Question

I am trying to get the following logic to work:

public class Wrapper {
    public static class General {
        public void copy(General g) {
            // copy fields
        }
        // General contents.
    }

    public static class Specific extends General {
        // Specific contents.
    }

    public static class GeneralLogic<T extends General> {
        public T load(ResultSet rs) {
            General o = new General();
            // Do stuff
            return new General();
        }
    }

    public static class SpecificLogic<T extends Specific> extends GeneralLogic<T> {
        @Override
        public T load(ResultSet rs) {
            Specific o = new Specific();
            o.copy(super.load(rs));
            // Do stuff
            return new Specific();
        }
    }
}

Both return lines result in a compile error: Type mismatch: cannot convert from Wrapper.General to T (Wrapper.Specific to T for the second return).

Answer 1

If T is a subtype of General, an instance of General isn’t necessarily a T. If you want to return a T, you’ll have to get it from another factory method (as an argument or abstract method), or by accepting a Class<? extends T> and instantiate it via reflection.