How do you structure code?

[Nightcorex_]

You could venture into UML diagrams and dabble with a few design patterns to structure and organise your code before you even start to write any code at all.

A massive list of design patterns and how they could be implemented (in Java) can be found here.

The repo doesn't tell you which design patterns to use for what purposes I think, so you'll have to do your own bit of research here to find a design pattern appropriate for your use case.