import java.util.*;

public class Client {
  public static void main(String[] args) {
    TreeSet treeSet = new TreeSet();

    treeSet.add("1");
    treeSet.add("2");
    treeSet.add("3");
    treeSet.add("4");

    Iterator iterator = treeSet.iterator();

    while(iterator.hasNext()) {
      System.out.println(iterator.next());
    }
  }
}

C:\JavaHello\Iterator>java Client
1
2
3
4

C:\JavaHello\Iterator>