how to transfer a file using UDP communication in java

//client code
public class UDPSend {
DatagramSocket clientSocket;
    DatagramPacket sendPacket, receivePacket;
    private InputStream inStream = null;
    String checkSumValue;
    public void communicate() {
        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];
        String path = "text1.txt", checkSumValue;
        try {
            FileReader fr = new FileReader("client.properties");
            Properties prop = new Properties();
            prop.load(fr);
            String serverHostname = prop.getProperty("ipAddress");
            int port = Integer.parseInt(prop.getProperty("port"));
            System.out.println("Host: " + serverHostname);
            System.out.println("Port: " + port);
//            BufferedReader inFromUser =
//                    new BufferedReader(new InputStreamReader(System.in));
            clientSocket = new DatagramSocket();
            InetAddress ipAddress = InetAddress.getByName(serverHostname);
            System.out.println("Attempting to connect to " + ipAddress + " via UDP " + port);
            checkSumValue = doMD5(path);
            sendData = checkSumValue.getBytes();
            sendPacket = new DatagramPacket(sendData, sendData.length, ipAddress, port);
            clientSocket.send(sendPacket);
            System.out.println("Client Checksum sent to server : " + new String(sendPacket.getData()));
//            int reply;
            receivePacket = new DatagramPacket(receiveData, receiveData.length);
            clientSocket.receive(receivePacket);
            String res =new String(receivePacket.getData());
//            reply=receivePacket.getData();
//             reply=Integer.parseInt(res);
            System.out.println("Reply: "+ res);

            if (res.equalsIgnoreCase("n")) {
            fileTransfer(path);
            clientSocket.close();
            }
            else{
            Thread.sleep(2000);
            clientSocket.close();
            }
        } catch (Exception ex) {
            System.out.println("Exception :" + ex.getMessage());
        }
    }

    public String doMD5(String path) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            FileInputStream fis = new FileInputStream(path);

            byte[] dataBytes = new byte[1024];

            int nread = 0;
            while ((nread = fis.read(dataBytes)) != -1) {
                md.update(dataBytes, 0, nread);
            };
            byte[] mdbytes = md.digest();
            StringBuffer sb = new StringBuffer();
            for (int i = 0; i < mdbytes.length; i++) {
                sb.append(Integer.toString((mdbytes[i] & 0xff) + 0x100, 16).substring(1));
            }
            checkSumValue = sb.toString();
            System.out.println("Digest(in hex format):: " + checkSumValue);
        } catch (Exception ex) {
            System.out.println("Exception : " + ex.getMessage());
        }
        return checkSumValue;
    }
    public void fileTransfer(String path){
        try {
            FileInputStream fstream = new FileInputStream("text1.txt");
            DataInputStream in = new DataInputStream(fstream);
            BufferedReader br = new BufferedReader(new InputStreamReader(in));

            File file = new File("text1.txt");
            FileInputStream fis = new FileInputStream(file);
            byte[] fsize = new byte[(int) file.length()];
            int size = fis.read(fsize);
            System.out.println("Size = " + size);
            InetAddress addr = InetAddress.getByName("localhost");
            byte[] buf = new byte[10000];
            String DataLine;
            while ((DataLine = br.readLine()) != null) {
//                DatagramPacket packet = new DatagramPacket(DataLine.getBytes(), DataLine.length(), addr, 4555);
                receivePacket=new DatagramPacket(DataLine.getBytes(), DataLine.length());
                System.out.println(DataLine);
                DatagramSocket socket = new DatagramSocket();
                socket.send(receivePacket);
                System.out.println("Sent Packet: "+new String(receivePacket.getData()));
            }
        } catch (Exception ex) {
            System.out.println("Exception in file: " + ex.getMessage());
        }
    }
}

//server code
public class UDPReceive {

    String checkSumValue;
    DatagramSocket serverSocket;
    DatagramPacket receivePacket, sendpacket;
    private OutputStream ouStream = null;

    public void communicate() {
        try {
            String path = "text1.txt";
            FileReader fr = new FileReader("server.properties");
            Properties prop = new Properties();
            prop.load(fr);
            int port = Integer.parseInt(prop.getProperty("port"));
            serverSocket = new DatagramSocket(port);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            receivePacket = new DatagramPacket(receiveData, receiveData.length);
            serverSocket.receive(receivePacket);
            InetAddress IPAddress = receivePacket.getAddress();
            System.out.println("Waiting for datagram packet. . .\n");
            checkSumValue = new String(doMD5(path));
            String msg = new String(receivePacket.getData());
            System.out.println("Checksum of Client: " + msg);
            if (checkSumValue.equalsIgnoreCase(msg)) {
                System.out.println("The file didn't update");
                String reply = "y";
//                String r=new String(reply.);
//                ouStream.write(reply);
                sendData = reply.getBytes();
                sendpacket = new DatagramPacket(sendData, sendData.length);
                serverSocket.send(sendpacket);
                System.out.println("Reply :" + new String(sendData));
            } else {
                String reply = "n";
//                ouStream.write(reply);
                sendData = reply.getBytes();
                System.out.println("Reply :" + new String(sendData));
                sendpacket = new DatagramPacket(sendData, sendData.length);
                serverSocket.send(sendpacket);
                System.out.println("The file updated");
                fileTransfer(path);
            }
        } catch (Exception ex) {
            System.out.println("Error msg: " + ex.getMessage());
        }

    }

    public String doMD5(String path) {
        try {
            MessageDigest md = MessageDigest.getInstance("MD5");
            FileInputStream fis = new FileInputStream(path);
            byte[] dataBytes = new byte[1024];
            int nread = 0;
            while ((nread = fis.read(dataBytes)) != -1) {
                md.update(dataBytes, 0, nread);
            };
            byte[] mdbytes = md.digest();
            StringBuffer sb = new StringBuffer();
            for (int i = 0; i < mdbytes.length; i++) {
                sb.append(Integer.toString((mdbytes[i] & 0xff) + 0x100, 16).substring(1));
            }
            checkSumValue = sb.toString();
            System.out.println("Digest(in hex format):: " + checkSumValue);
        } catch (Exception ex) {
            System.out.println("Error msg: " + ex);
        }
        return checkSumValue;
    }

    public void fileTransfer(String path) {
        try {
            FileWriter fw = new FileWriter(new File(
                    "text1.txt"));
//            fw.write("hi");
//            DatagramSocket Socket = new DatagramSocket(port);
            byte[] receiveData = new byte[1000000];
            while (receiveData != null) {
                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                serverSocket.receive(receivePacket);
                String sentence = new String(receivePacket.getData());
                fw.write(sentence.trim());
                fw.flush();
                System.out.printf("RECEIVED: %s ", new String(receivePacket.getData()));
            }

            fw.flush();
            fw.close();
            serverSocket.close();

        } catch (Exception e) {
            System.err.println(e);
        }
        }