Java is pretty easy to decompile and it's not a huge amount of effort to poke into the generated JVM code and start doing things. If you have a decent idea of how VMs work, C-like languages work, and how object dispatch works it's really not that hard. Also the early modding scene for Minecraft was really fun. I was a huge Minecraft player at the time and was early into the deobfuscating -> modding scene and the community was one of the most fun computing communities I've been in. Due to how focused it was on the game and its output it wasn't bogged down in nearly as much bikeshedding and philosophy as most FOSS projects get. Honestly one of the highlights of the coding I've done in my life.