> I was working on a Spring Boot REST API a few months ago and the senior engineer had defined every single service class as an interface and then implemented that interface in an actual @Service. The interfaces weren’t extended by multiple classes, this wasn’t an external library or a dependant, and there were no plans to implement additional classes later.

Reason I avoid working in Java and especially the "senior java engineers".

> The logic from the senior engineer was that the interface represented the “contract” and the class the business logic.

This is basically religious dogma