Java NavigableMap get lower key

import java.util.NavigableMap;
import java.util.TreeMap;

public class Main {
  public static void main(String args[]) {

    NavigableMap<String, Integer> nav = new TreeMap<String, Integer>();
    nav.put("1", 1);
    nav.put("2", 2);
    nav.put("3", 3);
    nav.put("4", 4);
    nav.put("5", 5);
    //from  www.ja v a2s. c  o  m
    System.out.println(nav);
    
    System.out.println(nav.lowerKey("3"));
    
    System.out.println(nav.lowerKey("Not Exist"));
  }
}