Introduction to Android

Android apps are written in the Java programming language. The Android SDK tools compile your code—along with any data and resource files—into an APK: an Android package, which is an archive file with an .apk suffix. One APK file contains all the contents of an Android app and is the file that Android-powered devices use to install the app.

Once installed on a device, each Android app lives in its own security sandbox:

  • The Android operating system is a multi-user Linux system in which each app is a different user.
  • By default, the system assigns each app a unique Linux user ID (the ID is used only by the system and is unknown to the app). The system sets permissions for all the files in an app so that only the user ID assigned to that app can access them.
  • Each process has its own virtual machine (VM), so an app’s code runs in isolation from other apps.
  • By default, every app runs in its own Linux process. Android starts the process when any of the app’s components need to be executed, then shuts down the process when it’s no longer needed or when the system must recover memory for other apps.

In this way, the Android system implements the principle of least privilege. That is, each app, by default, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an app cannot access parts of the system for which it is not given permission.

However, there are ways for an app to share data with other apps and for an app to access system services:

  • It’s possible to arrange for two apps to share the same Linux user ID, in which case they are able to access each other’s files. To conserve system resources, apps with the same user ID can also arrange to run in the same Linux process and share the same VM (the apps must also be signed with the same certificate).
  • An app can request permission to access device data such as the user’s contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. The user has to explicitly grant these permissions.

App Components


App components are the essential building blocks of an Android app. Each component is a different point through which the system can enter your app. Not all components are actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific role—each one is a unique building block that helps define your app’s overall behavior.

There are four different types of app components. Each type serves a distinct purpose and has a distinct lifecycle that defines how the component is created and destroyed.

Here are the four types of app components:

  • Activities

An activity represents a single screen with a user interface. For example,  contacts  app might have one activity that shows a list of all contacts, another activity to create new contact , and another activity for showing list of dialed numbers. Although the activities work together to form a cohesive user experience in the email app, each one is independent of the others. As such, a different app can start any one of these activities (if the app allows it).

  • Services

A service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide a user interface. For example, a service might play music in the background while the user is in a different app, or it might fetch data over the network without disturbing user interaction with another activity.

  • Content providers

A content provider manages a shared set of app data. You can store the data in the file system, an SQLite database, on the web, or any other persistent storage location your app can access. Through the content provider, other apps can query or even modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the user’s contact information. As such, any app with the proper permissions can query part of the content provider to read and write information about a particular person. Content providers are also useful for reading and writing data that is private to your app and not shared.

  • Broadcast receivers

A broadcast receiver is a component that responds to system-wide broadcast announcements. Many broadcasts originate from the system—for example, a broadcast announcing that the battery is low, or a screen shot is captured.  Although broadcast receivers don’t display a user interface, they may create a status bar notification to alert the user when a broadcast event occurs.

A unique aspect of the Android system design is that any app can start another app’s component. For example, if you want the user to capture a photo with the device camera, there’s probably another app that does that and your app can use it, instead of developing an activity to capture a photo yourself. You don’t need to incorporate or even link to the code from the camera app. Instead, you can simply start the activity in the camera app that captures a photo. When complete, the photo is even returned to your app so you can use it. To the user, it seems as if the camera is actually a part of your app.

Because the system runs each app in a separate process with file permissions that restrict access to other apps, your app cannot directly activate a component from another app. The Android system, however, can. So, to activate a component in another app, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the component for you.

The Manifest File

Before the Android system can start an app component, the system must know that the component exists by reading the app’s AndroidManifest.xml file (the “manifest” file). Your app must declare all its components in this file, which must be at the root of the app project directory.

The manifest does a number of things in addition to declaring the app’s components, such as:

  • Identify any user permissions the app requires, such as Internet access or read-access to the user’s contacts.
  • Declare the minimum API Level required by the app, based on which APIs the app uses.
  • Declare hardware and software features used or required by the app, such as a camera, bluetooth services, or a multitouch screen.
  • API libraries the app needs to be linked against (other than the Android framework APIs), such as the Google Maps library.
  • And more

Declaring components

The primary task of the manifest is to inform the system about the app’s components. For example, a manifest file can declare an activity as follows:

<?xml version=”1.0″ encoding=”utf-8″?>
<manifest … >
<application android:icon=”@drawable/app_icon.png” … >
<activity android:name=”com.example.project.ExampleActivity”
android:label=”@string/example_label” … >


In the <application> element, the android:icon attribute points to resources for an icon that identifies the app.

In the <activity> element, the android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attribute specifies a string to use as the user-visible label for the activity.

You must declare all app components this way:

  • <activity> elements for activities
  • <service> elements for services
  • <receiver> elements for broadcast receivers
  • <provider> elements for content providers

Activities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().

App Resources

An Android app is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the app. For example, you should define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using app resources makes it easy to update various characteristics of your app without modifying code and—by providing sets of alternative resources—enables you to optimize your app for a variety of device configurations (such as different languages and screen sizes).

For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your app code or from other resources defined in XML. For example, if your app contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo, which you can use to reference the image and insert it in your user interface.

One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings in XML, you can translate the strings into other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory’s name (such as res/values-fr/ for French string values) and the user’s language setting, the Android system applies the appropriate language strings to your UI.

Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration for which those resources should be used. As another example, you should often create different layouts for your activities, depending on the device’s screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate qualifier to each layout’s directory name. Then, the system automatically applies the appropriate layout depending on the current device orientation.

Android Emulator

Android Emulator is used to run, debug and test the android application. If you don’t have the real device, it can be the best way to run, debug and test the application.

It uses an open source processor emulator technology called QEMU.