Android Basics: Application Components

Application components are the basic building blocks of an Android application. These components are loosely coupled. The application manifest file AndroidManifest.xml describes each component of the application and how they interact with each other.

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

Activities

An activity represents a single user interface on the screen. For example, an email application might have one activity that displays a list of new emails, another activity for composing emails, and another activity for reading emails. If the application has more than one activity, then one of them should be marked as the activity presented when the application is launched.

It is implemented as a subclass of the Activity class as follows:

 public   class MainActivity extends Activity { 
 
 }

Services

A service is a component that runs in the background to perform long-running operations. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking the user from interacting with the activity.

Implementing a service as a subclass of a service class is as follows:

 public   class MyService extends Service { 
 
 }

Broadcast Receiver

Broadcast receivers simply respond to broadcast messages from other applications or from the system. For example, an application can also initiate a broadcast to let other applications know that some data has been downloaded to the device and is available for them to use, so it is the broadcast receiver that will intercept this communication and take appropriate action.

Broadcast receivers are implemented as subclasses of the BroadcastReceiver class, and each message is broadcast as an Intent object.

 public   class MyReceiver extends BroadcastReceiver { 
 
 }

Content Provider

Content provider components provide data from one application to other upon request. Such requests are handled by methods of the ContentResolver class. The data can be stored in the file system, a database or elsewhere.

Content providers implement subclasses of the ContentProvider class and must implement a standard set of APIs that enable other applications to perform transactions.

 public   class MyContentProvider extends ContentProvider { 
 
 }

We will cover the details of the application components through these tags, also in separate chapters.

Add-ons

Additional components can be used to construct the above entities, their logic and the connections between them. These components include: