Java Data Layer: Repository + Room + Retrofit

Instructions

The data layer is the single source of truth for domain entities. The UI must never talk to Retrofit or Room directly.

1. Domain Contracts

Declare repositories as interfaces in the domain layer. Return domain entities only.

public interface ArticleRepository {
    LiveData<List<Article>> observeArticles();
    ListenableFuture<Void> refresh();
    ListenableFuture<Article> getById(long id);
}

2. Room DAO (Java annotations)