Reverse Engineering : Convert class or jar to Java files

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