it is possible due to reverse engineering.

you can find one of such s/w from net named DJ java decompiler.

The process is called de- compilation. I am attaching a tool here which is
pretty good. It is a utility called JAD. You have to run it like this: jad
<class-file-name.class> It will create a file  called <class-file-name.jad> You
can rename it to <class-file-name.java> However, the variable names that you
give (object names or string names) are not converted as they are, but the tool
converts the names into something that it can. e.g. if your variable name is Car
oMFCar, it might write it like Car car. Still, it is a pretty good tool. It is
possible because, the sun has openly declared structure of class  files in it's
specification. So anybody can decompile the class file if one has access to
them.

it is interesting. could it possible for you to tell us how to use it. any
example.

I already sent the the tool with my last mail. I am sending it back again.

1) Extract the jad.exe and place it in any directory.
Add that directory name to your classpath.
2) If you have a .class file  and want to get the .java file of that, say u have
myFirst.class file then either add the directory which contains
myFirst.class
to your classpath or go to that directory and issue the following command
jad myFirst.class
You will find myFirst.jad file created in the same directory. This is
nothing
but your .java file

I hope that was what you were asking for