1 Application Fundamentals Android applications are written in the Java programming language. piled Java code — along with any data and resource files required by the application — is bundled by the aapt tool into an Android package , an archive file marked by an .apk suffix. This file is the vehicle for distributing the application and installing iton mobile devices; it's the file users download to their devices. All the code ina single .apk file is considered tobeone application . In many ways, each Android application lives in its own world: 1. By default, every application runs in its own Linux process. Android starts the process when any of the application's code needs tobe executed, and shuts down the process when it's no longer needed and system resources are required by other applications. 2. Each process has its own virtual machine (VM), so application code runs in isolation from the code of all other applications. 3. By default, each application is assigned a unique Linux user ID. Permissions are set so that the application's files are visible only to that user and only to the application itself — although there are ways to export them to other applications as well. It's possible to arrange for two applications to share the same user ID, in which case they will be able to see each other's files. To conserve system resources, applications with the same ID can also arrange to run in the same Linux process, sharing the same VM. ponents A central feature of Android is that one application can make use of elements of other applications (provided those applications permit it). For example, if your application needs to display a scrolling list of images and another application has developed a suitable scroller and made it available to others, you can call upon that scroller todo the work, rather than develop your own. Your application doesn't incorporate the code of the other application or link to it. Rather, it simply starts up that piece of the other applicati
