Generic methods or Wildcard types?{Resolved}

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.

Code:

---

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);
  }
 }

---

You mean this line:
directory.add(new Employee("Shipping","9878","Bill","Gates"));
throws a ClassCastException? It shouldn't.

Or am I misunderstanding your problem?

Yeah i thought i understood why but i was thinking about somthing else. I just doesn't make any sense to me. If the Collection is marked to hold a specific type then is should be legal to insert that type into the Collection. I simplified the code by taking out the super class.

Code:

---

import java.util.*;

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

class Aux{
  public static void listEmployee(Collection<Employee> e){
  for(Employee employee: e){
    System.out.print(employee.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);
  }
 }

---

If native types are used then everything is fine. I think somthing needs to be done to a user defined type when using that type as a type parameter to a Collection.

Code:

---

import java.util.*;

 public class Directory{
  public static void main(String[] args){
  Set<String> s = new TreeSet<String>();
  s.add(new String("Hello"));
  s.add(new String("Goodbye"));
  printElements(s);
  addElements(s);
  }
  public static void printElements(Collection<String> s){
  for(Object o :s){
    System.out.println(o);
  }
  }
  public static void addElements(Collection<String> s){
    s.add(new String("Welcome"));
  }
 }

---

I took the original code and went into the debugger. I believe the problem is that you insert the object into a TreeSet, which requires an ordering imposed on its element. Since you don't supply a custom comparator, it attempts to cast the Employees to Comparable, which throws the exception.

Edit: I seem to be right. After replacing the TreeSet with a HashSet, the thing worked.

Thanks CB. :thumb: Element ordering is not necessary so i should probably be able to use. List<Employee> directory = new ArrayList<Employee>(); Yes?

Yup everything runs fine. Thanks CB.