code: public static String dup(CharSequence cs, int num) {
          if (isEmpty(cs) || num <= 0)
              return "";
          StringBuilder sb = new StringBuilder(cs.length() * num);
          for (int i = 0; i < num; i++)
              sb.append(cs);
          return sb.toString();
      }
docs: 复制字符串
  
  @param cs
  字符串
  @param num
  数量
  @return 新字符串