Android Tutorial 26 : Activity In Android

•An activity presents a visual user interface for one focused endeavor the  user can undertake.

•An application might consist of just one activity or many.

•Each one is implemented as a subclass of the Activity base class

An Activity is an application component that provides a screen with which users can interact in order to do something, such as dial the phone, take a photo, send an email, or view a map. Each activity is given a window in which to draw its user interface.  

An application usually consists of multiple activities that are loosely bound to each other. Typically, one activity in an application is specified as the "main" activity, which is presented to the user when launching the application for the first time. Each activity can then start another activity in order to perform different actions. Each time a new activity starts, the previous activity is stopped, but the system preserves the activity in a stack (the "back stack"). When a new activity starts, it is pushed onto the back stack and takes user focus.. 

Declaring Activity In Manifest

<manifest ... >
  <application ... >
      <activity android:name=".ExampleActivity" />
      ...
  </application ...> 

</manifest >

Using Intent Filters  In Activity Tag

<activity android:name=".ExampleActivity" android:icon="@drawable/app_icon">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>

</activity> 

The <action> element specifies that this is the "main" entry point to the application.

 The <category> element specifies that this activity should be listed in the
system's application launcher (to allow users to launch this activity). 

Starting An Activity:

You can start another activity by calling startActivity(), passing it an Intent that describes the activity you want to start. The intent specifies either the exact activity you want to start or describes the type of action you want to perform

     Intent intent = new Intent(this, SignInActivity.class);
startActivity(intent); 

Implementing the lifecycle callbacks:

When an activity transitions into and out of the different states described above, it is notified through various callback methods. All of the callback methods are hooks that you can override to do appropriate work when the state of your activity changes. The following skeleton activity includes each of the fundamental lifecycle methods:

public class ExampleActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // The activity is being created.
    }
    @Override
    protected void onStart() {
        super.onStart();
        // The activity is about to become visible.
    }
    @Override
    protected void onResume() {
        super.onResume();
        // The activity has become visible (it is now "resumed").
    }
    @Override
    protected void onPause() {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
    }
    @Override
    protected void onStop() {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        // The activity is about to be destroyed.
    }
}