abstract method used without implementation

问题

I was learning the Room Database for Android app development through a tutorial and came across an abstract method being used without its implementation. Following are the files:

AppDatabase.java

@Database(entities = {Contact.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
    private static final String DATABASE_NAME = "database.db";
    private static AppDatabase database;

    **public abstract ContactDao getContactDao();**

    public static AppDatabase getInstance(Context context) {
    if(database == null)
    {
        database = Room.databaseBuilder(context, AppDatabase.class, DATABASE_NAME)
            .allowMainThreadQueries().build();
    }
    return database;
  }
}

The method getContactDao is being used in another file:

ContactsFragment

public class ContactsFragment extends Fragment {

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState){
View view = inflater.inflate(R.layout.fragment_contacts, container, false);

AppDatabase database = AppDatabase.getInstance(getContext());
**ContactDao contactoDao = database.getContactDao();**
return view;
}
}

Here is the ContactDao class:

    public interface ContactDao {

        @Query("SELECT * FROM contact")
        List<Contact> getAll();

        @Insert(onConflict = OnConflictStrategy.REPLACE)
        void insert(Contact contact);

        @Update
        void update(Contact contact);

        @Delete
        void delete(Contact contact);

    }

My question is what does the getContactDao method do? And shouldn't the abstract class be implemented before being used?

来源：https://stackoverflow.com/questions/48790457/abstract-method-used-without-implementation

标签

java

android

android-room