Learn Developing An Android Applications!!

What is Android?

Android is the world's most famous operating system for mobile devices and tablets. It is an open-source running system, created by means of Google, and available to all types of builders with various know-how levels, ranging from rookie to professional.
(The time period 'open source' sounds pretty familiar, would not it? Well, open-source means software program with source available for modification and sure to an open-source license agreement. More about open source terminology can be located here.
From a developer's perspective, Android is a Linux-based operating system for smartphones and tablets. It consists of a contact display screen person interface, widgets, camera, network information monitoring, and all the other facets that allow a phone cellphone to be called a smartphone. Android is a platform that helps a number of applications, available through the Android Play Store. The Android platform additionally approves end-users to develop, install, and use their own applications on top of the Android framework. The Android framework is licensed below the Apache License, with Android application builders holding the proper to distribute their applications under their customized license.

Like most software, Android is released in versions. Google has also assigned names to its versions since April 2009. Below are all the versions of Android released to date:

Version No.	Name	For:
1.0	Android Beta	Phone
1.1	Android	Phone
1.5	Cupcake	Phone
1.6	Donut	Phone
2.0/2.1	Eclair	Phone
2.2.x	Froyo	Phone
2.3.x	Gingerbread	Phone
3.x	Honeycomb	Tablet
4.0.x	Ice Cream Sandwich	Phone and Tablet
4.1/4.2	Jelly Bean	Phone and Tablet

As we can see in the table above, various versions of Android are supported on phones and tablets. There are umpteen Android devices available in the market from manufacturers like Samsung, HTC, Motorola and others. Google itself also has phones made in conjunction with OEMs, branded as the Nexus series.

Understanding Android

To begin development on Android even at the application level, I think it is paramount to understand the basic internal architecture. Knowing how things are arranged inside helps us understand the application framework better, so we can can design the application in a better way.
Android is an OS based on Linux. Hence, deep inside, Android is pretty similar to Linux. To begin our dive into the Android internals, let us look at an architectural diagram.

The above diagram illustrates the Android architecture. As you can see, it is a software stack above the hardware that is provided by the OEMs. Let's start with the topmost layer, i.e., the applications.

Applications

The diagram shows four basic apps (App 1, App 2, App 3 and App 4), just to give the idea that there can be multiple apps sitting on top of Android. These apps are like any user interface you use on Android; for example, when you use a music player, the GUI on which there are buttons to play, pause, seek, etc is an application. Similarly, is an app for making calls, a camera app, and so on. All these apps are not necessarily from Google. Anyone can develop an app and make it available to everyone through Google Play Store. These apps are developed in Java, and are installed directly, without the need to integrate with Android OS.

Application Framework

Scratching in addition below the applications, we attain the application framework, which application builders can leverage in developing Android applications. The framework gives a large set of APIs used by means of developers for various preferred purposes so that they do not have to code every basic task. The framework consists of sure entities; major ones are:

Activity Manager This manages the things to do that govern the application life cycle and has several states. An application may have multiple activities, which have their personal lifestyles cycles. However, there is one main activity that starts off-evolved when the application is launched. Generally, every activity in an application is given a window that has its personal layout and user interface. An recreation is stopped when every other starts and receives back to the window that initiated it thru an activity callback.

Notification Manager This manager allows the applications to create customized alerts

Views are used to create layouts, which include elements such as grids, lists, buttons, etc.

Resource Managers Applications do require external resources, such as graphics, external strings, etc. All these resources are managed through the resource manager, which makes them on hand in a standardized way.

Content Provider Applications additionally share data. From time to time, one application might also need some data from every other application. For example, an worldwide calling application will want to access the user's address book. This get entry to to some other application's data is enabled by the content material providers. 

Libraries

This layer holds the Android native libraries. These libraries are written in C/C+ and offer capabilities similar to the above layer, while sitting on top of the kernel. A few of the major native libraries include

• Surface Manager: Manages the display and compositing window-ING manager. - Media framework: Supports various audio and video formats and codecs including their playback and recording.
• System C Libraries: Standard C library like lib targeted for ARM or embedded devices.
• Open GL ES Libraries : These are the graphics libraries for rendering 2D and 3D graphics.
• SQLite : A database engine for Android.

