I have a simple question but hard to answer:
to mate a exe file with a jar is super easy (no guide needed for that), BUT those can be extracted so you see the code,
is there a way to make jar to exe that "lock:s" it to be extracted?
ProGuard is a well-known one, and it's very often used for Android apps (I think Google's tools for Android apps support it). But you can also use it for other Java applications than Android apps.
Beware that this ofcourse does not make your code 100% secure. Do not store secrets, such as passwords or encryption keys, in your code and think they are safe when you obfuscate the code. A determined hacker can still reverse engineer and find the secrets you tried to hide. The only thing that obfuscation does is make it harder for a hacker, but not impossible.
About using ProGuard: You just have to read the documentation and learn how to use it.