Javawasdesignedfromthestarttobeobject-oriented.Object-orientedprogramming(OOP)isapopularprogrammingapproachthatisreplacingtraditionalproceduralprogrammingtechniques.Oneofthecentralissuesinsoftwaredevelopmentishowtoreusecode.Object-orientedprogrammingprovidesgreatflexibility,modularity,clarity,andreusabilitythroughencapsulation,inheritance,andpolymorphism.CompanionWebsite25CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributed

JavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicDistributedcomputinginvolvesseveralcomputersworkingtogetheronanetwork.Javaisdesignedtomakedistributedcomputingeasy.SincenetworkingcapabilityisinherentlyintegratedintoJava,writingnetworkprogramsislikesendingandreceivingdatatoandfromafile.

CompanionWebsite26CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpreted

JavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicYouneedaninterpretertorunJavaprograms.TheprogramsarecompiledintotheJavaVirtualMachinecodecalledbytecode.Thebytecodeismachine-independentandcanrunonanymachinethathasaJavainterpreter,whichispartoftheJavaVirtualMachine(JVM).CompanionWebsite27CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobust

JavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicJavacompilerscandetectmanyproblemsthatwouldfirstshowupatexecutiontimeinotherlanguages.Javahaseliminatedcertaintypesoferror-proneprogrammingconstructsfoundinotherlanguages.Javahasaruntimeexception-handlingfeaturetoprovideprogrammingsupportforrobustness.CompanionWebsite28CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecure

JavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicJavaimplementsseveralsecuritymechanismstoprotectyoursystemagainstharmcausedbystrayprograms.CompanionWebsite29CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-Neutral

JavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamicWriteonce,runanywhereWithaJavaVirtualMachine(JVM),youcanwriteoneprogramthatwillrunonanyplatform.CompanionWebsite30CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortable

Java'sPerformanceJavaIsMultithreadedJavaIsDynamicBecauseJavaisarchitectureneutral,Javaprogramsareportable.Theycanberunonanyplatformwithoutbeingrecompiled.CompanionWebsite31CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformance

JavaIsMultithreadedJavaIsDynamicJava’sperformanceBecauseJavaisarchitectureneutral,Javaprogramsareportable.Theycanberunonanyplatformwithoutbeingrecompiled.CompanionWebsite32CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreaded

JavaIsDynamicMultithreadprogrammingissmoothlyintegratedinJava,whereasinotherlanguagesyouhavetocallproceduresspecifictotheoperatingsystemtoenablemultithreading.CompanionWebsite33CharacteristicsofJavaJavaIsCharacteristicsofJavaJavaIsSimpleJavaIsObject-OrientedJavaIsDistributedJavaIsInterpretedJavaIsRobustJavaIsSecureJavaIsArchitecture-NeutralJavaIsPortableJava'sPerformanceJavaIsMultithreadedJavaIsDynamic

Javawasdesignedtoadapttoanevolvingenvironment.Newcodecanbeloadedontheflywithoutrecompilation.Thereisnoneedfordeveloperstocreate,andforuserstoinstall,majornewsoftwareversions.Newfeaturescanbeincorporatedtransparentlyasneeded.

CompanionWebsite34CharacteristicsofJavaJavaIsJDKVersionsJDK1.02(1995)JDK1.1(1996)JDK1.2(1998)JDK1.3(2000)JDK1.4(2002)JDK1.5(2004)a.k.a.JDK5orJava5JDK1.6(2006)a.k.a.JDK6orJava6JDK1.7(possibly2010)a.k.a.JDK7orJava735JDKVersionsJDK1.02(1995)35JDKEditionsJavaStandardEdition(J2SE)J2SEcanbeusedtodevelopclient-sidestandaloneapplicationsorapplets.JavaEnterpriseEdition(J2EE)J2EEcanbeusedtodevelopserver-sideapplicationssuchasJavaservletsandJavaServerPages.JavaMicroEdition(J2ME).J2MEcanbeusedtodevelopapplicationsformobiledevicessuchascellphones.ThisbookusesJ2SEtointroduceJavaprogramming.

