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Android applications are written in the Java programming language. The compiled

Java code — along with any data and resource files required by the application — is

bundled by the aapt tool into an Android package, an archive file marked by an .apk

suffix. This file is the vehicle for distributing the application and installing it on mobile

devices; it's the file users download to their devices. All the code in a single .apk file is

considered to be one application.

In many ways, each Android application lives in its own world:

1. By default, every application runs in its own Linux process. Android starts the

process when any of the application's code needs to be executed, and shuts down the

process when it's no longer needed and system resources are required by other

It's possible to arrange for two applications to share the same user ID, in which case

they will be able to see each other's files. To conserve system resources, applications

with the same ID can also arrange to run in the same Linux process, sharing the same

VM.

Application Components

A central feature of Android is that one application can make use of elements of other

applications (provided those applications permit it). For example, if your application

needs to display a scrolling list of images and another application has developed a

suitable scroller and made it available to others, you can call upon that scroller to do the

work, rather than develop your own. Your application doesn't incorporate the code of the

essential components that the system can instantiate and run as needed. There are four

types of components:

Activities

An activity presents a visual user interface for one focused endeavor the user can

undertake. For example, an activity might present a list of menu items users can choose

from or it might display photographs along with their captions. A text messaging

application might have one activity that shows a list of contacts to send messages to, a

second activity to write the message to the chosen contact, and other activities to review

old messages or change settings. Though they work together to form a cohesive user

interface, each activity is independent of the others. Each one is implemented as a

the current activity start the next one.

Each activity is given a default window to draw in. Typically, the window fills the

screen, but it might be smaller than the screen and float on top of other windows. An

activity can also make use of additional windows — for example, a pop-up dialog that

calls for a user response in the midst of the activity, or a window that presents users with

vital information when they select a particular item on-screen.

The visual content of the window is provided by a hierarchy of views — objects

derived from the base View class. Each view controls a particular rectangular space

within the window. Parent views contain and organize the layout of their children. Leaf

views (those at the bottom of the hierarchy) draw in the rectangles they control and

Activity.setContentView() method. The content view is the View object at the root of the

hierarchy. (See the separate User Interface document for more information on views and

the hierarchy.)

Services

A service doesn't have a visual user interface, but rather runs in the background for an

indefinite period of time. For example, a service might play background music as the

user attends to other matters, or it might fetch data over the network or calculate

something and provide the result to activities that need it. Each service extends the

Service base class.

A prime example is a media player playing songs from a play list. The player

application would probably have one or more activities that allow the user to choose

songs and start playing them. However, the music playback itself would not be handled

by an activity because users will expect the music to keep playing even after they leave

the player and begin something different. To keep the music going, the media player

activity could start a service to run in the background. The system would then keep the

to pause, rewind, stop, and restart the playback.

Like activities and the other components, services run in the main thread of the

application process. So that they won't block other components or the user interface,

they often spawn another thread for time-consuming tasks (like music playback). See

Processes and Threads, later.

A broadcast receiver is a component that does nothing but receive and react to

broadcast announcements. Many broadcasts originate in system code — for example,

announcements that the timezone has changed, that the battery is low, that a picture has

been taken, or that the user changed a language preference. Applications can also initiate

activity in response to the information they receive, or they may use the

NotificationManager to alert the user. Notifications can get the user's attention in various

ways — flashing the backlight, vibrating the device, playing a sound, and so on. They