java.lang.NumberFormatException: null in Integer.parseInt

Question

Okay, the following are 2 classes that I am using in creating a simple TFTP (Trivial File Transfer Protocol) program. I am running a server instance and a client instance of this program. What I am trying to accomplish is this:

The client connects to the server and sends a message of a specific file that it wants. The server locates the file and sends a response (either a 1 or 0... 1 meaning the file is there, and 0 meaning that it is not). Then the server will send the content of the file to the client application. The files I am trying to send are just simple text files.

Right now, I am able to receive the name of the text file that the client wants, but then when I go to send back the response, I am not getting anything returned. Also below are the methods that the server and the client both run.

This is the server instance

SwingUtilities.invokeLater(new Runnable(){
    public void run(){
        String fileRequest = UDPReceiver(SERVER_PORT_NUMBER);
        outputArea.append("\n" + "File Requested: " + fileRequest + "\n");
        outputArea.append("Determining if file exists...\n");
        String checkFile = SHARED_DIR + "\\" + fileRequest;
        outputArea.append("Checking location: " + checkFile + "\n");

        boolean check = fileCheck(checkFile);
        if(check == true){
            outputArea.append("File location verified..." + "\n");
            outputArea.append("Initiating transfer...." + "\n\n");

            UDPSender(CLIENT_HOSTNAME, CLIENT_PORT_NUMBER, "1");

            }
            else{
                outputArea.append("File does not exist..." + "\n");
                outputArea.append("Exiting run..." + "\n");
            }
       } 
  });

The client instance.

SwingUtilities.invokeLater(new Runnable(){
    public void run(){
        UDPSender(SERVER_HOSTNAME, SERVER_PORT_NUMBER, FILE_REQUEST);

        String message = UDPReceiver(CLIENT_PORT_NUMBER);
        outputArea.append("\n\n" + message + "\n");

        if(message == "1"){
            // File exists
            outputArea.append("\n");
            outputArea.append("File verified..." + "\n");
            outputArea.append("Transfer initiated..." + "\n");
        }
        else{
            // File doesn't exist
            outputArea.append("\n");
            outputArea.append("File does not exist..." + "\n");
            outputArea.append("Terminating connection...");
        }
    } 
});

Here are the Sender and Receiver methods.

 private void UDPSender(String hostname, String port, String message){
   DatagramSocket socket = null;
   try{
       // Create a datagram socket, look for the first available port
       socket = new DatagramSocket();

       outputArea.append("Using local port: " + socket.getLocalPort() + "\n");
       ByteArrayOutputStream bOut = new ByteArrayOutputStream();
       PrintStream pOut = new PrintStream(bOut);
       pOut.print(message);
       // Convert printstream to byte array
       byte[] bArray = bOut.toByteArray();
       // Create a DatagramPacket, containing a maximum buffer of 256 bytes
       DatagramPacket packet = new DatagramPacket(bArray, bArray.length);

       outputArea.append("Looking for hostname " + hostname + "\n");
       // Get the InetAddress object
       InetAddress remote_addr = InetAddress.getByName(hostname);
       // Check its IP Number
       outputArea.append("Hostname has IP Address = " + remote_addr.getHostAddress() + "\n");
       // Configure the DatagramPacket
       packet.setAddress(remote_addr);
       packet.setPort(Integer.parseInt(port));
       // Send the packet
       socket.send(packet);
       outputArea.append("Packet sent at: " + new Date() + "\n");

       // Display packet information
       outputArea.append("Sent by: " + remote_addr.getHostAddress() + "\n");
       outputArea.append("Sent from: " + packet.getPort() + "\n");
       socket.close();
   }
   catch(UnknownHostException ue){
       outputArea.append("Unknown host: " + hostname + "\n");
       outputArea.append("Unknown host: " + ue + "\n");
   }
   catch(IOException e){
       outputArea.append("Error: " + e + "\n");
   }
}


private String UDPReceiver(String portNum){
   String message = "";
   DatagramSocket socket = null;
   try{
       // Create a DatagramSocket
       socket = new DatagramSocket(Integer.parseInt(portNum));

       outputArea.append("Listening on local port " + socket.getLocalPort() + "\n");

       // Create a DatagramPacket, containing a maximum buffer of 256 bytes
       DatagramPacket packet = new DatagramPacket(new byte[256], 256);

       // Receive a packet - remember by default this is a blocking operation
       socket.receive(packet);

       outputArea.append("Packet received at " + new Date() + "\n");
       // Display packet information
       InetAddress remote_addr = packet.getAddress();
       outputArea.append("Sender: " + remote_addr.getHostAddress() + "\n");
       outputArea.append("From Port: " + packet.getPort() + "\n");

       CLIENT_HOSTNAME = remote_addr.getHostAddress();
       //CLIENT_PORT_NUMBER = Integer.toString(packet.getPort());

       // Display packet contents, by reading from byte array
       ByteArrayInputStream bin = new ByteArrayInputStream(packet.getData());

       // Display only up to the length of the original UDP packet
       for(int i = 0; i < packet.getLength(); i++){
           int data = bin.read();
           if(data == -1){
               break;
           }
           else{
               message = message + (char)data;
               //outputArea.append(Character.toString((char)data));
           }
       }
       socket.close();
       return message;
   }
   catch(IOException e){
       outputArea.append("Error: " + e + "\n");
       message = "Error: " + e;
       return message;
   }
}

Any help that you folks can offer would be greatly appreciated. The main thing I am trying to figure out is how to be able to get the server and client to be able to send messages back and forth. Thanks in advance guys.

EDIT:

I am also getting an error now in Netbeans when I run this project. I think it has something to do with this line of code in the UDPReceiver method:

socket = new DatagramSocket(Integer.parseInt(portNum));

But I can't figure out what is wrong with that.

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: null
at java.lang.Integer.parseInt(Integer.java:454)
at java.lang.Integer.parseInt(Integer.java:527)
at tftp_gui.main.UDPReceiver(main.java:508)
at tftp_gui.main.access$800(main.java:20)
at tftp_gui.main$10.run(main.java:374)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:721)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:682)
at java.awt.EventQueue$3.run(EventQueue.java:680)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:691)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

Answer 1

you approach is wrong, as it is requires opened client port, anyway your particular problem is because you're not sending back result when file is not found:

check:

if(check == true){
    outputArea.append("File location verified..." + "\n");
    outputArea.append("Initiating transfer...." + "\n\n");
    UDPSender(CLIENT_HOSTNAME, CLIENT_PORT_NUMBER, "1");
}
else {
    outputArea.append("File does not exist..." + "\n");
    outputArea.append("Exiting run..." + "\n");
    UDPSender(CLIENT_HOSTNAME, CLIENT_PORT_NUMBER, "0"); // here you should send 0
}

also, you have small problem here: if(message == "1") it should look like if ("1".equals(message))

Your error is self explainable - you're haven't declared proper CLIENT_PORT_NUMBER or SERVER_PORT_NUMBER, here is what I've used for testing:

private final static String SERVER_PORT_NUMBER = "1234";
private static String CLIENT_HOSTNAME;
private static final String CLIENT_PORT_NUMBER = "2345";
private static final String FILE_REQUEST = "a.txt";
private static final String SHARED_DIR = "d:/";
private static final String SERVER_HOSTNAME = "localhost";