android populer

public class MainActivity extends Activity {
}

Services :
          A service 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.For example : Running an alarm in background
A service is implemented as a subclass of Service.

Content providers :
          They manages a shared set of application data.Data can be stored in SQLite Database,File systems,web or any other medium where application can access.
Content Providers allows to modify and use these stored data.For example : storing users contacts details,where proper permission one can get the contacts.
A content provider is implemented as a subclass of ContentProvider.

Broadcast receivers :
          They responds to systems  broadcast announcements.Some of the native Broadcast are volume control, light, switch on and off, screen on and off,bluetooth etc.A broadcast receiver is just a "gateway" to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.
A broadcast receiver is implemented as a subclass of BroadcastReceiver.

Who Activates A component ?
         The above components are activated by a message called Intents.
Intents bind individual components to each other at runtime, whether the component belongs to your application or another.
An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of component—an intent can be either explicit or implicit, respectively.

         The Content Provider, is not activated by intents. It is activated when there is request from ContentResolver. These Resolvers handles direct transaction with the providers so they donot need further any method calls. This leaves a layer of abstraction between the content provider and the component requesting information (for security).

What is an AndroidManifest file ?
          This is a root directory of Android application where all the components are declared.Other than this they also have

• user permissions the application requires, such as read and write external storage
• Declare the minimum API level
• Declare hardware and software features such as bluetooth or a printer
• API libraries etc...

 

in java file:

 
public class MainActivity extends Activity {
  //Create broadcast object
    BroadcastReceiver mybroadcast = new BroadcastReceiver() {
  
  //When Event is published, onReceive method is called
  @Override
  public void onReceive(Context context, Intent intent) {
   // TODO Auto-generated method stub
       Log.i("[BroadcastReceiver]", "MyReceiver");

          if(intent.getAction().equals(Intent.ACTION_SCREEN_ON)){
              Log.i("[BroadcastReceiver]", "Screen ON");
          }
          else if(intent.getAction().equals(Intent.ACTION_SCREEN_OFF)){
              Log.i("[BroadcastReceiver]", "Screen OFF");
          }
   }};

   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
 
       registerReceiver(mybroadcast, new IntentFilter(Intent.ACTION_SCREEN_ON));
       registerReceiver(mybroadcast, new IntentFilter(Intent.ACTION_SCREEN_OFF));
   }  
}

See Introduction for Broadcast Receiver here:

 When you need to call a broadcast (suppose here battery value) you need to register the broadcast and when the event is published it will call onReceive method which have context and particular intent as parameter.

 
public class Broadcast extends Activity {
  //Create broadcast object
    BroadcastReceiver mybroadcast = new BroadcastReceiver() {
   
 //When Event is published, onReceive method is called
    @Override
 public void onReceive(Context context, Intent intent) {
     //Get battery percentage
      int batterylevel = intent.getIntExtra("level", 0);     
     //get progressbar
      ProgressBar myprogressbar = (ProgressBar) findViewById(R.id.progressbar);
      myprogressbar.setProgress(batterylevel);
      TextView tv = (TextView) findViewById(R.id.textfield);
    //Set TextView with text
      tv.setText("Battery Level: " + Integer.toString(batterylevel) + "%");
     }
 };
 
    @Override
   public void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_broadcast);    
      registerReceiver(mybroadcast, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
    } 
} 

The arguments of the registerReceiver() method
receiver :: The BroadcastReceiver you want to register
filter :: The IntentFilter object that specifies which event your receiver should listen to.

To learn more on event and usages click here...