36JDKEditionsJavaStandardEditPopularJavaIDEsNetBeansOpenSourcebySunEclipseOpenSourcebyIBM37PopularJavaIDEsNetBeansOpenASimpleJavaProgram//ThisprogramprintsWelcometoJava!publicclassWelcome{ publicstaticvoidmain(String[]args){System.out.println("WelcometoJava!");}}RunWelcomeListing1.1IMPORTANTNOTE:(1)Toenablethebuttons,youmustdownloadtheentireslidefileslide.zipandunzipthefilesintoadirectory(e.g.,c:\slide).(2)YoumusthaveinstalledJDKandsetJDK’sbindirectoryinyourenvironmentpath(e.g.,c:\ProgramFiles\java\jdk1.6.0_14\bininyourenvironmentpath.38ASimpleJavaProgram//ThisprCreatingandEditingUsingNotePadTouseNotePad,typenotepadWelcome.javafromtheDOSprompt.39CreatingandEditingUsingNotCreatingandEditingUsingWordPadTouseWordPad,typewriteWelcome.javafromtheDOSprompt.40CreatingandEditingUsingWorCreating,Compiling,andRunningPrograms41Creating,Compiling,andRunniCompilingJavaSourceCodeYoucanportasourceprogramtoanymachinewithappropriatecompilers.Thesourceprogrammustberecompiled,however,becausetheobjectprogramcanonlyrunonaspecificmachine.Nowadayscomputersarenetworkedtoworktogether.Javawasdesignedtorunobjectprogramsonanyplatform.WithJava,youwritetheprogramonce,andcompilethesourceprogramintoaspecialtypeofobjectcode,knownasbytecode.ThebytecodecanthenrunonanycomputerwithaJavaVirtualMachine,asshownbelow.JavaVirtualMachineisasoftwarethatinterpretsJavabytecode.42CompilingJavaSourceCodeYouTraceaProgramExecution//ThisprogramprintsWelcometoJava!publicclassWelcome{ publicstaticvoidmain(String[]args){System.out.println("WelcometoJava!");}}Entermainmethodanimation43TraceaProgramExecution//ThiTraceaProgramExecution//ThisprogramprintsWelcometoJava!publicclassWelcome{ publicstaticvoidmain(String[]args){System.out.println("WelcometoJava!");}}Executestatementanimation44TraceaProgramExecution//ThiTraceaProgramExecution//ThisprogramprintsWelcometoJava!publicclassWelcome{ publicstaticvoidmain(String[]args){System.out.println("WelcometoJava!");}}animationprintamessagetotheconsole45TraceaProgramExecution//ThiTwoMoreSimpleExamplesRunWelcome1RunComputeExpression46TwoMoreSimpleExamplesRunWelSupplementsontheCompanionWebsiteSeeSupplementI.BforinstallingandconfiguringJDKSeeSupplementI.CforcompilingandrunningJavafromthecommandwindowfordetails/liang/intro8eCompanionWebsite47SupplementsontheCompanionWCompilingandRunningJavafromtheCommandWindowSetpathtoJDKbindirectorysetpath=c:\ProgramFiles\java\jdk1.6.0\binSetclasspathtoincludethecurrentdirectorysetclasspath=.CompilejavacWelcome.javaRunjavaWelcomeCompanionWebsite48CompilingandRunningJavafroCompilingandRunningJavafromTextPadSeeSupplementII.AontheWebsitefordetailsCompanionWebsite49CompilingandRunningJavafroCompilingandRunningJavafromJBuilderSeeSupplementII.HontheWebsitefordetailsCompanionWebsite50CompilingandRunningJavafroCompilingandRunningJavafromNetBeansSeeSupplementI.DontheWebsitefordetailsCompanionWebsite51CompilingandRunningJavafroAnatomyofaJavaProgramCommentsReservedwordsModifiersStatementsBlocksClassesMethodsThemainmethod52AnatomyofaJavaProgramCommeCommentsLinecomment:Alinecommentisprecededbytwoslashes(//)inaline.Paragraphcomment:Aparagraphcommentisenclosedbetween/*and*/inoneormultiplelines.javadoccomment:javadoccommentsbeginwith/**andendwith*/.Theyareusedfordocumentingclasses,data,andmethods.TheycanbeextractedintoanHTMLfileusingJDK'sjavadoccommand.ThreetypesofcommentsinJava.53CommentsLinecomment:AlinecReservedWordsReservedwordsorkeywordsarewordsthathaveaspecificmeaningtothecompilerandcannotbeusedforotherpurposesintheprogram.Forexample,whenthecompilerseesthewordclass,itunderstandsthatthewordafterclassisthenamefortheclass.OtherreservedwordsinListing1.1arepublic,static,andvoid.Theirusewillbeintroducedlaterinthebook.54ReservedWordsReservedwordsoModifiersJavausescertainreservedwordscalledmodifiersthatspecifythepropertiesofthedata,methods,andclassesandhowtheycanbeused.Examplesofmodifiersarepublicandstatic.Othermodifiersareprivate,final,abstract,andprotected.Apublicdatum,method,orclasscanbeaccessedbyotherprograms.Aprivatedatumormethodcannotbeaccessedbyotherprograms.ModifiersarediscussedinChapter6,“ObjectsandClasses.”55ModifiersJavausescertainresStatementsAstatementrepresentsanactionorasequenceofactions.ThestatementSystem.out.println("WelcometoJava!")intheprograminListing1.1isastatementtodisplaythegreeting"WelcometoJava!"EverystatementinJavaendswithasemicolon(;).56StatementsAstatementrepresenBlocksApairofbracesinaprogramformsablockthatgroupscomponentsofaprogram.

