Previously, we have seen how to extract contents of .img file and also how to convert dex files to classes/jar.
In this post, I'm going to show a method to covert class/classes/jar file to java file(s).
To do so, we are going to use a tool, JD-GUI. It is a Java Decompiler, which supports all versions of Java, including java-7.
Download the application from here, and extract it to a proper location.
Now, open the terminal (in case of Linux) and run the following command:

./jd-gui

It will open the GUI window of Java decompiler.

Just drag-and-drop a class/jar file into this window. In the left panel, we can find the corresponding java files, arranged in proper package format.
To save a single file, select it and choose "Save Source" from the top menu. And to save entire package, choose "Save all sources".

Troubleshooting:
You may ran into one of these problems, while trying to open jd-gui. Install the packages specified accordingly.
Error 1:

./jd-gui: error while loading shared libraries: libgtk-x11-2.0.so.0: cannot open shared object file: No such file or directory

Solution:

sudo apt-get install libgtk2.0-0:i386

Error 2:

./jd-gui: error while loading shared libraries: libXxf86vm.so.1: cannot open shared object file: No such file or directory

Solution:

sudo apt-get install libxxf86vm1:i386

Error 3:

./jd-gui: error while loading shared libraries: libSM.so.6: cannot open shared object file: No such file or directory

Solution:

sudo apt-get install libsm6:i386

Error 4:

/jd-gui: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

Solution:

sudo apt-get install lib32stdc++6

In my previous post, we have seen how to extract the contents of img file.
After extraction, you will find that most of the files have ".dex" extension. These are Compiled Android application code files.
In order to convert them into executable format (.class or .jar), you can use dex2jar tool.
Extract it to a proper location and open the terminal to this location.
Now run the following command:

./d2j-dex2jar.sh <Path-to-dex_file>

It will bundle the dex files into a jar file, and stores it in the current directory.
dex2jar can also be used to convert dex files into variuos other formats.
For detailed info, click here.

Unyaffs is a program to extract files from a YAFFS2 file system image. Currently it can only extract images created by mkyaffs2image.

Download the source from here.

Compiling:

Extract the contents into a suitable place and run the following command

make

Usage:

unyaffs [options] <image_file_name> [<extract_directory>]
Options:
-d               detection of flash layout, no extraction
-b               spare contains bad block information
-c <chunk size>  set chunk size in KByte (default: autodetect, max: 16)
-s <spare size>  set spare size in Byte  (default: autodetect, max: 512)
-t               list image contents
-v               verbose output
-V               print version
Source: Official github repository

Recently, I installed ubuntu updates and my browser asked me for flash player, which was previously installed. i tried to install using the regular command

sudo apt-get install flashplugin-installer

But, it says

flashplugin-installer is already the newest version

Finally, I found the workaround. Previous installation has to removed first (which anyway is not working)
Run the following commands :

sudo apt-get purge flashplugin-installer
sudo apt-get install flashplugin-installer

Viola!! It works now.

Today, I tried to install an application from command line and I found this error:

Failure [INSTALL_FAILED_UID_CHANGED]

After Googling for a while, I found the reason.
This happens when there is application data from older version is still present. To resolve this, clear/delete the following directory:

/data/data/<PACKAGE_NAME>

To do this, open terminal and type the following commands:

adb shell
cd /data/data/
rm -r <PACKAGE_NAME>
exit

Now try to install the application.
Viola.

By default, localhost (127.0.0.1) points to /var/www/ directory in Ubuntu. I prefer the location to be in my home directory.
To change the default location, open the file:

/etc/apache2/sites-available/000-default.conf

 and change it to look as follows:
(My wokspace is located at /home/chaithanya/www)

<VirtualHost *:80>
        ServerAdmin webmaster@localhost
        DocumentRoot /home/chaithanya/www

        <Directory /home/chaithanya/www/>
                Options Indexes FollowSymLinks
                AllowOverride All
                Require all granted
        </Directory>
        ErrorLog /var/log/apache2/error.log
        CustomLog /var/log/apache2/access.log common
</VirtualHost> 

Note: This works for Ubuntu 13.10+

When I tried to install Ubuntu 12.04 on my PC, it halted with 4/5 dots under Ubuntu. It's stuck at that point and no progress. Sometimes you may see some distorted image or white screen. All these problems are due to incompatible video drivers.

To get pass behind this screen and boot into the system, hold down SHIFT button while booting from CD/USB and press F6. Now select "nomodeset" option to prevent video drivers from loading.

Now you can install Ubuntu successfully. But you may face the problem again while booting (after installation). Then boot in "Recovery Mode" and install graphics drivers from GRUB menu or command line.