Activity in Android

What is Activity in Android
Activity is a one of the most important component in android.User interacted with UI and perform some action in application.

Life Cycle Method of Activity
They have callback method below
1-onCreate(Bundle onSaveInstance); //Called when the activity is first created

 2-onStart() // Called when the activity is becoming visible to the user.

 3-onResume()//Called when the activity will start interacting with the user. At this point your activity is at the top of the activity stack, with user input going to it

 4-onPause() // Called when the system is about to start resuming a previous activity. This is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, etc. Implementations of this method must be very quick because the next activity will not be resumed until this method returns.
Followed by either onResume() if the activity returns back to the front, or onStop() if it becomes invisible to the user.

5-onStop() // Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this one. This may happen either because a new activity is being started, an existing one is being brought in front of this one, or this one is being destroyed.
Followed by either onRestart() if this activity is coming back to interact with the user, or onDestroy() if this activity is going away

6-onRestart() // Called after your activity has been stopped, prior to it being started again.
Always followed by onStart()

7-onDestroy() // The final call you receive before your activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.

 Killable? 
They are not Killable below
  onCreate(Bundle onSaveInstance)
  onStart()
  onResume()
  onRestart()

onPause() are Killable Pre-HoneyComb

They are killable below
onStop()
onDestroy()

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