What is Android?

Android is an open source and Linux-based Operating System for mobile devices such as
smartphones and tablet computers. Android was developed by the Open Handset Alliance, led by
Google, and other companies.

Android offers a unified approach to application development for mobile devices which means
developers need only develop for Android, and their applications should be able to run on
different devices powered by Android.

The first beta version of the Android Software Development Kit (SDK) was released by Google
in 2007 where as the first commercial version, Android 1.0, was released in September 2008.

On June 27, 2012, at the Google I/O conference, Google announced the next Android version,
4.1 Jelly Bean. Jelly Bean is an incremental update, with the primary aim of improving the user
interface, both in terms of functionality and performance.

The source code for Android is available under free and open source software licenses. Google
publishes most of the code under the Apache License version 2.0 and the rest, Linux kernel
changes, under the GNU General Public License version 2.
Why Android ?
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

SMS and MMS

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.
Android Applications
Android applications are usually developed in the Java language using the Android Software
Development Kit.

Once developed, Android applications can be packaged easily and sold out either through a store
such as Google Play, SlideME, Opera Mobile Store, Mobango, F-droid and the Amazon
Appstore.

Android powers hundreds of millions of mobile devices in more than 190 countries around the
world. It's the largest installed base of any mobile platform and growing fast. Every day more
than 1 million new Android devices are activated worldwide.

This tutorial has been written with an aim to teach you how to develop and package Android
application. We will start from environment setup for Android application programming and then
drill down to look into various aspects of Android applications.

Categories of Android applications

There are many android applications in the market. The top categories are −
History of Android
The code names of android ranges from A to N currently, such as Aestro, Blender, Cupcake,
Donut, Eclair, Froyo, Gingerbread, Honeycomb, Ice Cream Sandwitch, Jelly Bean, KitKat,
Lollipop and Marshmallow. Let's understand the android history in a sequence.
What is API level?
API Level is an integer value that uniquely identifies the framework API revision
offered by a version of the Android platform.

Platform Version API VERSION_CODE

Level

Android 6.0 23 MARSHMALLOW

Android 5.1 22 LOLLIPOP_MR1

Android 5.0 21 LOLLIPOP

Android 4.4W 20 KITKAT_WATCH KitKat for

Wearables Only

Android 4.4 19 KITKAT

Android 4.3 18 JELLY_BEAN_MR2

Android 4.2, 4.2.2 17 JELLY_BEAN_MR1

Android 4.1, 4.1.1 16 JELLY_BEAN

Android 4.0.3, 15 ICE_CREAM_SANDWICH_MR1

4.0.4

Android 4.0, 4.0.1, 14 ICE_CREAM_SANDWICH

4.0.2

Android 3.2 13 HONEYCOMB_MR2

Android 3.1.x 12 HONEYCOMB_MR1

Android 3.0.x 11 HONEYCOMB

Android 2.3.4 10 GINGERBREAD_MR1

Android 2.3.3

Android 2.3.2 9 GINGERBREAD

Android 2.3.1

Android 2.3

Android 2.2.x 8 FROYO

Android 2.1.x 7 ECLAIR_MR1

Android 2.0.1 6 ECLAIR_0_1

Android 2.0 5 ÉCLAIR

Android 1.6 4 DONUT

Android 1.5 3 CUPCAKE

Android 1.1 2 BASE_1_1

Android 1.0 1 BASE

You will be glad to know that you can start your Android application
development on either of the following operating systems −

 Microsoft Windows XP or later version.

 Mac OS X 10.5.8 or later version with Intel chip.

 Linux including GNU C Library 2.7 or later.

Second point is that all the required tools to develop Android applications are
freely available and can be downloaded from the Web. Following is the list of
software's you will need before you start your Android application
programming.

 Java JDK5 or later version

 Android Studio

Here last two components are optional and if you are working on Windows
machine then these components make your life easy while doing Java based
application development. So let us have a look how to proceed to set required
environment.

Set-up Java Development Kit (JDK)

You can download the latest version of Java JDK from Oracle's Java site
− Java SE Downloads. You will find instructions for installing JDK in
downloaded files, follow the given instructions to install and configure the
setup. Finally set PATH and JAVA_HOME environment variables to refer to the
directory that contains java and javac, typically java_install_dir/bin and
java_install_dir respectively.

If you are running Windows and installed the JDK in C:\jdk1.8.0_102, you
would have to put the following line in your C:\autoexec.bat file.
set PATH=C:\jdk1.8.0_102\bin;%PATH%
set JAVA_HOME=C:\jdk1.8.0_102

Alternatively, you could also right-click on My Computer, select Properties,

then Advanced, then Environment Variables. Then, you would update the
PATH value and press the OK button.
On Linux, if the SDK is installed in /usr/local/jdk1.8.0_102 and you use the
C shell, you would put the following code into your .cshrc file.
setenv PATH /usr/local/jdk1.8.0_102/bin:$PATH
setenv JAVA_HOME /usr/local/jdk1.8.0_102

Alternatively, if you use Android studio, then it will know automatically where
you have installed your Java.

Android IDEs
There are so many sophisticated Technologies are available to develop
android applications, the familiar technologies, which are predominantly
using tools as follows

 Android Studio

 Eclipse IDE(Deprecated)
Android Architecture
Android operating system is a stack of software components which is roughly
divided into five sections and four main layers as shown below in the
architecture diagram.

