-> Receive messages-e.g. Trigger alarm upon event
-> Focused thing a user can do-e.g. Play a game
-> Background process-e.g. Play music
-> Store & retrieve data-e.g. Open a phone contact
Every application is consists one or more elements depicted above and they are instantiated by the operating system as per requirements. Moreover, other applications can use them too. Since multiple functionalities play out in OS each component pass through a life cycle begins from creation, focused, defocused and destroyed and in due course default behavior is overridden for graceful operation like saving variables or restoring UI elements.
With the exception of Store & retrieve data functionality, each component is activated by an asynchronous messages called an Intent and Intent containing a Bundle of supporting information depicting the component so in return this provides a system of passing information between components. Another thing is Activity since all activities are specifying an interaction with a user, a window is created automatically with each activity.
Create a project and an activity in Android programming
You can create an Android project using Eclipse Integrated Development Environment (IDE) during Android programming. This way you can ensure proper setup of the supporting files during Android Programming.
-> In Eclipse select file-->New-->Android Project
-> Fill the project name field
-> Choose a Build Target from the options provided
-> Fill the Application name
-> Fill the package name
-> In order to create the main activity in same step just check Create Activity field and fill in a name of the activity.
Read More:- Android App Development—Costs Factors