关于JAVA领域的外文翻译Word文件下载.doc

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Asthe1980sapproached,programmerswerewitnessinganotherspurtintheevolutionofprogramminglanguage.ComputertechnologyadvancedbeyondthecapabilitiesoftheCprogramminglanguage.Theproblemwasn’tnew.Itoccurredpreviouslyandcausedthedemiseofgenerationsofprogramminglanguages.Theproblemwasthatprogramswerebecomingtoocomplicatedtodesign,write,andmanagetokeepupwiththecapabilitiesofcomputers.ItwasaroundthistimethatadesignconceptbasedonSimula67andSmalltalk（fromthelate1960s）movedprogrammingtothenextevolutionarystep.Thiswastheperiodwhenobject-orientedprogramming（OOP）,andwithitanewprogramminglanguagecalledC++,tookprogrammersbystorm.

In1979,BjarneStroustrupofBellLaboratoriesinNewJerseyenhancedtheCprogramminglanguagetoincludeobject-orientedfeatures.HecalledthelanguageC++.（The++istheincrementaloperatorintheCprogramminglanguage.）C++istrulyanenhancementoftheCprogramminglanguage,anditbeganasapreprocessorlanguagethatwastranslatedintoCsyntaxbeforetheprogramwasprocessedbythecompiler.

Stroustrupbuiltontheconceptofaclass（takenfromSimula67andSmalltalk）,fromwhichinstancesofobjectsarecreated.Aclasscontainsdatamembersandmemberfunctionsthatdefineanobject’sdataandfunctionality.Healsointroducedtheconceptofinheritance,whichenabledaclasstoinheritsomeoralldatamembersandmemberfunctionsfromoneormoreotherclasses—allofwhichcomplementstheconceptsofobject-orientedprogramming.

By1988,ANSIofficialsstandardizedStroustrup’sC++specification.

2EnterJava

JustasC++wasbecomingthelanguageofchoiceforbuildingindustrial-strengthapplications,anothergrowthspurtintheevolutionofprogramminglanguagewasbudding,fertilizedbythelatestdisruptivetechnology—theWorldWideWeb.TheInternethadbeenawell-keptsecretfordecadesbeforetheNationalScienceFoundation（whooversawtheInternet）removedbarriersthatpreventedcommercialization.Until1991whenitwasopenedtocommerce,theInternetwasthealmostexclusivedomainofgovernmentagenciesandtheacademiccommunity.Oncethebarriertocommercializationwaslifted,theWorldWideWeb—oneofseveralservicesofferedontheInternet—becameavirtualcommunitycenterwherevisitorscouldgetfreeinformationaboutpracticallyanythingandbrowsethroughthousandsofvirtualstores.

BrowserspowertheWorldWideWeb.AbrowsertranslatesASCIItextfileswritteninHTMLintoaninteractivedisplaythatcanbeinterpretedonanymachine.AslongasthebrowseriscompatiblewiththecorrectversionofHTMLandHTTPimplementation,anycomputerrunningthebrowsercanusethesameHTMLdocumentwithouthavingtomodifyitforaparticulartypeofcomputer,whichwassomethingunheardofatthetime.ProgramswritteninCorC++aremachinedependentandcannotrunonadifferent

machineunlesstheprogramisrecompiled.

ThesuccessoftheInternetgaverenewedfocustodevelopingamachine-independentprogramminglanguage.AndthesameyeartheInternetwascommercialized,fivetechnologistsatSunMicrosystemssetouttodojustthat.JamesGosling,PatrickNaughton,ChrisWarth,EdFrank,andMikeSheridanspent18monthsdevelopingtheprogramminglanguagetheycalledOak,whichwasrenamedJavawhenthisnewlanguagemadeitsdebutin1995.Javawentthroughnumerousiterationsbetween1991and1995,duringwhichtimemanyothertechnologistsatSunmadesubstantialcontributionstothelanguage.TheseincludedBillJoy,ArthurvanHoff,JonathanPayne,FrankYelin,andTimLindholm.

AlthoughJavaiscloselyassociatedwiththeInternet,itwasdevelopedasalanguageforprogrammingsoftwarethatcouldbeembeddedintoelectronicdevicesregardlessofthetypeofCPUusedbythedevice.ThisisknownastheEmbeddedJavaplatformandisincontinuoususetodayforclosedsystems.

TheJavateamfromSunsucceededincreatingaportableprogramminglanguage,somethingthathadeludedprogrammerssincecomputerswerefirstprogrammed.Theirsuccess,however,wasfarbeyondtheirwildestdreams.ThesameconceptusedtomakeJavaprogramsportabletoelectronicdevicesalsocouldbeusedtomakeJavaprogramsrunoncomputersrunningMicrosoftWindows,UNIX,andMacintosh.Timingwasperfect.TheInternet/intranethadwhettedcorporateAmerica’sappetiteforcost-effective,portableprogramsthatcouldreplacemission-criticalapplicationswithinthecorporation.AndJavahadprovenitselfasaprogramminglanguageusedtosuccessfullydevelopmachine-independentapplications.

