When you use the Room persistence library to store your app's data, you interact with the stored data by defining data access objects, or DAOs. Each DAO includes methods that offer abstract access to your app's database. At compile time, Room automatically generates implementations of the DAOs that you define.
By using DAOs to access your app's database instead of query builders or direct queries, you can preserve separation of concerns, a critical architectural principle. DAOs also make it easier for you to mock database access when you test your app.
Anatomy of a DAO
You can define each DAO as either an interface or an abstract class. For basic
use cases, you usually use an interface. In either case, you must always
annotate your DAOs with @Dao. DAOs
don't have properties, but they do define one or more methods for interacting
with the data in your app's database.
The following code is an example of a simple DAO that defines methods for
inserting, deleting, and selecting User objects in a Room database:
Kotlin
@Dao