Conclusion: Write "static" at the beginning of the method created separately from the main method.
For example, create a method with the method name abc int abc(int a,int b) If you get an error saying "Non-static variables cannot be referenced from a static context" when trying to compile static int abc(int a,int b) Just write "static" in the head and no error will occur.
My post, a memorandum of lessons learned from the very first steps in programming, It's like a learning record that a programming transcendental beginner left for himself.
I think that it is completely useless for people with high skills, but I hope that it will help those who have the same problem at the same level.
It may change suddenly because it is the first article posting.