Android Application Fundamentals

Application components are the essential building blocks of an Android application. These components are loosely coupled by the application manifest file AndroidManifest.xml that describes each component of the application and how they interact.

There are following four main components that can be used within an Android application:

Components

Description

Activities
They dictate
the UI and handle the user interaction to the smartphone screen
Services
They handle
background processing associated with an application.
Broadcast
Receivers
They handle
communication between Android OS and applications.
Content
Providers
They handle
data and database management issues.

Activities
An activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. If an application has more than one activity, then one of them should be marked as the activity that is presented when the application is launched.
An activity is implemented as a subclass of Activity class as follows: