Could not find or load main class error (Solved and Explained)

Описание: A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class. I have not only fixed the issue but explained the core reason of this issue.

Reasons why Java cannot find the class
1. you made a mistake with the classname argument
2. the application's classpath is incorrectly specified
3. the wrong directory is on the classpath
4. the subdirectory path doesn't match the FQN
5. dependencies missing from the classpath
6. the class has been declared in the wrong package

Still can't find the problem?
Drop your problem in the comment section, I will try to help you out :)