Java Collection - 如何按键对地图值进行排序

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class Main {

  public static void main(String[] args) {
    HashMap<String, String> hm = new HashMap<String, String>();
    hm.put("4", "four");
    hm.put("3", "three");
    hm.put("1", "one");
    hm.put("2", "two");

    Map<String, String> treeMap = new TreeMap<String, String>(hm);
    System.out.println(treeMap);
  }
}