What is the most important part of your project code? Of course, it is the domain logic code. We have to protect it and look after it. You say "Yeah... I heard about Ports and Adapters or Clean Architecture". It is not enough. Let's look into your domain code and make it really clear and safe.
I will be talking about names, types, languages and techniques available in JVM world which help us making our domain code better and cleaner.