一个把List<String>转化为以","隔开的字符串的方法

 public static String listToString(List<String> stringList){         if (stringList==null) {             return null;         }         StringBuilder result=new StringBuilder();         boolean flag=false;         for (String string : stringList) {             if (flag) {                 result.append(",");             }else {                 flag=true;             }             result.append(string);         }         return result.toString();     }