UDP Echo Server

udpserver.java

import java.io.*;
import java.net.*;

class UDPServer {
  public static void main(String args[]) throws Exception {
    if (args.length < 1) {
      System.out.println("usage: java UDPServer <port>");
    } else {
      int listen = Integer.parseInt(args[0]);
      try {
        DatagramSocket serverSocket = new DatagramSocket(listen);

        byte[] receiveData = new byte[1024];
        byte[] sendData  = new byte[1024];

        System.out.println("listening on port " + listen);

        while(true) {
          receiveData = new byte[1024];

          DatagramPacket receivePacket =
             new DatagramPacket(receiveData, receiveData.length);

          System.out.println ("Waiting for datagram packet");

          serverSocket.receive(receivePacket);

          String sentence = new String(receivePacket.getData());

          InetAddress IPAddress = receivePacket.getAddress();

          int port = receivePacket.getPort();

          System.out.println ("From: " + IPAddress + ":" + port);
          System.out.println ("Message: " + sentence);

          String capitalizedSentence = sentence.toUpperCase();

          sendData = capitalizedSentence.getBytes();

          DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);

          serverSocket.send(sendPacket);
        }
      } catch (SocketException ex) {
        System.out.println("UDP Port " + listen + " is occupied.");
        System.exit(1);
      }
    }
  }
}