Style Guide

Please read Clean Architecture. Or at least understand the gist of it. Or at least understand how to work with it.

0. The Google Style Guide (comes everywhere)
1. Use @Nullable instead of Optional. Nobody likes Optional.
2. Use org.checkerframework.checker.nullness.qual.NonNull instead of lombok.NonNull, as we want checks to be at compile-time, not at run-time.
3. Use org.checkerframework.checker.nullness.qual.Nullable instead of javax.annotation.Nullable, as the latter is pretty much deprecated.