Android Runtime

The Android runtime consists of the Vidal Virtual Machine. It is essentially a digital machine for embedded devices, which like any different virtual machine is a byte code interpreter. When we say it is for embedded devices, it capacity it is low on memory, comparatively slower and runs on battery power. Besides the Vidal Virtual Machine, it also consists of the core libraries, which are Java libraries and are available for all devices.

Kernel

The Android OS is derived from Linux Kernel 2.6 and is actually created from Linux source, compiled for mobile devices. The memory management, process management etc. are mostly similar. The kernel acts as a Hardware Abstraction Layer between hardware and the Android software stack.

Android SDK

As already mentioned, Android is open source and hence the source code is available for all developers. In totality it is called the Android SDK. You can download, build and work on Android in a number of different ways-it all depends on what you want to do. If your goal is to develop an Android application, you don't necessarily need to download all the source. Google recommends the Eclipse IDE, for which there is an available Android Developer Tools (ADT) plugin, through which you can install the specific SDK, create projects, launch emulators, debug, etc. You can see more details of Eclipse and ADT through Android's official website for developers - http://developer.android.com/sdk/index.html

Android Development for Windows Users

Android as of now does not support building on Windows, so if you want to modify the Android OS itself, you'll have to use Linux (see building the Android OS). However, on Windows, we do have tools and plugins for application and native Android development. And here we will talk about setting up basic Android development tools on Windows.

Downloading the Android SDK and developer tools

Google provides a convenient bundle to download and setup Android for Windows developers, which you can download here, under the name ADT bundle for Windows. The exact name of the file you download will depend on your OS architecture (32 vs 64 bit), but for my case (64 Bit Win 7), I see the following zip file downloaded: adt-bundle-windows-x86_64.zip. Extracting the zip file, I have contents as in the following snapshot:

First of all, we have Eclipse, which is the IDE for writing source. As an IDE it offers the surroundings for creating Android Applications. Android Applications are developed exceptionally in Java. Next, we have the 'SDK', which does now not consist of any of the sources. However, it holds the already constructed platform tools, tools, images, and some platform-specific libraries. When we say, building Android is no longer supported on Windows, we mean that we can not assemble system images and tools. However, different sources needed for application development are be available through the SDK Manager, which is the third entity present in the extracted zip file.

So, let's download the sources! Double click on the SDK Manager. exe. You'll see some thing like this:

This is the SDK Manager, via which we can install or delete whatever version of SDK we want. As you can see, it mentions we have the Android SDK Tools and Android SDK Platform Tools installed.
The latest Android available, as of the writing of this article, is 4.2, but with the SDK we can download and install any of the previous versions too. Now let us play around with the latest Android-i.e., 4.2, which is also known as Jelly Bean.
Select the check box for "Android 4.2(API 17)", which will select everything required for and under Android 4.2.

In all, as we can see, SDK Manager found 6 packages that need installation. Click the "Install 6 packages" button. We see another dialog box for package descriptions and license.

Select "Accept All" and click Install, which will initiate the download and then installation. When done, you will see "Installed" in front of all the packages selected.
It's now time to launch Eclipse, but first, we need to make sure that we have the Java Developer Kit (JDK) installed. If you don't have it, you can download it from Oracle here. I have JDK 7 installed in my case. Next, we need to launch Eclipse from the executable present in the Eclipse directory. If we obtained Eclipse via the instructions in this article, it should already have the ADT plugin installed. (Otherwise, you can download the ADT separately by following the instructions here.) Eclipse generally asks for a workspace path where it will create and maintain projects.
Here is what the newly launched Eclipse looks like:

Eclipse also includes the SDK Manager from where you can manage the SDK packages. Check out the Window menu and select "Android SDK Manager". To see what the emulator looks like, go to the Window menu and select "Android Virtual Device Manager". From there, we can create our virtual device or use one of the standard devices available. To create a new virtual device, click on 'New' as we see in the following snapshot:

Another window will pop up to take inputs for device type, target processor, memory, etc.

