UDPServer.java

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;

public class UDPServer{
  // Server UDP socket runs at this port
  public final static int SERVICE_PORT=50001;
 
  public static void main(String[] args) throws IOException{
    try{
      // Instantiate a new DatagramSocket to receive responses from the client
      DatagramSocket serverSocket = new DatagramSocket(SERVICE_PORT);
      
      /* Create buffers to hold sending and receiving data.
      It temporarily stores data in case of communication delays */
      byte[] receivingDataBuffer = new byte[1024];
      byte[] sendingDataBuffer = new byte[1024];
      
      /* Instantiate a UDP packet to store the 
      client data using the buffer for receiving data*/
      DatagramPacket inputPacket = new DatagramPacket(receivingDataBuffer, receivingDataBuffer.length);
      System.out.println("Waiting for a client to connect...");
      
      // Receive data from the client and store in inputPacket
      serverSocket.receive(inputPacket);
      
      // Printing out the client sent data
      String receivedData = new String(inputPacket.getData());
      System.out.println("Sent from the client: "+receivedData);
      
      /* 
      * Convert client sent data string to upper case,
      * Convert it to bytes
      *  and store it in the corresponding buffer. */
      sendingDataBuffer = receivedData.toUpperCase().getBytes();
      
      // Obtain client's IP address and the port
      InetAddress senderAddress = inputPacket.getAddress();
      int senderPort = inputPacket.getPort();
      
      // Create new UDP packet with data to send to the client
      DatagramPacket outputPacket = new DatagramPacket(
        sendingDataBuffer, sendingDataBuffer.length,
        senderAddress,senderPort
      );
      
      // Send the created packet to client
      serverSocket.send(outputPacket);
      // Close the socket connection
      serverSocket.close();
    }
    catch (SocketException e){
      e.printStackTrace();
    }
  }
}