Introduction To ANDROID

HISTORY :-

Android Os is based on Linux-kernel. It was initially developed by Android Inc. and it was latter backed by Google and was bought in 2005.

It is a open source platform hence most of the code was released under open source Apache Licence.  So anyone who is interested to use Android can do so by downloading the full source code.

 The main advantage of Android is that it offer Unified approach towards Application Development. In a survey it has been found that 71% of the Mobile Developers Uses Android. Some companies that uses Android platform are Sony, Motorola, Samsung, Micromax  etc. and recently Nokia have also joined the group. 

Android Versions:-

Android Versions	API level	Nick name
Android 1.0	1	N/A
Android 1.1	2	N/A
Android 1.5	3	Cupcake
Android 1.6	4	Donut
Android 2.0	5	Eclair
Android 2.0.1	6	Eclair
Android 2.1	7	Eclair
Android 2.2-2.2.3	8	Froyo
Android 2.3-2.3.2	9	Gingerbread
Android 2.3.3-2.3.7	10	GingerBread
Android 3.0	11	Honeycomb
Android 3.1	12	Honeycomb
Android 3.2	13	Honeycomb
Android 4.0-4.0.2	14	Ice Cream Sandwich
Android 4.0.3-4.0.4	15	Ice Cream Sandwich
Android 4.1	16	Jelly bean
Android 4.2	17	Jelly bean
Android 4.3	18	Jelly bean
Android 4.4	19	Kitkat

Architecture of Android:-

FIGURE:- Architecture of Android stack

The Android OS is roughly divided into five sections in four main layers : -

1. Linux Kernal: Android is built on top of Linux. The reasons of selecting Linux are security,           portability and features.

2  2. Libraries: The Native libraries are taken from Open Source community. These libraries are             written either in C/C++.

3 3. Dalvik is the process virtual machine (VM). Programs are commonly written                              in Java and compiled to bytecode. They are then converted from Java Virtual Machine-                  compatible .class files to Dalvik-compatible .dex (Dalvik Executable) files before installation         on a device. The compact Dalvik Executable format is designed to be suitable for systems

4 4. Application Framework: The application framework is a rich environment that provides numerous services for example it provide the ecosystem of capabilities your application can tap into, such as location, sensors, WiFi, telephony, and so on.

5 5.Applications: The applications that developers create are used by the end users. It come preinstalled on a device or it can be downloaded from one of the many Android market. 

Android Activity LifeCycle:-

Activity is usually a single screen that user sees on the device at one time. Launching an activity can be quite expensive. It may involve creating a new Linux process, allocating memory for all the UI objects, inflating all the objects from XML layouts, and setting up the whole screen. Since we’re doing a lot of work to launch an activity, it would be a waste to just toss it out once the user leaves that screen. To avoid this waste, the activity life cycle is managed via Activity Manager.

            Figure 2 :-   Activity Life-Cycle

Starting State :-  

When an activity doesn't exist in memory, it is in a starting state. While an activity starts, the activity will go through a whole set of callback methods. After the activity starts it will go to running state. This transition from starting state to running state is one of the most expensive operations in terms of computing time, and this also directly affects the battery life of the device .

Running State :-

The Activity that is currently on the screen and Interacting with the User.

Paused State :-

When an activity is not in focus (i.e., not interacting with the user) but still visible on the screen, we say it’s in a paused state.

Stopped State :-

When an activity is not visible, but still in memory, we say it’s in a stopped state. Stopped activity could be brought back to the front to become a Running activity again. Or, it could be destroyed and removed from memory.

Destroy State :-

A destroyed activity is no longer in memory. The Activity Manager decided that this activity is no longer needed and has removed it. Before the activity is destroyed, it can perform certain actions, such as save any unsaved information.

In the next post I will show how to install ADT  (Android Development Tool)