Android Application

Android Applications

Android apps can be written using Kotlin, the Java programming language, and C++ languages. The Android SDK tools compile your code along with any data and resource files into an APK or an Android App Bundle.
An Android package, which is an archive file with an .apk suffix, contains the contents of an Android app required at runtime, and it is the file that Android-powered devices use to install the app.
The Android system implements the principle of least privilege. That is, each app, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an app can't access parts of the system it is not given permission for.
However, there are ways for an app to share data with other apps and for an app to access system services:
• It's possible to arrange for two apps to share the same Linux user ID, in which case they are able to access each other's files. To conserve system resources, apps with the same user ID can also arrange to run in the same Linux process and share the same VM. The apps must also be signed with the same certificate.
• An app can request permission to access device data such as the device's location, camera, and Bluetooth connection. The user has to explicitly grant these permissions. For more information about permissions, see Permissions on Android.

App components

App components are the essential building blocks of an Android app. Each component is an entry point through which the system or a user can enter your app. Some components depend on others.
There are four types of app components:
• Activities
• Services
• Broadcast receivers
• Content providers
Each type serves a distinct purpose and has a distinct lifecycle that defines how a component is created and destroyed. The following sections describe the four types of app components.

Activities

An activity is the entry point for interacting with the user. It represents a single screen with a user interface. For example, an email app might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. Although the activities work together to form a cohesive user experience in the email app, each one is independent of the others.
A different app can start any one of these activities if the email app allows it. For example, a camera app might start the activity in the email app for composing a new email to let the user share a picture.

Services

A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons. It is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface.

Broadcast receivers

A broadcast receiver is a component that lets the system deliver events to the app outside of a regular user flow so the app can respond to system-wide broadcast announcements. Because broadcast receivers are another well-defined entry into the app, the system can deliver broadcasts even to apps that aren't currently running.

Content providers

A content provider manages a shared set of app data that you can store in the file system, in a SQLite database, on the web, or on any other persistent storage location that your app can access. Through the content provider, other apps can query or modify the data, if the content provider permits it.