将 InputStream 转换为 BigInteger

public static void main(String[] args) throws IOException {
    // TODO Auto-generated method stub
    // Load file with 17 million digit long number
    BufferedReader Br = new BufferedReader (new FileReader("test2.txt"));
    String Line = Br.readLine();

      try {

         // create a new file with an ObjectOutputStream
         FileOutputStream out = new FileOutputStream("test.txt");
         ObjectOutputStream oout = new ObjectOutputStream(out);

         // write the number into a new file
         oout.writeObject(Line);

         // close the stream
         oout.close();

         // create an ObjectInputStream for the new file
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("test.txt"));

         // convert new file into a BigInteger
         BigInteger Big = (BigInteger) ois.readObject();


      } catch (Exception ex) {
         ex.printStackTrace();
      }

   }

这是我为学习如何使用 Input/OutputStream 而制作的程序。一切正常，只是在尝试将我的文件转换为 BigInteger 时出错。
java.lang.String 不能在 ReadOutPutStream.main 中转换为 java.math.BigInteger
我是新手，所以我可能犯了一个简单的错误，我做错了什么？