Android apps can be written using Kotlin, Java, and C++ languages. 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.

Each Android app lives in its own security sandbox, protected by the following Android security features:

  • 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. The Android system starts the process when any of the app’s components need to be executed, and then shuts down the process when it’s no longer needed or when the system must recover memory for other apps.


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 device’s location, camera, and Bluetooth connection. The user has to explicitly grant these permissions.



The rest of this document introduces the following concepts:

  • The core framework components that define your app.
  • The manifest file in which you declare the components and the required device features for your app.
  • Resources that are separate from the app code and that allow your app to gracefully optimize its behavior for a variety of device configurations.

App components :

App components are the essential building blocks of an Android app. Each component is an entry point through which the system or a user can enter your app. Some components depend on others.

There are four different types of app components:

  • Activities
  • Services
  • Broadcast receivers
  • Content providers

Each type serves a distinct purpose and has a distinct lifecycle that defines how the component is created and destroyed. The following sections describe the four types of app components.

Activities :

An activity is the entry point for interacting with the user. It represents a single screen with a user interface. For example, an email app might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. 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 email app allows it. For example, a camera app can start the activity in the email app that composes new mail to allow the user to share a picture. An activity facilitates the following key interactions between system and app:

  • Keeping track of what the user currently cares about (what is on screen) to ensure that the system keeps running the process that is hosting the activity.
  • Knowing that previously used processes contain things the user may return to (stopped activities), and thus more highly prioritize keeping those processes around.
  • Helping the app handle having its process killed so the user can return to activities with their previous state restored.
  • Providing a way for apps to implement user flows between each other, and for the system to coordinate these flows. (The most classic example here being share.)

You implement an activity as a subclass of the Activity class. For more information about the Activity class, see the Activities developer guide.

Features of Android :

Android is a powerful operating system competing with Apple 4GS and supports great features. Few of them are listed below –


Sr.No. Feature & Description
1 Beautiful UI

Android OS basic screen provides a beautiful and intuitive user interface.

2 Connectivity

GSM/EDGE, IDEN, CDMA, EV-DO, UMTS, Bluetooth, Wi-Fi, LTE, NFC and WiMAX.

3 Storage

SQLite, a lightweight relational database, is used for data storage purposes.

4 Media support

H.263, H.264, MPEG-4 SP, AMR, AMR-WB, AAC, HE-AAC, AAC 5.1, MP3, MIDI, Ogg Vorbis, WAV, JPEG, PNG, GIF, and BMP.

5 Messaging


6 Web browser

Based on the open-source WebKit layout engine, coupled with Chrome’s V8 JavaScript engine supporting HTML5 and CSS3.

7 Multi-touch

Android has native support for multi-touch which was initially made available in handsets such as the HTC Hero.

8 Multi-tasking

User can jump from one task to another and same time various application can run simultaneously.

9 Resizable widgets

Widgets are resizable, so users can expand them to show more content or shrink them to save space.

10 Multi-Language

Supports single direction and bi-directional text.


11 GCM

Google Cloud Messaging (GCM) is a service that lets developers send short message data to their users on Android devices, without needing a proprietary sync solution.

12 Wi-Fi Direct

A technology that lets apps discover and pair directly, over a high-bandwidth peer-to-peer connection.

13 Android Beam

A popular NFC-based technology that lets users instantly share, just by touching two NFC-enabled phones together.

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