code: public boolean startWith(String start) throws IOException{
          char[] cs = start.toCharArray();
          int i = 0;
          for(;i < cs.length; i ++){
              if(peek(i) != cs[i]){
                  return false;
              }
          }
          return true;
      }
docs: 是否以 start 开始
  @param start
  @return true, 如果的确以指定字符串开始
  @throws IOException