Activities in Android Studio

The Activity class is a crucial component of an Android app, and the way activities are launched and put together is a fundamental part of the platform's application model. Unlike programming paradigms in which apps are launched with a main() method, the Android system initiates code in an Activity instance by invoking specific callback methods that correspond to specific stages of its lifecycle.

Declare activities

To declare your activity, open your manifest file and add an <activity> element as a child of the <application> element. For example:

Declare intent filters

Intent filters are a very powerful feature of the Android platform. They provide the ability to launch an activity based not only on an explicit request, but also an implicit one.

Managing the activity lifecycle

Over the course of its lifetime, an activity goes through a number of states. You use a series of callbacks to handle transitions between states. The following sections introduce these callbacks.

onCreate()

You must implement this callback, which fires when the system creates your activity. Your implementation should initialize the essential components of your activityonStart()

As onCreate() exits, the activity enters the Started state, and the activity becomes visible to the user. onResume()

The system invokes this callback just before the activity starts interacting with the user. 

The onPause() callback always follows onResume().

onPause()

The system calls onPause() when the activity loses focus and enters a Paused state. 

onStop()

The system calls onStop() when the activity is no longer visible to the user. 

onRestart()

The system invokes this callback when an activity in the Stopped state is about to restart. onDestroy()

The system invokes this callback before an activity is destroyed.