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AndroidAndroid,asasystem,isaJava-basedoperatingsystemthatrunsontheLinux2.6kernel.Thesystemisverylightweightandfullfeatured.AndroidapplicationsaredevelopedusingJavaandcanbeportedrathereasilytothenewplatform.IfyouhavenotyetdownloadedJavaorareunsureaboutwhichversionyouneed,IdetailtheinstallationofthedevelopmentenvironmentinChapter2.OtherfeaturesofAndroidincludeanaccelerated3-Dgraphicsengine(basedonhardwaresupport),databasesupportpoweredbySQLite,andanintegratedwebbrowser.IfyouarefamiliarwithJavaprogrammingorareanOOPdeveloperofanysort,youarelikelyusedtoprogrammaticuserinterface(UI)development—thatis,UIplacementwhichishandleddirectlywithintheprogramcode.Android,whilerecognizingandallowingforprogrammaticUIdevelopment,alsosupportsthenewer,XML-basedUIlayout.XMLUIlayoutisafairlynewconcepttotheaveragedesktopdeveloper.IwillcoverboththeXMLUIlayoutandtheprogrammaticUIdevelopmentinthesupportingchaptersofthisbook.OneofthemoreexcitingandcompellingfeaturesofAndroidisthat,becauseofitsarchitecture,third-partyapplications—includingthosethatare“homegrown”—areexecutedwiththesamesystempriorityasthosethatarebundledwiththecoresystem.Thisisamajordeparturefrommostsystems,whichgiveembeddedsystemappsagreaterexecutionprioritythanthethreadpriorityavailabletoappscreatedbythird-partydevelopers.Also,eachapplicationisexecutedwithinitsownthreadusingaverylightweightvirtualmachine.AsidefromtheverygenerousSDKandthewell-formedlibrariesthatareavailabletoustodevelopwith,themostexcitingfeatureforAndroiddevelopersisthatwenowhaveaccesstoanythingtheoperatingsystemhasaccessto.Inotherwords,ifyouwanttocreateanapplicationthatdialsthephone,youhaveaccesstothephone’sdialer;ifyouwanttocreateanapplicationthatutilizesthephone’sinternalGPS(ifequipped),youhaveaccesstoit.Thepotentialfordeveloperstocreatedynamicandintriguingapplicationsisnowwideopen.OntopofallthefeaturesthatareavailablefromtheAndroidsideoftheequation,Googlehasthrowninsomeverytantalizingfeaturesofitsown.DevelopersofAndroidapplicationswillbeabletotietheirapplicationsintoexistingGoogleofferingssuchasGoogleMapsandtheomnipresentGoogleSearch.SupposeyouwanttowriteanapplicationthatpullsupaGooglemapofwhereanincomingcallisemanatingfrom,oryouwanttobeabletostorecommonsearchresultswithyourcontacts;thedoorsofpossibilityhavebeenflungwideopenwithAndroid.Chapter2beginsyourjourneytoAndroiddevelopment.Youwilllearnthehow’sandwhy’sofusingspecificdevelopmentenvironmentsorintegrateddevelopmentenvironments(IDE),andyouwilldownloadandinstalltheJavaIDEEclipse.ApplicationComponentsAcentralfeatureofAndroidisthatoneapplicationcanmakeuseofelementsofotherapplications(providedthoseapplicationspermitit).Forexample,ifyourapplicationneedstodisplayascrollinglistofimagesandanotherapplicationhasdevelopedasuitablescrollerandmadeitavailabletoothers,youcancalluponthatscrollertodothework,ratherthandevelopyourown.Yourapplicationdoesn'tincorporatethecodeoftheotherapplicationorlinktoit.Rather,itsimplystartsupthatpieceoftheotherapplicationwhentheneedarises.Forthistowork,thesystemmustbeabletostartanapplicationprocesswhenanypartofitisneeded,andinstantiatetheJavaobjectsforthatpart.Therefore,unlikeapplicationsonmostothersystems,Androidapplicationsdon'thaveasingleentrypointforeverythingintheapplication(nomain()function,forexample).Rather,theyhaveessentialcomponentsthatthesystemcaninstantiateandrunasneeded.Therearefourtypesofcomponents: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