The set is an Interface, so  we can't make an object. You need to import TreeSet or SortedSet class and thus follow the steps.

import java.util.TreeSet;

public class setExample {

     public static void main(String args[])

    {

        Set s=new TreeSet();

        s.add(10);

        s.add(10);

        s.add(20);

       

        System.out.println(s);

    }

}