Soo... I made a bytecode editor for Java just for fun. On the left you can see the original decompiled class, on the right a slightly changed class which still runs normally. Below you can see the pure ASM bytecode :)
It's pretty awesome for learning how bytecode works (together with a listing like https://en.wikipedia.org/wiki/...)! It also has an AutoComplete feature and some syntax highlighting (which I'm still working on).

What's special about it? It's a pure text parser! It assembles a full class from just a String!
If you want to look at it: https://github.com/SuspiciousActivi...
(I'm new to GitHub, don't judge me :D)