Activity Basics

•  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.
• When an activity is stopped because a new activity starts, it is notified of this change in state through the activity's lifecycle callback methods.
• When stopped your activity should release any large objects, such as network or database connections. When the activity resumes, you can reacquire the necessary resources and resume actions that were interrupted. These state transitions are all part of the activity lifecycle.

Declaring the activity in the manifest

• Use <intent-filter> element—in order to declare how other application components may activate it.
• 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).

Activity lifecycle callbacks

An activity can exist in essentially three states:    

Resumed :

The activity is in the foreground of the screen and has user focus. (This state is also sometimes referred to as "running".)

Paused :

Another activity is in the foreground and has focus, but this one is still visible. That is, another activity is visible on top of this one and that activity is partially transparent or doesn't cover the entire screen. A paused activity is completely alive (the Activity object is retained in memory, it maintains all state and member information, and remains attached to the window manager), but can be killed by the system in extremely low memory situations.

Stopped :

The activity is completely obscured by another activity (the activity is now in the "background"). A stopped activity is also still alive (the Activity object is retained in memory, it maintains all state and member information, but is not attached to the window manager). However, it is no longer visible to the user and it can be killed by the system when memory is needed elsewhere.

•   If an activity is paused or stopped, the system can drop it from memory either by asking it to finish (calling its finish() method), or simply killing its process. When the activity is opened again (after being finished or killed), it must be created all over.

• if your activity has a thread running in the background to download data from the network, it might create that thread in onCreate() and then stop the thread in onDestroy().

• onPause() is called when the device goes to sleep or when a dialog appears.

• The order of lifecycle callbacks is well defined, particularly when the two activities are in the same process and one is starting the other. Here's the order of operations that occur when Activity A starts Acivity B:
  
   - >  Activity A's onPause() method executes.
   - >  Activity B's onCreate(), onStart(), and onResume() methods execute in sequence. (Activity B now has user focus.)
   - >  Then, if Activity A is no longer visible on screen, its onStop() method executes.

 Some device configurations can change during runtime (such as screen orientation, keyboard availability, and language). When such a change occurs, Android restarts the running Activity (onDestroy() is called, followed by onCreate()). The restart behavior is designed to help your application adapt to new configurations by automatically reloading your application with alternative resources that match the new device configuration.