3JavaVirtualMachine

WritingJavaprogramsissimilartowritingC++programsinthattheprogrammerwritessourcecodethatcontainsinstructionsintoaneditor,orinanintegrateddevelopment

environment,andthenthesourcecodeiscompiled.However,that’swhereJavaandC++

partways.ThecompilingandlinkingprocessofaC++programresultsinanexecutable

thatcanberunonanappropriatemachine.Incontrast,theJavacompilerconvertsJava

sourcecodeintobytecodethatisexecutedbytheJavaVirtualMachine（JVM）.

Machine-specificinstructionsarenotincludedinbytecode.Instead,theyalreadyresideintheJVM,whichismachinespecific.ThismeansthatthebytecodemightcontainfewerinstructionsthatneedtobetranslatedthanacomparableC++program.

AlthoughtheJavacompilergeneratesbytecodethatmustbeinterpretedbytheJVM

atruntime,thenumberofinstructionsthatneedtranslationareusuallyminimaland

havealreadybeenoptimizedbytheJavacompiler.

4BacktotheFuture:

J2ME

RememberthatJavabeganasaprogramminglanguagetocreateprogramsforembeddedsystems—microcomputersfoundinconsumerandindustrialproductssuchasthoseusedtocontrolautomobilesandappliances.ThedevelopmentteamatSunworkedonJavaintheearly1990stoaddresstheprogrammingneedsofthefledglingembeddedcomputermarket,butthateffortwassidetrackedbymorecompellingopportunitiespresentedbytheInternet.

Asthoseopportunitieswereaddressed,anewbreedofportablecommunicationsdevicesopenedotheropportunitiesattheturnofthecentury.CellphonesexpandedJ2ME:

TheCompleteReferencefromvoicecommunicationsdevicestovoiceandtextcommunicationsdevices.Pocketelectronictelephonedirectoriesevolvedintopersonaldigitalassistants.Chipmakerswerereleasingnewproductsatthistimethatweredesignedtotransfercomputingpowerfromadesktopcomputerintomobilesmallcomputersthatcontrolledgaspumps,cabletelevisionboxes,andanassortmentofotherappliances.

ThetimewasrightforthenextevolutionofJava.However,insteadofbeefingupJavawithadditionalAPIs,theteamatSun,alongwiththeJavaCommunityProcessProgram,dismantledboththeJavaprogramminglanguageandtheJavaVirtualMachine.They

strippeddownJavaAPIsandtheJVMtotheminimumcodingrequiredtoprovide

intelligencetoembeddedsystemsandmicrocomputerdevices.Thiswasnecessary

becauseofresourceconstraintsimposeduponthehardwaredesignofthesedevices.The

resultoftheireffortsisJ2ME.J2MEisareducedversionoftheJavaAPIandJavaVirtualMachinethatisdesignedtooperatewithinthesparseresourcesavailableinthenewbreedofembeddedcomputersandmicrocomputers.

5HowJ2MEIsOrganized

Traditionalcomputingdevicesusefairlystandardhardwareconfigurationssuchasadisplay,keyboard,mouse,andlargeamountsofmemoryandpermanentstorage.However,thenewbreedofcomputingdeviceslackshardwareconfigurationcontinuityamongdevices.Somedevicesdon’thaveadisplay,permanentstorage,keyboard,ormouse.Andmemoryavailabilityisinconsistentamongsmallcomputingdevices.ThelackofuniformhardwareconfigurationamongthesmallcomputingdevicesposesaformidablechallengefortheJavaCommunityProcessProgram,whichischargedwithdevelopingstandardsfortheJVMandtheJ2MEforsmallcomputingdevices.

J2MEmustservicemanydifferentkindsofsmallcomputingdevices,includingscreenphones,digitalset-topboxesusedforcabletelevision,cellphones,andpersonaldigitalassistants.ThechallengefortheJavaCommunityProcessProgramistodevelopaJavastandardthatcanbeimplementedonsmallcomputingdevicesthathavenonstandard

hardwareconfigurations.TheJavaCommunityProcessProgramhasusedatwofoldapproachtoaddressingtheneedsofsmallcomputingdevices.First,theydefinedtheJavarun-timeenvironmentandcoreclassesthatoperateoneachdevice.Thisisreferredtoastheconfiguration.AconfigurationdefinestheJavaVirtualMachineforaparticularsmallcomputingdevice.Therearetwoconfigurations,oneforhandhelddevicesandtheotherforplug-indevices.Next,theJavaCommunityProcessProgramdefinedaprofileforcategoriesofsmallcomputingdevices.Aprofileconsistsofclassesthatenabledeveloperstoimplementfeaturesfoundonarelatedgroupofsmallcomputingdevices.

6J2MEconfigurations

TherearetwoconfigurationsforJ2MEasofthiswriting.TheseareConnectedLimitedDeviceConfiguration（CLDC）andtheConnectedDeviceConfiguration（CDC）.TheCLDCisdesignedfor16-bitor32-bitsmallcomputingdeviceswithlimitedamountsofmemory.

CLDCdevicesusuallyhavebetween160KBand512KBofavailablememoryandar