Saving Data with Room

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Accessing data using Room DOAs

When you use the Room persistence library to store your app’s data, you interact with stored data by defining data access objects(DAO)

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.

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Anatomy of a DAO

Each DAO can be defined as either an interface or an abstract class. Whichever class you decide to use, you must always annotate @Dao
DAOs do not 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:

@Dao
public interface UserDao {
    @Insert
    void insertAll(User... users);

    @Delete
    void delete(User user);

    @Query("SELECT * FROM user")
    List<User> getAll();
}  

NOTE: There are two types of DAO methods that define database interactions:

• Convenience methods that let you insert, update, and delete rows in your database without writing any SQL code.
• Query methods that let you write your own SQL query to interact with the database. – - – -

Source: Accessing data using Room DOAs