无法从Timer函数访问类

Question

我正在开发一个COM端口应用程序，我需要为从COM端口接收到Java控制台的数据设置一个Timer函数。 我的代码是

public void Print_data_function() {
 try{
      serialPort = (SerialPort)portId.open("SimpleReadApp",10000);
     }
    catch(PortInUseException ae)
     { 
        System.out.println(ae+"   port in use beta");
     }   
    timer.schedule(new Serial_Port_Operation(), 2000,1000); /* Application breaks up here when debugging is done*/
    System.out.println("Application Started");
    try{       
       inputStream = serialPort.getInputStream();
    } catch(IOException dd)
    {
        System.out.println("IOE exception");
    }      
}
class Serial_Port_Operation extends TimerTask
{         
  @Override
  public void run()
  {
      boolean data_recieved = false;
      byte[] readBuffer = new byte[1];

        try {

            while (inputStream.available() > 0) {
                int numBytes = inputStream.read(readBuffer);
                data = data + new String(readBuffer);
                data_recieved = true;
            }if(data_recieved == false)
          {
              if ( data.length() > 0)
              {
                    print_data();
              }
          } 
        } catch (IOException e) {System.out.println(e);}

  }

控制台上显示的错误是

COM2
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at serial_printer.Serial_Printer.Print_data_function(Serial_Printer.java:202)
at serial_printer.Serial_Printer.com_portsActionPerformed(Serial_Printer.java:137)
at serial_printer.Serial_Printer.access$000(Serial_Printer.java:21)
at serial_printer.Serial_Printer$1.actionPerformed(Serial_Printer.java:87)

如何成功访问Timer函数中的类。