TreeSet is a container which put objects in sorted order. In order to sort data , object being added should be of type Comparable.But the class Person does not implements the Comparable Interface, therefore the classcastexception occurs.

import java.util.*;
public class Test1{
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 implements Comparable{ 
   Person(int i){}  
    public int compareTo(Object arg0) {  
    return -1;  
    }
}

By implementing Comparable class and its method, you can remove the exception