Generic methods or Wildcard types?{Resolved}

[Dillinger4]

I've been recently converting some legacy code to use Generics and ive been running into problems. Below is an example of one wall ive run into. A ClassCastException is throw where new Employee Ojects are added to the Set. I understand why but i can't figure out a work around using Wildcards. Does anyone have any suggestions? Perhaps i should use Generics instead? Thanks.

 import java.util.*; 

 class Person{
  public String fname; 
  private String lname;
  public Person(String fname, String lname){
   this.fname = fname; 
   this.lname = lname; 
  } 
 }

 class Employee extends Person{
  private String dept; 
  private String empid; 
  public Employee(String dept, String empid, String fname, String lname){
   super(fname,lname); 
   this.dept = dept; 
   this.empid = empid; 
  }
 }

 class Aux{
  public static void listEmployee(Collection<? extends Person> person){
   for(Person p: person){
    System.out.print(p.fname);
   }
  }
 }

 public class Directory{
  public static void main(String[] args){
   Set<Employee> directory = new TreeSet<Employee>(); 
   directory.add(new Employee("Shipping","9878","Bill","Gates"));
   directory.add(new Employee("Mailroom","6546","John","Holmes"));
   Aux.listEmployee(directory);
  }
 }