God is the creator of Humans, and has no inner Human class, because no human is a god. Try it with three separate classes, and it should work fine. You just need to know the minimum about classes and inheritance to complete the task.

Yes. This could have been explained better.

There's no requirement in Java for classes to be in seperate files. It's obviously best practice in the real world but for simple academic exercises such as these it's perfectly acceptable to have all the classes in one file.

As far as I know you can define multiple classes in one file as long as only one of them is public (although it's very messy).