code: public int poll() throws IOException{
          peekindex = 0;
          int v = -1;
          if(cache.size() <= 0){
              v = is.read();
          } else {
              v = cache.poll();
          }
          if(v == -1){
              end = true;
          }
          if(v == '\n'){
              col = 0;
              row ++;
          } else {
              col ++;
          }
          return v;
      }
docs: 读取头部字节, 并删除
  @return 头部字节
  @throws IOException