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);
  }
 }

[CornedBee]

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

Or am I misunderstanding your problem?

[Dillinger4]

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.

 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);
  }
 }

[Dillinger4]

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.

 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")); 
  }
 }

[CornedBee]

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.

[Dillinger4]

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

[Dillinger4]

Yup everything runs fine. Thanks CB.