1  import java.util.Set;

  2  import java.util.TreeSet;

  3  

  4  /**

  5     This program tests a tree set with a class that implements the Comparable interface.

  6  */

  7  public class TreeSetDemo

  8  {

  9     public static void main(String[] args)

 10     {

 11        Country country1 = new Country("Uruguay", 176220);

 12        Country country2 = new Country("Thailand", 514000);

 13        Country country3 = new Country("Belgium", 30510);

 14  

 15        Set<Country> countries = new TreeSet<Country>();

 16  

 17        countries.add(country1);

 18        countries.add(country2);

 19        countries.add(country3);

 20        countries.add(country1); // Has no effect

 21        countries.remove(country3); 

 22        countries.remove(country3); // Has no effect

 23        countries.add(country3);

 24  

 25        System.out.println(countries);

 26     }

 27  }