Linux kernel
At the bottom of the layers is Linux - Linux 3.6 with approximately 115
patches. This provides a level of abstraction between the device hardware
and it contains all the essential hardware drivers like camera, keypad, display
etc. Also, the kernel handles all the things that Linux is really good at such
as networking and a vast array of device drivers, which take the pain out of
interfacing to peripheral hardware.
Libraries
On top of Linux kernel there is a set of libraries including open-source Web
browser engine WebKit, well known library libc, SQLite database which is a
useful repository for storage and sharing of application data, libraries to play
and record audio and video, SSL libraries responsible for Internet security
etc.

Android Libraries
This category encompasses those Java-based libraries that are specific to
Android development. Examples of libraries in this category include the
application framework libraries in addition to those that facilitate user
interface building, graphics drawing and database access. A summary of
some key core Android libraries available to the Android developer is as
follows −

 android.app − Provides access to the application model and is the cornerstone

of all Android applications.

 android.content − Facilitates content access, publishing and messaging

between applications and application components.

 android.database − Used to access data published by content providers and

includes SQLite database management classes.

 android.opengl − A Java interface to the OpenGL ES 3D graphics rendering API.

 android.os − Provides applications with access to standard operating system

services including messages, system services and inter-process communication.

 android.text − Used to render and manipulate text on a device display.

 android.view − The fundamental building blocks of application user interfaces.

 android.widget − A rich collection of pre-built user interface components such

as buttons, labels, list views, layout managers, radio buttons etc.

 android.webkit − A set of classes intended to allow web-browsing capabilities

to be built into applications.
Having covered the Java-based core libraries in the Android runtime, it is now
time to turn our attention to the C/C++ based libraries contained in this layer
of the Android software stack.

Android Runtime
This is the third section of the architecture and available on the second layer
from the bottom. This section provides a key component called Dalvik
Virtual Machine which is a kind of Java Virtual Machine specially designed
and optimized for Android.

The Dalvik VM makes use of Linux core features like memory management
and multi-threading, which is intrinsic in the Java language. The Dalvik VM
enables every Android application to run in its own process, with its own
instance of the Dalvik virtual machine.

The Android runtime also provides a set of core libraries which enable Android
application developers to write Android applications using standard Java
programming language.

Application Framework
The Application Framework layer provides many higher-level services to
applications in the form of Java classes. Application developers are allowed
to make use of these services in their applications.

The Android framework includes the following key services −

 Activity Manager − Controls all aspects of the application lifecycle and activity
stack.

 Content Providers − Allows applications to publish and share data with other
applications.

 Resource Manager − Provides access to non-code embedded resources such as

strings, color settings and user interface layouts.

 Notifications Manager − Allows applications to display alerts and notifications

to the user.
  View System − An extensible set of views used to create application user
interfaces.

Applications
You will find all the Android application at the top layer. You will write your
application to be installed on this layer only. Examples of such applications
are Contacts Books, Browser, Games etc.

Application components are the essential building blocks of an Android

application. These components are loosely coupled by the application
manifest file AndroidManifest.xml that describes each component of the
application and how they interact.

There are following four main components that can be used within an Android
application −

Sr.No Components & Description

Activities
1
They dictate the UI and handle the user interaction to the smart phone
screen.

Services
2
They handle background processing associated with an application.

Broadcast Receivers
3
They handle communication between Android OS and applications.

Content Providers
4
They handle data and database management issues.
Activities
An activity represents a single screen with a user interface,in-short Activity
performs actions on the screen. For example, an email application might have
one activity that shows a list of new emails, another activity to compose an
email, and another activity for reading emails. If an application has more than
one activity, then one of them should be marked as the activity that is
presented when the application is launched.

An activity is implemented as a subclass of Activity class as follows −

public class MainActivity extends Activity {

Services
A service is a component that runs in the background to perform long-running
operations. For example, a service might play music in the background while
the user is in a different application, or it might fetch data over the network
without blocking user interaction with an activity.

A service is implemented as a subclass of Service class as follows −

public class MyService extends Service {

Broadcast Receivers
Broadcast Receivers simply respond to broadcast messages from other
applications or from the system. For example, applications can also initiate
broadcasts to let other applications know that some data has been
downloaded to the device and is available for them to use, so this is broadcast
receiver who will intercept this communication and will initiate appropriate
action.

A broadcast receiver is implemented as a subclass

of BroadcastReceiverclass and each message is broadcaster as
an Intent object.

public class MyReceiver extends BroadcastReceiver {

 public void onReceive(context,intent){}

Content Providers
A content provider component supplies data from one application to others
on request. Such requests are handled by the methods of
the ContentResolverclass. The data may be stored in the file system, the
database or somewhere else entirely.

A content provider is implemented as a subclass of ContentProvider class

and must implement a standard set of APIs that enable other applications to
perform transactions.

public class MyContentProvider extends ContentProvider {

public void onCreate(){}

We will go through these tags in detail while covering application components

in individual chapters.

Additional Components
There are additional components which will be used in the construction of
above mentioned entities, their logic, and wiring between them. These
components are −

S.No Components & Description

Fragments
1
Represents a portion of user interface in an Activity.

Views
2
UI elements that are drawn on-screen including buttons, lists forms etc.

3 Layouts
 View hierarchies that control screen format and appearance of the views.

Intents
4
Messages wiring components together.

Resources
5
External elements, such as strings, constants and drawable pictures.

Manifest
6
Configuration file for the application.