import java.util.Iterator; import java.util.Set; import java.util.TreeSet; public class TreeTypeSet { public static void main(String[] args) { // TODO Auto-generated method stub Set set = new TreeSet(); set.add("2"); set.add(3); set.add("1"); Iterator it = set.iterator(); while (it.hasNext()) { System.out.print(it.next()+" "); } } } Output is