You can provide customized device specifications for a virtual device. Once the device is created by clicking 'OK', it will be available in the list on "Android Virtual Device Manager" window. To launch the emulator for your own defined virtual device, select it and click "Start".
You can also select pre-loaded options that correspond to the specifications of existing Android devices. To see them in the Android Virtual Device Manager, go to the Device Definitions tab.

We select the first in our list, "Nexus S by Google", and add a virtual device by clicking "Create AVD". The following dialog box requires the "Target" and "CPU" to be specified along with the size of the SD Card.

 

We will assign Target as "Android 4.2 - API level 17" CPU as "ARM" and SD Card Size equals "1024 MiB" and click "OK". We can now see the newly defined virtual device in the AVD list

To launch the emulator, select it and click "Start". Here's what the emulator looks like:

We can now use our newly-created emulator for running our Windows Android apps. All we have to do is compile our code, then load the app onto the emulator.

Building the Android OS

The above instructions, for Windows users, will work great if all you want to do is create Android apps. However, you can do even more with Android, including modifying the Android OS to create Android ROMs or MODs-that's the beauty of open source! To do this, you'll need to download the complete available Android source from its repository and cross-compiling for the device. You'll also need to be using Linux, since building Android is not supposed on Windows machines. And before we move further, there are certain assumptions to get out of the way:
1. All the information is generic and should work for all Linux flavours, however these instructions have only been tried and tested on Ubuntu 11.04.
2. Because we had to pick one of the versions of Android, we chose Android 4.0.1. Hence, some commands might be specific to Android 4.0.1, but things might be slightly different for other versions.

Memory Requirements

It is always a wise idea to check the memory requirements before starting any project. The size of the Android SDK is around 8.5GB and you will need around 30GB free disk space to build it.

Prerequisite Installations

Prior to downloading the SDK and starting cross-compiling, there are certain prerequisites of the Android SDK we have to have. It is better to set up these before jumping into the Android SDK, as we all know prevention is much better than debugging! First of all, for the Android version we have chosen (i.e Android 4.0.1) cross compilation is well tested on 64 -bit machines, but the documentation says it is experimental on 32-bit machines.
1. JDK Android SDK building requires the JDK, so you must install the JDK. For Android versions 2.3.x and later, one needs to install Java 6. It is recommend to install Sun JDK only, rather than Open JDK. First, download the JDK 6. Then run the following commands. (Please Note: To avoid specifying a specific version, the installer binary name and directories are modified to be generic names.)

$ chmod +x jdk-6xxxx-linux-xxx.bin
$ sudo./jdk-6xxxx-linux-xxx.bin
$ sudo mv jdk1.6.xxxx /usr/lib/jvm/ 
$sudo update-alternatives -install /usr/bin/java java /usr/lib/jvm/jdk1.6.xxxx/bin/java 1
$ sudo update-alternatives -install /usr/bin/javac javac /usr/lib/jvm/jdk1.6.xxxx/bin/javac 1
$ sudo update-alternatives -install /usr/bin/javaws javaws /usr/lib/jvm/jdk1.6.xxxx/bin/javaws 1 
$ sudo update-alternatives -config java
$ sudo update-alternatives -config javac
$ sudo update-alternatives -config javaws

For versions prior to 2.3.x, Java 5 is required. 2. You also need to install the following packages:

$sudo apt-get install git-core gnupg flex bison gperf libsdl-dev libesd0-dev libwxgtk2.6-dev build-essential zip curl  
libncurses5-dev zlib1g-dev ant gcc-multilib g+-multilib
$sudo apt-get install libx11-dev:i386

These packages are needed for downloading and compiling the SDK. Once these get installed, you are ready to download the Android SDK. There may be some optional things to do, for example enabling caching, etc. But we will not go into much detail here as we don't really need to do those things.

Downloading the Android SDK

The entire Android SDK is stored in a git repository that also maintains various other versions as well. We need to know the repository path and then download to a separate directory where we wish to store the complete source.
Let us first create a root source directory.

