Interfaces, by definition, do not actually have method implementations, so there is no "default" method here. You can understand more about java interfaces here

Part 2 of your issue here is that you may not need to implement some of those methods that your are writing. In spring boot, extending the MongoRepository class gives you the simple stuff for free. Like save, and findById.

Go through this module, it'll help with understanding

Here's the part about the MongoRepository specifically

Hope that helps, welcome to Spring Boot!