why am i not able to implement a class in a class ,which already extends a class

Question

This is my DBInfo class:

public class DBInfo  stars table
 static final String STARS = "stars";
 static final String FIRST_NAME = "first_name";
 static final String LAST_NAME = "last_name";

 static final String CREATE_STAR_TABLE = "CREATE TABLE " + STARS + 
                                                    "(" + _ID + " integer primary key autoincrement, " 
                                                        + FIRST_NAME + " text not null, "
                                                        + LAST_NAME + " text not null);";

 //stars in movies
 static final String STARS_IN_MOVIES = "stars_in_movies";
 static final String STAR_ID = "star_id";
 static final String MOVIE_ID = "movie_id";
 static final String CREATE_STAR_IN_MOVIE_TABLE = "CREATE TABLE " + STARS_IN_MOVIES + 
                                                        "(" + STAR_ID + " integer not null, "
                                                            + MOVIE_ID + " integer not null, " 
                                                            + "FOREIGN KEY (" + MOVIE_ID + ") REFERENCES "+ MOVIES +" (" + _ID + ") "
                                                            + "FOREIGN KEY (" + STAR_ID + ") REFERENCES "+ STARS +" (" + _ID + "));";

 //statistics table
 static final String STATISTICS = "statistics";
 static final String RIGHT = "right";
 static final String WRONG = "wrong";
 static final String CREATE_STATISTICS_TABLE = "CREATE TABLE " + STATISTICS + 
                                                    "(" + _ID + " integer primary key autoincrement, " 
                                                    + RIGHT + " int not null default 0, "
                                                    + WRONG + " int not null default 0);";

}

And now I want to implement this class toy DBAdapter class which already extends SQLiteOpenHelper

public class DbAdapter extends SQLiteOpenHelper implements DBInfo{

    private static final String DATABASE_NAME = "quizdb";
    private static final int DATABASE_VERSION = 9;

    private static final String MOVIES_FILE = "movies.csv";
    private static final String STARS_FILE = "stars.csv";
    private static final String STARS_IN_MOVIES_FILE = "stars_in_movies.csv";

    private static SQLiteDatabase mDb;
    private static Context mContext;

    public static SQLiteDatabase getSQLiteDatabase(Context ctx){
        if(mDb == null){
            new DbAdapter(ctx);
        }
        return mDb;
    }

And I am getting the following error:

The type DBInfo cannot be a superinterface of DbAdapter; a superinterface must be an interface

Answer 1

You cannot implement class .

Make DBInfo an interface

Answer 2

implements DBInfo

This won't work since you can implement only interface and your DBInfo is class . So you have only choice:

• Change your DBInfo class to interface

In your case only way is to change it to interface ¹ or change whole application logic . By the way it's very good practise to create interface that will hold static constants:

public interface DBInfo {

   public static final String STARS = "stars";
   public static final String FIRST_NAME = "first_name";
   public static final String LAST_NAME = "last_name";
}

¹ Because Java doesn't support multi-inheritance as was suggested.

Answer 3

You can only implement interfaces, not classes. This would seem like a situation where you should try composition instead of inheritance (have an instance of DBInfo in DbAdapter instead of inheriting it).