$mkdir android

Further, we will initialize and configure a tool 'repo', which is used to work with git conveniently. To download it, we need a 'bin' directory in our home, which should be added to the path.

$mkdir bin
$export PATH=$PATH:~/bin
$curl https://dl-ssl.google.com/dl/googlesource/git-repo/repo > bin/repo
$chmod a+x bin/repo

Now we have the repo installed and must initialise it with the path of git where the Android SDK lies. There is a master branch of git, which holds the latest Android version. The master branch in git is the main source repository, which means any new release or new update would be part of this master branch. However, if we want a specific Android version, we need to initialise with a specific branch.
Here in this article, we will go for a specific branch, since the master will keep on changing.

$repo init -u https://android.googlesource.com/platform/manifest -b android-4.0.1_r1

It will ask you for a name and email id, and you must provide them.
To get all the Android SDK files, you just now need to do:

$repo sync

It may take some time to download the complete source. In the end, we get

Syncing work tree: 100% (221/221), done.

This indicates the sources are downloaded. Let's have a look at the folder structure from the topmost level; here is what I have:

$ls
abi build development  external libcore packages  system
bionic cts device frameworks  Makefile  prebuilt
bootable  dalvik  docs hardware ndk sdk

Building Android
Now that we have the source, the next step is to build it. The first factor we want to determine before the building 
is what platform are we going to run this Android on? It is necessary to decide this. The options could be the type of 
your device or an emulator.

When flashing a phone, the worst case it that you may brick the smartphone in the event of a necessary bug. As a 
result, it is a good idea to develop on Android thru an emulator-so we will do that.
First of all, we want to set the environment variables, which are specified in a shell script.

$ source build/envsetup.sh
including device/samsung/maguro/vendorsetup.sh
including device/samsung/tuna/vendorsetup.sh
including device/ti/panda/vendorsetup.sh
including sdk/bash_completion/adb.bash
Next, we need to specify the target for which we need to cross-compile, using the command 'lunch'. But first of all, what is cross-compiling and why we need to do it? Well, cross-compiling is a compiling process that is done for a different platform, to create an executable or library to be used on a that different platform. Therefore, we would have two platforms,
host : On which we have the sources and are being cross-compiled. In our case, this is our Linux machine.
target: The platform for which the sources are being compiled for, and the compiled image or library will be used on the target platform. In this case, this would be our Android emulator.
This is required for embedded systems, as not all platforms support compiling and debugging. Examples of such platforms are ARM, MIPS, etc. For the target emulator and development build, we do
$lunch full-eng
=
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.0.1
TARGET_PRODUCT=full
TARGET_BUILD_VARIANT=eng
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a
HOST_ARCH=x86
HOST_OS=linux
HOST_BUILD_TYPE=release
BUILD_ID=ITL41D
=
Here 'full' is for emulator and 'eng' is because we want a development build. To see other available options, do:
$lunch
You're building on Linux

Lunch menu. pick a combo: 1. full-eng 2. full_x86-eng 3. vbox_x86-eng 4. full_maguro-userdebug 5. full_tuna-userdebug 6. full_panda-en
Next, we just have to build the code using 'make'. GNU make has the ability to run parallel tasks. How many tasks to run in parallel is determined by the '-j' option. The usage is make -jN, where N is the number For example, to run 4 parallel tasks, our command would be
$make -j4
It is common to set this number between 1 to 2 times the number of hardware threads the computer supports, which is generally the number of processors. To know how many processors we have in our system, we need to peek into the cpuinfo. To get the CPU info, we have command.
$cat /proc/cpuinfo
However, to get to know the number of processors in a single command, we can use
$grep -c ^processor /proc/cpuinfo
The building of Android also takes quite a lot of time when done for the first time. Note, all the images, libraries and applications built are placed in the directory out/target/product/generic/
Running on the Emulator
To launch the emulator with our built Android, just run the following command
$emulator
This will automatically pick the built images and binaries for the recently built Android. You can have the look and feel of the emulator environment, which is pretty similar to a device.