code: public String next() {
          if (maxLen <= 0 || minLen <= 0 || minLen > maxLen)
              return null;
          char[] buf = new char[R.random(minLen, maxLen)];
          for (int i = 0; i < buf.length; i++)
              buf[i] = CharGenerator.next();
          return new String(buf);
      }
docs: 根据设置的max和min的长度,生成随机字符串.
  <p/>
  若max或min小于0,则返回null
  
  @return 生成的字符串