Thread:LOTRMod/@comment-29954640-20170223220430/@comment-25101690-20170228184755

Don't open individual class files. When java compiles code into a jar library, cross-references within the code are resolved and bound to the code. A class file only makes sense inside the jar because taking it out of context would break these references. Moreover, each mod has references to the Minecraft source and other libraries of the Forge environment. To decompile it perfectly, your decompiler would need all these libraries or their source code. You will also need FML to decompile the Minecraft code itself because Minecraft is well obfuscated to make decompiling much harder.

Don't decompile the mod, except to take a look at the source yourself. To make a submod, you can simply add the compiled mod jar as library to your mod. That allows you to use reference it and use its public methods. With Java Reflection and the FML tools you might access all of it. Then you can write your own mod in the same way as LOTR and extend LOTR's classes to use their functionality.

All this is in no way an easy task and I can't really recommend it to anyone. I can't even guarantee that it will work like I describe it. If you have as much devotion, creativity and time as Mevans himself, you will be better off doing it completely yourself and only using the mod to see how Mevans performed difficult parts of the coding. If you lack the devotion and skills, you will probably never finish the project.