Parametric polymorphism struggles

Question

In addition to implementing a Bag & List for an assignment, the next step is to create an ordered version. A requirement is to specify the parametric interface OrderedCollection with the correct type parameters and constraints. My problem is with implementing it.

An interface Collection <E> exists and is defined as

public interface Collection<E> extends Iterable<E>{
  public void add(E e);
  public void remove(E e);
  public boolean contains(Object e);
  public void clear();
  public int size();
  public boolean isEmpty();
  public Object[] toArray();
}

It is implemented by the classes

public class UnorderedList<E> implements Collection<E>
public class UnorderedBag<E> extends UnorderedList<E> implements Collection<E>

I have the structures working, and am now trying to implement the sorted version. To do this and satisfy part of the requirements, I created OrderedCollection as

public interface OrderedCollection <E extends Comparable<E>> {
  public int compareTo(E e);
}

because it is extending the methods already defined in Collection, and the only new functionality required is a compareTo() method.

However, when I try to implement OrderedList by the declaration

public class OrderedList<E> extends UnorderedList<E> implements OrderedCollection<E>

I get an error stating that

Bound mismatch: The type E is not a valid substitute for the bounded parameter <E
extends Comparable<E>> of the type OrderedCollection<E>

As I understand the error message, I need to specify a parameter type that is a valid substitute for the one given in the interface declaration. However, I've tried

OrderedCollection<E extends Comparable<E>>

as the implements declarer, but then I get a warning that a syntax error exists on extends.

How do I satisfy the requirements here?

Answer 1

In your declaration of the OrderedList class, the generic type of OrderedList needs to match the restriction that OrderedCollection expects.

public class OrderedList<E extends Comparable<E>> 
             extends UnorderedList<E> 
             implements OrderedCollection<E>