57BlocksApairofbracesinaprClassesTheclassistheessentialJavaconstruct.Aclassisatemplateorblueprintforobjects.ToprograminJava,youmustunderstandclassesandbeabletowriteandusethem.Themysteryoftheclasswillcontinuetobeunveiledthroughoutthisbook.Fornow,though,understandthataprogramisdefinedbyusingoneormoreclasses.58ClassesTheclassistheessentMethodsWhatisSystem.out.println?Itisamethod:acollectionofstatementsthatperformsasequenceofoperationstodisplayamessageontheconsole.Itcanbeusedevenwithoutfullyunderstandingthedetailsofhowitworks.Itisusedbyinvokingastatementwithastringargument.Thestringargumentisenclosedwithinparentheses.Inthiscase,theargumentis"WelcometoJava!"Youcancallthesameprintlnmethodwithadifferentargumenttoprintadifferentmessage.59MethodsWhatisSystem.out.prinmainMethodThemainmethodprovidesthecontrolofprogramflow.TheJavainterpreterexecutestheapplicationbyinvokingthemainmethod.

Themainmethodlookslikethis:

publicstaticvoidmain(String[]args){//Statements;}60mainMethodThemainmethodproDisplayingTextinaMessageDialogBoxyoucanusetheshowMessageDialogmethodintheJOptionPaneclass.JOptionPaneisoneofthemanypredefinedclassesintheJavasystem,whichcanbereusedratherthan“reinventingthewheel.”RunWelcomeInMessageDialogBoxIMPORTANTNOTE:Toenablethebuttons,youmustdownloadtheentireslidefileslide.zipandunzipthefilesintoadirectory(e.g.,c:\slide).61DisplayingTextinaMessageDTheshowMessageDialogMethodJOptionPane.showMessageDialog(null,"WelcometoJava!","DisplayMessage",JOptionPane.INFORMATION_MESSAGE);62TheshowMessageDialogMethodJTwoWaystoInvoketheMethodThereareseveralwaystousetheshowMessageDialogmethod.Forthetimebeing,allyouneedtoknowaretwowaystoinvokeit.Oneistouseastatementasshownintheexample:JOptionPane.showMessageDialog(null,x,y,JOptionPane.INFORMATION_MESSAGE);wherexisastringforthetexttobedisplayed,andyisastringforthetitleofthemessagedialogbox.Theotheristouseastatementlikethis:JOptionPane.showMessageDialog(null,x);wherexisastringforthetexttobedisplayed.63TwoWaystoInvoketheMethodTChapter1IntroductiontoComputers,Programs,andJava64Chapter1IntroductiontoCompObjectivesToreviewcomputerbasics,programs,andoperatingsystems(§§1.2-1.4).ToexploretherelationshipbetweenJavaandtheWorldWideWeb(§1.5).TodistinguishthetermsAPI,IDE,andJDK(§1.6).TowriteasimpleJavaprogram(§1.7).Todisplayoutputontheconsole(§1.7).ToexplainthebasicsyntaxofaJavaprogram(§1.7).Tocreate,compile,andrunJavaprograms(§1.8).