I once had the error  exception in main  subroutine. All of your code  seems
to be correct;however,you ended up compiling in the wrong subfolder path.

A way to correct this is to go to the Windows Explorer (if you have Windows OS)
and search for the file name, (aim_basic.java) that is the same name as the
class.

If you don't have the filename, then save the java  filename as aim_basic.java
and compile it from there.

Hopefully, one of these solutions could help.