code: public static String listToString(Map<?, ?> map){
  		if (map == null){
  			return "null";
  		}
  		TreeMap<?, ?> tm = new TreeMap<Object, Object>(map);
  		StringBuilder sb = new StringBuilder();
  		for (Map.Entry<?, ?> item: tm.entrySet()){
  			Object k = item.getKey();
  			Object v = item.getValue();
  			sb.append(k == null ? "null" : k.toString());
  			sb.append("\t= ");
  			sb.append(v == null ? "null" : v.toString());
  			sb.append('\n');
  		}
  		return sb.toString();
  	}
docs: Print the content of the Map in a newly created String,
  entries are sorted by key,  formatted as "key\t= value\n".<br>
  把Map里的内容列在String里，Map里的每个entry占一行，按key排序，
  每行的格式为“key\t= value\n”。
  
  @param map	The Map object for which the content need to be listed.<br>
  需要列出内容的Map对象
  @return 		A String that holds formated content of the Map.<br>
  含有格式化过的Map内容的String