code: public static String removeFirst(CharSequence str) {
          if (str == null)
              return null;
          if (str.length() > 1)
              return str.subSequence(1, str.length()).toString();
          return "";
      }
docs: 截去第一个字符
  <p>
  比如:
  <ul>
  <li>removeFirst("12345") => 2345
  <li>removeFirst("A") => ""
  </ul>
  
  @param str
  字符串
  @return 新字符串