Java TreeMap如何按照key或value降序排列呢？

//key进行降序排序
//构造函数中传入外部比较器
Map<String,String> map = new TreeMap<String,String>(new Comparator<String>(){ 
   public int compare(String obj1,String obj2){ 
    //降序排序 
    return obj2.compareTo(obj1); 
   } 
}); 
map.put("one", "1"); 
map.put("two", "2"); 
map.put("a", "3"); 
Set<String> keySet = map.keySet(); 
Iterator<String> iter = keySet.iterator(); 
while(iter.hasNext()){ 
    String key = iter.next(); 
    System.out.println(key+":"+map.get(key)); 
}

//value进行降序排列
//自定义一个list,用于存放value值，然后传入比较器对value值进行比较
List<Map.Entry<String,String>> mappingList = null; 
Map<String,String> map2 = new TreeMap<String,String>(); 
map2.put("1","one"); 
map2.put("2","two"); 
map2.put("3","a"); 

//借助ArrayList构造函数把map.entrySet()转换成list 
mappingList = new ArrayList<Map.Entry<String,String>>(map2.entrySet()); 
//通过比较器实现比较排序 
Collections.sort(mappingList, new Comparator<Map.Entry<String,String>>(){ 
  public int compare(Map.Entry<String,String> mapping1,Map.Entry<String,String> mapping2){  
      return mapping1.getValue().compareTo(mapping2.getValue()); 
  }
});
for(Map.Entry<String,String> mapping:mappingList){  
    System.out.println(mapping.getKey()+":"+mapping.getValue()); 
}