Question

I have a super class DBAdapter, it have method fetchAll() which selects all records from the table of the DB to List<DBObject>. I have inherit this class with CategoryDBAdapter, this class knows about class Category extended from DBObject.

When I call:

CategoryDBAdapter categoryDbAdapter = new CategoryDBAdapter(dbHelper);

List<Category> categories = categoryDbAdapter.fetchAll();

my code can’t cast List<DBObject> to the List<Category>. What is the common used way to resolve this collision?

Answer 1

You could change your DBAdapter and DBCategoryAdapter classes to use generics:

class DBAdapter<T extends DBObject> {
    List<T> fetchAll() {
        ...
    }
}

class DBCategoryAdapter extends DBAdapter<Category> {

}

You would also need to adapt the content of the fetchAll() method, how easy this is depends on your implementation.