Main component of android application

�� Activities: The building block of the user interface is the activity. You

can think of an activity as being the Android analogue for the window

or dialog box in a desktop application. While it is possible for activities

to not have a user interface, most likely your “headless” code will be

packaged in the form of content providers or services.



�� Content providers: Content providers provide a level of abstraction for

any data stored on the device that is accessible by multiple

applications. The Android development model encourages you to

make your own data available to other applications, as well as your

own. Building a content provider lets you do that, while maintaining

complete control over how your data is accessed.



�� Services: Activities and content providers are short-lived and can be

shut down at any time. Services, on the other hand, are designed to

keep running, if needed, independent of any activity. You might use a

service for checking for updates to an RSS feed or to play back music

even if the controlling activity is no longer operating.



�� Intents: Intents are system messages, running around the inside of the

device, notifying applications of various events, from hardware state

changes (e.g., an SD card was inserted), to incoming data (e.g., an

SMS message arrived), to application events (e.g., your activity was

launched from the device’s main menu). Not only can you respond to

intents, but you can create your own to launch other activities or to let

you know when specific situations arise (e.g., raise such-and-so intent

when the user gets within 100 meters of this-and-such location).