Now as in most everything else, there are MANY ways to go about class editing. I'm going to show you a fast and sort of 'dirty' way (at least for now) that will have you editing class files and getting those efforts 'in' game quickly. It's not the way I normally mod, but it's effective. I won't be going into GREAT detail. I won't be teaching you Java. I won't be (here anyway) going into the huge amount of changes that can (and cannot) be made, nor the huge problems that can be ran into. But it will work, and if you follow along with me and stick it out, in the end, you'll see the good results.
What you will need:
Java SDK 1.3.1_20 installed upon your computer... this is an old Java SDK, yet it's a complete must.
Get it here: http://java.sun.com/products/archive/j2se/1.3.1_20/
Mediafire download: http://www.mediafire.com/download/yvbm7 ... .1.20.7z#!
Take care to get the SDK -NOT- the JRE! The JRE will NOT work for you. Getting the installation notes and readme may also be helpful.
Basically you can just install it into your computer's C: drive.

You will also need 'these' tools:
NEW LINK MOVED TO BOTTOM OF POST!
They are QTims Modding Tools (the original IL2 hacker) that broke the code.
These Tools contains the following:
A Class Resolver: Decrypts Game's Java classes in order to manipulate them.
Usage:
Java -jar classresolver.jar exampleDir\. (you have to put inside the hashed java class in order to get decrypted, you know the AD3889RS0A1 like files) When its finished you'll see a com folder outside. Inside it you will find the decrypted file)
A Class Hasher: Hashes back (encrypts) Java files in Il2's hashed format.
Usage:
Java -jar classhasher.jar exampleDir\. (you have to put the modified java file INSIDE the com folder. You HAVE TO always work INSIDE the com folder and when its time to hash the file back you should copy the whole com folder containing the file. When its finished you'll see a hashed (or many if you did many files together) files outside.
You use these tools, the Resolver, to open up IL2's java files so you can edit them. You use the Hasher to zip them back into the java format that the game (IL2) can read.
Also included with these tools are the older 'extracted' and 'resolved' files for the game. You can use these, OR you can simply copy the contents of your 4.09M UP2.01 installation's FILES over and use the Resolver on them to get a newer batch of java classes to work with. Note, none of the folders and not all of the java's inside UP's FILES will resolve. But---you'll get the bulk of them, and really that's all you need to start working with java classes.
The next thing you'll need is a Java Editor/compiler/recompiler. NOTE: There are several free and paid for editors that can (more or less) allow you to read and (somewhat) edit java class files. The easiest of these is a payware/shareware proggy called DJ. And you can use 'this' proggy here that's got a fix to it ---if you want to. It's not my proggy and I'm not telling you what to do, use it or not, at your own risk. All I'm saying is that 'here' is a link to it.
NEW LINK MOVED TO BOTTOM OF THIS POST!
If you download and use it that's your business and not mine.
A picture of the DJ type decompiler/recompiler (center open program.)

Now with 'these' files, already you have a way to work with the game's java. There's much more to it, but that's for another post.
QTim's Tools:
http://www.mediafire.com/?a8t5jg1aaxp38v4
DJ Decompiler
http://www.mediafire.com/?vgbvagtbanp9bty