Question

I have an abstract class that gets implemented by Java and by Android. This class has a method that needs to return a generic Cursor so that the implementations can return their respective cursors.

My abstract class:

public abstract class DatabaseAdapter {

protected abstract com.domain.database.Cursor<?> executeQuery(String command);      

}

The cursor type:

public final class Cursor<T> {

private T value = null;

public void setValue(T value) {
    this.value = value;
}

public T getValue() {
    return value;
}
    }

As stated earlier, I want the implementing method to return either a Java cursor or and Android cursor depending upon the situation. I can create the abstract classes just fine but when I try to implement, I get a compile error. I am apparently not understanding polymorphism very well here.

The implementing method

public Cursor executeQuery(String query){
Cursor cursor = db.rawQuery(query, selectionArgs);
return cursor;      
}

where Cursor is android.database.Cursor

Answer 1

I guess you can remove com.domain.database.Cursor, but I may be wrong.

My solution is to make DatabaseAdapter generic, not Cursor.

/**
 * @param C the type of the Cursor this adapter returns
 */
public abstract class DatabaseAdapter<C> {
    protected abstract C executeQuery(String command);      
}

Then you implement like this:

public class AndroidDatabaseAdapter extends DatabaseAdapter<android.database.Cursor> {
    public Cursor executeQuery(String query){
        Cursor cursor = db.rawQuery(query, selectionArgs);
        return cursor;
    }
}