Android Application Fundamentals

•  Android applications are written in the Java programming language.
• The Android SDK tools compile the code—along with any data and resource files—into an Android package, an archive file with an .apk suffix.
• By using the aapt tool, the data and resources bundled into a package.
•   aapt- android asset packaging tool.
• apk- android package kit.
• The .apk file is used to install the applications on Android-powered devices.
• Each Android application lives in its own world:
•  By default, every application runs in its own Linux process.
• Android starts the process when any of the application's code needs to be executed, and shuts down the process when it's no longer needed or when some system resources are required by other applications.
•  Each process has its own virtual machine (VM), so application code runs in isolation from the code of all other applications.
•  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 to the application itself.
• 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.
• An application can request permission to access device data such as the user's contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissions must be granted by the user at install time.