Anddev.org • Index page. Home. Android Developers Blog. Highest Voted 'android' Questions. Application Fundamentals. Android apps are written in the Java programming language. The Android SDK tools compile your code—along with any data and resource files—into an APK: an Android package, which is an archive file with an .apk suffix. One APK file contains all the contents of an Android app and is the file that Android-powered devices use to install the app. Once installed on a device, each Android app lives in its own security sandbox: The Android operating system is a multi-user Linux system in which each app is a different user. By default, the system assigns each app a unique Linux user ID (the ID is used only by the system and is unknown to the app). The system sets permissions for all the files in an app so that only the user ID assigned to that app can access them.
In this way, the Android system implements the principle of least privilege. However, there are ways for an app to share data with other apps and for an app to access system services: The core framework components that define your app. Programming Android (9781449389697): Zigurd Mednieks, Laird Dornin, G. Blake Meike, Masumi Nakamura. Series: Android Basics « | mybringback.