Adding an element to a Treeset( was Plz clear My doubt ......)

[kumaresan Natarajan]

import java.util.*;
public class SetTest{
    public static void main(String a[]){
       Set s = new TreeSet();
       s.add(new Person(20));
       s.add(new Person(10));
       System.out.println(s);
    }
}
class Person{
   Person(int i){}
}

I am tring to compile this programe it throws classcastException...
Exception in thread "main" java.lang.ClassCastException
at java.util.TreeMap.compare(TreeMap.java:1085)
at java.util.TreeMap.put(TreeMap.java:463)
at java.util.TreeSet.add(TreeSet.java:209)
at SetTest.main(SetTest.java:6)
Press any key to continue . . .

Why its throws ClassCastException at Runtime...? Can anyone help me...

Thanks in advance
Kumaresan
[ July 28, 2006: Message edited by: Barry Gaunt ]

[Stary Kapec]

Hi,

You use TreeSet. It is a sorted set. Elements you put in it are compared with eath other to be placed in the right order (default would be natural order). So they must implement Comparable, moreover they must also be mutually comparable.
Make the class Person comparable to solve the compilation problem.
[ July 28, 2006: Message edited by: Jasiek Motyka ]

[Barry Gaunt]

Please, Kumaresan, use a meaningful topic title and use

tags to surround formatted code.

[Barry Gaunt]

So, following on from Jasiek's post, you need a person class more or less like this:

class Person implements Comparable {
   private int id;
   Person(int i){
    id = i;
   }
   
   public int compareTo(Object o) {
       Person p = (Person)o; // throwing a CastClassException is OK
       if (id == p.id) {
           return 0;
       }
       return (id < p.id) ? -1 : 1;       
   }
   
   public String toString() {
       return Integer.toString(id);
   }
}    

[Nishita Jain]

"u forgot to implement the Comparable interface "
Read this:
java.util.TreeSet.TreeSet()

Constructs a new, empty set, sorted according to the elements' natural order. All elements
inserted into the set must implement the Comparable interface. Furthermore, all such elements
must be mutually comparable: e1.compareTo(e2) must not throw a ClassCastException for any
elements e1 and e2 in the set. If the user attempts to add an element to the set that violates this
constraint (for example, the user attempts to add a string element to a set whose elements are
integers), the add(Object) call will throw a ClassCastException.

[ July 28, 2006: Message edited by: Nishita Jain ]
[ August 03, 2006: Message edited by: Nishita Jain ]

[Barry Gaunt]

hi, i modified your Person class and wrote like this now its running successfully.

Really? You can substitute your Person class for the original Person class and everything works?
[ July 28, 2006: Message edited by: Barry Gaunt ]

[kumaresan Natarajan]

Thanks Jasiek,Barry , Jain .....[ ]

[Nishita Jain]

Hello,
i made changes in that code ya i agree that you cant replace that with original code...it was my mistake
actually the focus point was "u forgot to implement the Comparable interface "..

thanks