Adding null to empty TreeSet raising NullPointerException

Question

import java.util.TreeSet;
class Test 
{
    public static void main(String[] args) 
    {
        TreeSet t=new TreeSet();
        t.add(null);
        System.out.println(t);
    }
}

output: NullPointerException. I read in many articles that empty TreeSet will accept null for first time but am getting NullPointerException...am using java7..can any body clarify my doubt....

Answer 1

The documentation for TreeSet#add in Java 7 states:

NullPointerException - if the specified element is null and this set uses natural ordering, or its comparator does not permit null elements

So since you have not specified a custom comparator implementation that can handle null values, you get the NPE.

Edit: It was possible to add a null element as the first element of a TreeSet/TreeMap in Java 6, but it was considered a bug: