Type converters

allow access to custom data types that are stored in a single database column. Room disallows object refrences because they take place on the UI thread and are expensive.

Create a TypeConverter:

You can annotate the methods of arbitrary class using the @TypeConverter annotation. You can use type converters to define conversions between data types in your plain old Java object (POJO), and column types in a SQLite database. You can reference annotated classes from different scopes using the @TypeConverters annotation, for example from a @Database, @DAO, or @Entity

public class Converters {
    @TypeConverter
    public static Date fromTimestamp(Long value) {
        return value == null ? null : new Date(value);
    }

    @TypeConverter
    public static Long dateToTimestamp(Date date) {
        return date == null ? null : date.getTime();
    }
}

To use the converter annotate with Database

@Database(entities = {User.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

Use: User.java

@Entity
public class User {
    ...
    private Date birthday;
}

UserDao.java

@Dao
public interface UserDao {
    ...
    @Query("SELECT * FROM user WHERE birthday BETWEEN :from AND :to")
    List<User> findUsersBornBetweenDates(Date from, Date to);
}

Dao

@Entity
@TypeConverters(DateConverter.class)
class MyEntity {
    Date birthday;
}