Activity in android

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. If you have worked with C, C++ or Java programming language then you must have seen that your program starts from main() function. Very similar way, Android system initiates its program with in anActivity starting with a call on onCreate() callback method. There is a sequence of callback methods that start up an activity and a sequence of callback methods that tear down an activity as shown in the below Activity lifecycle diagram: (image courtesy : android.com )
The

 

The Activity class defines the following callbacks i.e. events. You don't need to implement all the callbacks methods. However, it's important that you understand each one and implement those that ensure your app behaves the way users expect.

  

Call Back                                                           Description

onCreate()                                             This is the first callback and called when the activity is first created.

onStart()                                                This callback is called when the activity becomes visible to the user.

onResume()                                           This is called when the user starts interacting with the application.

onPause()                                              The paused activity does not receive user input     

onStop()                                                This callback is called when the activity is no longer visible.

onDestroy()                                           This callback is called before the activity is destroyed by the system

onRestart()                                             This callback is called when the activity restarts after stopping it.

Examples.....

package com.example.helloworld;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;

public class MainActivity extends Activity {
String msg = "Android : ";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.d(msg, "The onCreate() event");
}
/** Called when the activity is about to become visible. */
@Override
protected void onStart() {
super.onStart();
Log.d(msg, "The onStart() event");
}
/** Called when the activity has become visible. */
@Override
protected void onResume() {
super.onResume();
Log.d(msg, "The onResume() event");
}
/** Called when another activity is taking focus. */
@Override
protected void onPause() {
super.onPause();
Log.d(msg, "The onPause() event");
}
/** Called when the activity is no longer visible. */
@Override
protected void onStop() {
super.onStop();
Log.d(msg, "The onStop() event");
}
/** Called just before the activity is destroyed. */
@Override
public void onDestroy() {
super.onDestroy();
Log.d(msg, "The onDestroy() event");
}
}

An activity class loads all the UI component using the XML file available in res/layout folder of the project. Following statement loads UI components from res/layout/activity_main.xml file:

setContentView(R.layout.activity_main);