(GUI)TodisplayoutputusingtheJOptionPaneoutputdialogboxes(§1.9).65ObjectivesToreviewcomputerbWhatisaComputer?AcomputerconsistsofaCPU,memory,harddisk,floppydisk,monitor,printer,andcommunicationdevices.66WhatisaComputer?AcomputerCPUThecentralprocessingunit(CPU)isthebrainofacomputer.Itretrievesinstructionsfrommemoryandexecutesthem.TheCPUspeedismeasuredinmegahertz(MHz),with1megahertzequaling1millionpulsespersecond.ThespeedoftheCPUhasbeenimprovedcontinuously.IfyoubuyaPCnow,youcangetanIntelPentium4Processorat3gigahertz(1gigahertzis1000megahertz).67CPUThecentralprocessingunitMemoryMemoryistostoredataandprograminstructionsforCPUtoexecute.Amemoryunitisanorderedsequenceofbytes,eachholdseightbits.Aprogramanditsdatamustbebroughttomemorybeforetheycanbeexecuted.Amemorybyteisneverempty,butitsinitialcontentmaybemeaninglesstoyourprogram.Thecurrentcontentofamemorybyteislostwhenevernewinformationisplacedinit.68MemoryMemoryistostoredataHowDataisStored?Dataofvariouskinds,suchasnumbers,characters,andstrings,areencodedasaseriesofbits(zerosandones).Computersusezerosandonesbecausedigitaldeviceshavetwostablestates,whicharereferredtoaszeroandonebyconvention.Theprogrammersneednottobeconcernedabouttheencodinganddecodingofdata,whichisperformedautomaticallybythesystembasedontheencodingscheme.Theencodingschemevaries.Forexample,character‘J’isrepresentedby01001010inonebyte.Asmallnumbersuchasthreecanbestoredinasinglebyte.Ifcomputerneedstostorealargenumberthatcannotfitintoasinglebyte,itusesanumberofadjacentbytes.Notwodatacanshareorsplitasamebyte.Abyteistheminimumstorageunit.69HowDataisStored?DataofvarStorageDevicesMemoryisvolatile,becauseinformationislostwhenthepowerisoff.Programsanddataarepermanentlystoredonstoragedevicesandaremovedtomemorywhenthecomputeractuallyusesthem.Therearethreemaintypesofstoragedevices:Diskdrives(harddisksandfloppydisks),CDdrives(CD-RandCD-RW),andTapedrives.70StorageDevicesMemoryisvolatOutputDevices:MonitorThemonitordisplaysinformation(textandgraphics).Theresolutionanddotpitchdeterminethequalityofthedisplay.

71OutputDevices:MonitorThemonMonitorResolutionandDotPitchTheresolutionspecifiesthenumberofpixelspersquareinch.Pixels(shortfor“pictureelements”)aretinydotsthatformanimageonthescreen.Theresolutioncanbesetmanually.Thehighertheresolution,thesharperandclearertheimageis.However,theimagemaybeverysmallifyousethighresolutiononasmallscreenmonitor.PCmonitorsareusually15-inch,17-inch,19-inch,or21-inch.Fora15-inchmonitor,acomfortableresolutionsettingwouldbe640480(307,200pixels).resolutionThedotpitchistheamountofspacebetweenpixels.Thesmallerthedotpitch,thebetterthedisplay.

dotpitch72MonitorResolutionandDotPitCommunicationDevicesAregularmodemusesaphonelineandcantransferdatainaspeedupto56,000bps(bitspersecond).ADSL(digitalsubscriberline)alsousesaphonelineandcantransferdatainaspeed20timesfasterthanaregularmodem.AcablemodemusestheTVcablelinemaintainedbythecablecompany.AcablemodemisasfastasaDSL.Networkinter