Question

I have an abstract factory like so:

public abstract class DAOFactory {
    public abstract EntryDAO<EntryDTO> getEntryDAO();
    ...
}

With my DAO and DTO interfaces like so:

public interface EntryDTO extends GenericDTO {
}

public interface EntryDAO<T extends EntryDTO> extends GenericDAO<T, Serializable> {  
}

And my implementation like so:

public class EntryDTOImpl implements EntryDTO {
}

public class EntryDAOImpl<T extends EntryDTO> extends GenericDaoImpl<EntryDTOImpl, ObjectId> 
 implements EntryDAO<T> {
}

Now, if I create a factory and override the getEntryDAO method like so:

public class MyDAOFactory extends DAOFactory {   
    @Override
    public EntryDAO<EntryDTO> getEntryDAO() {
        try {
            return new EntryDAOImpl<EntryDTOImpl>();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
    }

I get a compile time error:

Type mismatch: cannot convert from EntryDAOImpl to EntryDAO

EDIT

I’ve updated my abstract factory like so:

public abstract <T extends EntryDTO> EntryDAO<T> getEntryDAO();

And made the change in my factory implementation:

@Override
public <T extends EntryDTO> EntryDAO<T> getEntryDAO() {
    try {
        return new EntryDAOImpl<EntryDTOImpl>();
    } catch (UnknownHostException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

Now I’m getting the compile time error:

Type mismatch: cannot convert from EntryDAOImpl<EntryDTOImpl> to EntryDao<T>

Answer 1

Define the abstract method this way (notice ? instead of T):

public abstract EntryDAO<? extends EntryDTO> getEntryDAO(); //should work for you

I recreated the scenario the way I understood it: returning subclass1<subclass2> as interface1<interface2>

For example, this works well:

public static List<? extends CharSequence> dostuff1() { 
    return new ArrayList<String>();
}

But this doesn’t work:

public <B extends CharSequence> List<B> dostuff2() {
    return new ArrayList<String>();
}

This gives the same compile-time error that you got: Type mismatch: cannot convert from ArrayList<String> to List<B>