Cannot Cast List ? extends Object as List Object

Question

I apologize if this is a common question but I could not seem to find the proper set of search terms that yielded an answer.
Basically I have an RMI application and on the 'middle-tier' I would like to pass around references to the implementation objects and only convert to the interface type when dealing with them over RMI (I never pass an object back to the middle-tier).
public interface Questionnaire implements Remote {
    public List<? extends Comment> getComments() throws RemoteException;
public class QuestionnaireImpl extends UnicastRemoteObject implements Questionnaire {
    public List<CommentImpl> getComments() {...}
}This works fine, the issue arises when I try to do the following:
public List<Comment> getComments(int questionnaireId) throws RemoteException {
    return classLoader.getQuestionnaire(questionnaireId).getComments();
}The compiler issues a Type mismatch: cannot convert from List<capture#8-of ? extends Comment> to List<Comment> Which I find perplexing as the compiler can assure that at the very least it is a List of Comment objects.

Answer 1

public List<? extends Comment> getComments() throws RemoteException;
public List<Comment> getComments(int questionnaireId) throws RemoteException {
return classLoader.getQuestionnaire(questionnaireId).getComments();
The compiler issues a Type mismatch: cannot convert from List<capture#8-of ? extends Comment> to List<Comment> Which I find perplexing as the compiler can assure that at the very least it is a List of Comment objects.Yes, however Java's generics work correctly to prevent you from shooting yourself in the foot. If you have a List<Superclass> pointing at a List<Subclass>, you would be entirely within your rights to try to store an object that is not castable to Subclass in it, this breaks type safety, so the compiler will not let you do it.
Instead, create a new List<Supertype> and copy over the elements from the list returned by getComments(). There is a constructor that will do this for you.