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英文原文ApplicationFundamentalsAndroidapplicationsarewrittenintheJavaprogramminglanguage.ThecompiledJavacode—alongwithanydataandresourcefilesrequiredbytheapplication—isbundledbytheaapttoolintoanAndroidpackage,anarchivefilemarkedbyan.apksuffix.Thisfileisthevehiclefordistributingtheapplicationandinstallingitonmobiledevices;it'sthefileusersdownloadtotheirdevices.Allthecodeinasingle.apkfileisconsideredtobeoneapplication.Inmanyways,eachAndroidapplicationlivesinitsownworld:(1)Bydefault,everyapplicationrunsinitsownLinuxprocess.Androidstartstheprocesswhenanyoftheapplication'scodeneedstobeexecuted,andshutsdowntheprocesswhenit'snolongerneededandsystemresourcesarerequiredbyotherapplications.(2)Eachprocesshasitsownvirtualmachine(VM),soapplicationcoderunsinisolationfromthecodeofallotherapplications.(3)Bydefault,eachapplicationisassignedauniqueLinuxuserID.Permissionsaresetsothattheapplication'sfilesarevisibleonlytothatuserandonlytotheapplicationitself—althoughtherearewaystoexportthemtootherapplicationsaswell.It'spossibletoarrangefortwoapplicationstosharethesameuserID,inwhichcasetheywillbeabletoseeeachother'sfiles.Toconservesystemresources,applicationswiththesameIDcanalsoarrangetoruninthesameLinuxprocess,sharingthesameVM.ApplicationComponentsAcentralfeatureofAndroidisthatoneapplicationcanmakeuseofelementsofotherapplications(providedthoseapplicationspermitit).Forexample,ifyourapplicationneedstodisplayascrollinglistofimagesandanotherapplicationhasdevelopedasuitablescrollerandmadeitavailabletoothers,youcancalluponthatscrollertodothework,ratherthandevelopyourown.Yourapplicationdoesn'tincorporatethecodeoftheotherapplicationorlinktoit.Rather,itsimplystartsupthatpieceoftheotherapplicationwhentheneedarises.Forthistowork,thesystemmustbeabletostartanapplicationprocesswhenanypartofitisneeded,andinstantiatetheJavaobjectsforthatpart.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdon'thaveasingleentrypointforeverythingintheapplication(nomain()function,forexample).Rather,theyhaveessentialcomponentsthatthesystemcaninstantiateandrunasneeded.Therearefourtypesofcomponents:(1)ActivitiesAnactivitypresentsavisualuserinterfaceforonefocusedendeavortheusercanundertake.Forexample,anactivitymightpresentalistofmenuitemsuserscanchoosefromoritmightdisplayphotographsalongwiththeircaptions.Atextmessagingapplicationmighthaveoneactivitythatshowsalistofcontactstosendmessagesto,asecondactivitytowritethemessagetothechosencontact,andotheractivitiestoreviewoldmessagesorchangesettings.Thoughtheyworktogethertoformacohesiveuserinterface,eachactivityisindependentoftheothers.EachoneisimplementedasasubclassoftheActivitybaseclass.Anapplicationmightconsistofjustoneactivityor,likethetextmessagingapplicationjustmentioned,itmaycontainseveral.Whattheactivitiesare,andhowmanytherearedepends,ofcourse,ontheapplicationanditsdesign.Typically,oneoftheactivitiesismarkedasthefirstonethatshouldbepresentedtotheuserwhentheapplicationislaunched.Movingfromoneactivitytoanotherisaccomplishedbyhavingthecurrentactivitystartthenextone.Eachactivityisgivenadefaultwindowtodrawin.Typically,thewindowfillsthescreen,butitmightbesmallerthanthescreenandfloatontopofotherwindows.Anactivitycanalsomakeuseofadditionalwindows—forexample,apop-updialogthatcallsforauserresponseinthemidstoftheactivity,orawindowthatpresentsuserswithvitalinformationwhentheyselectaparticularitemon-screen.Thevisualcontentofthewindowisprovidedbyahierarchyofviews—objectsderivedfromthebaseViewclass.Eachviewcontrolsaparticularrectangularspacewithinthewindow.Parentviewscontainandorganizethelayoutoftheirchildren.Leafviews(thoseatthebottomofthehierarchy)drawintherectanglestheycontrolandrespondtouseractionsdirectedatthatspace.Thus,viewsarewheretheactivity'sinteractionwiththeusertakesplace.Forexample,aviewmightdisplayasmallimageandinitiateanactionwhentheusertapsthatimage.Androidhasanumberofready-madeviewsthatyoucanuse—includingbuttons,textfields,scrollbars,menuitems,checkboxes,andmore.Aviewhierarchyisplacedwithinanactivity'swindowbytheActivity.setContentView()method.ThecontentviewistheViewobjectattherootofthehierarchy.(SeetheseparateUserInterfacedocumentformoreinformationonviewsandthehierarchy.)(2)ServicesAservicedoesn'thaveavisualuserinterface,butratherrunsinthebackgroundforanindefiniteperiodoftime.Forexample,aservicemightplaybackgroundmusicastheuserattendstoothermatters,oritmightfetchdataoverthenetworkorcalculatesomethingandprovidetheresulttoactivitiesthatneedit.EachserviceextendstheServicebaseclass.Aprimeexampleisamediaplayerplayingsongsfromaplaylist.Theplayerapplicationwouldprobablyhaveoneormoreactivitiesthatallowtheusertochoosesongsandstartplayingthem.However,themusicplaybackitselfwouldnotbehandledbyanactivitybecauseuserswillexpectthemusictokeepplayingevenaftertheyleavetheplayerandbeginsomethingdifferent.Tokeepthemusicgoing,themediaplayeractivitycouldstartaservicetoruninthebackground.Thesystemwouldthenkeepthemusicplaybackservicerunningevenaftertheactivitythatstarteditleavesthescreen.It'spossibletoconnectto(bindto)anong