Room Using Date field

You're converting from Date to Long (wrapper) and from long (primitive) to Date. I changed it to Long and it compiled. Besides, unboxing null in your converter produces a NPE.

public class DateConverter {

    @TypeConverter
    public static Date toDate(Long dateLong){
        return dateLong == null ? null: new Date(dateLong);
    }

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

See my complete example.

Refer to the documentation : https://developer.android.com/training/data-storage/room/referencing-data

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();
    }
}

Then map it to the database.

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

And the entity.

@Entity
public class User {
    private Date birthday;
}

I had this same problem (how to store time to Room), but I was using Calendar, so I made this: [note: This anwer is for Calendar ; the main reason is that Calendar is now supported]

edit: the main reason for this answer is that Date is deprecated, so here you go

  @TypeConverter
  public static Calendar toCalendar(Long l) {
    Calendar c = Calendar.getInstance();
    c.setTimeInMillis(l);
    return c;
  }

  @TypeConverter
  public static Long fromCalendar(Calendar c){
    return c == null ? null : c.getTime().getTime();
  }