server端
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;


public class UDPServer {
    public static void main(String args[]) throws Exception
    {
       DatagramSocket serverSocket = new DatagramSocket(9876);
          byte[] receiveData = new byte[1024];
          byte[] sendData = new byte[1024];
          while(true)
             {
                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                serverSocket.receive(receivePacket);
                String sentence = new String( receivePacket.getData());
                System.out.println("RECEIVED: " + sentence);
                InetAddress IPAddress = receivePacket.getAddress();
                int port = receivePacket.getPort();
                String capitalizedSentence = sentence.toUpperCase();
                sendData = capitalizedSentence.getBytes();
                DatagramPacket sendPacket =
                new DatagramPacket(sendData, sendData.length, IPAddress, port);
                serverSocket.send(sendPacket);
             }
    }
}


client端


import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;


public class UDPClient {

     public static void main(String args[]) throws Exception
       {
          BufferedReader inFromUser =
             new BufferedReader(new InputStreamReader(System.in));
          DatagramSocket clientSocket = new DatagramSocket();
          InetAddress IPAddress = InetAddress.getByName("localhost");
          byte[] sendData = new byte[1024];
          byte[] receiveData = new byte[1024];
          String sentence = inFromUser.readLine();
          sendData = sentence.getBytes();
          DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
          clientSocket.send(sendPacket);
          DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
          clientSocket.receive(receivePacket);
          String modifiedSentence = new String(receivePacket.getData());
          System.out.println("FROM SERVER:" + modifiedSentence);
          clientSocket.close();
       }
}