How to make decompiled code unreadable?

Question

Java

As I know, protecting Java code from decompilation is quite difficult, but I would like to know if there are any tools to make the code unreadable when decompiled?
For example, in Minecraft jar all classes have unreadable names (aa, ab, ac, aab, aaw, aay) and unreadable variables (nt4, er3, r24m). With what tool can this be done?

Answer 1

Google "java obfuscation"

Answer 2

as an option to implement as many pieces of code as possible, for example, in the "and-not" basis without branching based on bit masks, for example.
it will be just a hell of a mess, but a drop in performance every 100 times.