Android - Android activity life cycle

The life cycle is shown diagrammatically below

Your activity monitors and reacts to these events by instantiating methods that override the Activity class methods for each event:

onCreate

Called when your activity is first created. This is the place you normally create your views, open any persistent datafiles your activity needs to use, and in general initialize your activity. When calling onCreate, the Android framework is passed a Bundle object that contains any activity state saved from when the activity ran before.

onStart

Called just before your activity becomes visible on the screen. Once onStart completes, if your activity can become the foreground activity on the screen, control will transfer to onResume. If the activity cannot become the foreground activity for some reason, control transfers to the onStopmethod.

onResume

Called right after onStart if your activity is the foreground activity on the screen. At this point your activity is running and interacting with the user. You are receiving keyboard and touch inputs, and the screen is displaying your user interface. onResume is also called if your activity loses the foreground to another activity, and that activity eventually exits, popping your activity back to the foreground. This is where your activity would start (or resume) doing things that are needed to update the user interface (receiving location updates or running an animation, for example).

onPause

Called when Android is just about to resume a different activity, giving that activity the foreground. At this point your activity will no longer have access to the screen, so you should stop doing things that consume battery and CPU cycles unnecessarily. If you are running an animation, no one is going to be able to see it, so you might as well suspend it until you get the screen back. Your activity needs to take advantage of this method to store any state that you will need in case your activity gains the foreground again—and it is not guaranteed that your activity will resume. If the mobile device you are running on runs out of memory, there is no virtual memory on disk to use for expansion, so your activity may have to make way for a system process that needs memory. Once you exit this method, Android may kill your activity at any time without returning control to you.

onStop

Called when your activity is no longer visible, either because another activity has taken the foreground or because your activity is being destroyed.

onDestroy

The last chance for your activity to do any processing before it is destroyed. Normally you'd get to this point because the activity is done and the framework called its finish method. But as mentioned earlier, the method might be called because Android has decided it needs the resources your activity is consuming.

It is important to take advantage of these methods to provide the best user experience possible. 

All of these are hooks that you can override to do appropriate work when the activity changes state. All activities will implement onCreate(Bundle) to do their initial setup; many will also implement onPause() to commit changes to data and otherwise prepare to stop interacting with the user. You should always call up to your superclass when implementing these methods.

 public class Activity extends ApplicationContext {
     protected void onCreate(Bundle savedInstanceState);

     protected void onStart();

     protected void onRestart();

     protected void onResume();

     protected void onPause();

     protected void onStop();

     protected void onDestroy();
 }

Here is the short program which gives more idea about android life cycle :


package com.sujith.interest;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class LifeCycleActivity extends Activity {

private final static String TAG="LifeCycleActivity";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Log.i(TAG,"On Create");
}

@Override
protected void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
Log.i(TAG,"On Destroy");
}

@Override
protected void onPause() {
// TODO Auto-generated method stub
super.onPause();
Log.i(TAG,"On Pause");
}

@Override
protected void onRestart() {
// TODO Auto-generated method stub
super.onRestart();
Log.i(TAG,"On Restart");
}

@Override
protected void onResume() {
// TODO Auto-generated method stub
super.onResume();
Log.i(TAG,"On Resume");
}

@Override
protected void onStart() {
// TODO Auto-generated method stub
super.onStart();
Log.i(TAG,"On Start");
}

@Override
protected void onStop() {
// TODO Auto-generated method stub
super.onStop();
Log.i(TAG,"On Stop");
}
}

After running the above code , you will get following screen.

To see logcat message click DDMS from right top of eclipse click LogCat View .

You will get a screen like this.