Why does it give exception

[rimzy abdulkader]

import java.util.*;
public class test{
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){}
}
[ October 04, 2006: Message edited by: rimzy abdulkader ]

[Ali Gohar]

TreeSet requires that objects added to it should implements Comparable.

Check this out i have modified your code here

   
import java.util.Set;
import java.util.TreeSet;
 

public class test{
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{
int i;
Person(int i){
this.i = i;
}
public int compareTo(Object o) {
Person p = (Person)o;

return this.i - p.i;
}
     }

[Steven Gao Song]

You did not override the equals() and hashCode() methods.

I think you should override them in order to use TreeSet.

Am I right? or it is not necessary?

[Rajesh Kadle]

equals() and hashCode() methods should be overridden if we want to store
the objects in HashSet or LinkedHashSet or HashMap or LinkedHashMap.

If you want to store the objects in TreeSet or TreeMap you should can override compareTo() method. This will provide the NATURAL ORDERING.

We can also manage by not implementing 'compareTo()' but passing OUR IMPLEMENTATION of 'Comparator' INTERFACE to the TreeSet/TreeMap constructors. Comparator interface provides 'comparable(Object o1, Object o2) method which should be overridden.

We need not override equals() and hashCode() for TreeSet and TreeMap.

Ranchers please correct me.....