Here is an example of a UDP client-server program in Java:

UDP Server:

import java.net.*;

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 message = new String(receivePacket.getData(), 0, receivePacket.getLength());
            System.out.println("Received: " + message);

            InetAddress clientAddress = receivePacket.getAddress();
            int clientPort = receivePacket.getPort();
            String response = message.toUpperCase();
            sendData = response.getBytes();

            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, clientAddress, clientPort);
            serverSocket.send(sendPacket);
        }
    }
}

UDP Client:

import java.net.*;

public class UDPClient {
    public static void main(String[] args) throws Exception {
        DatagramSocket clientSocket = new DatagramSocket();

        InetAddress serverAddress = InetAddress.getByName("localhost");
        int serverPort = 9876;

        String message = "Hello, server!";
        byte[] sendData = message.getBytes();

        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, serverAddress, serverPort);
        clientSocket.send(sendPacket);

        byte[] receiveData = new byte[1024];
        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        clientSocket.receive(receivePacket);

        String response = new String(receivePacket.getData(), 0, receivePacket.getLength());
        System.out.println("Response from server: " + response);

        clientSocket.close();
    }
}

In this example, the server listens on port 9876 and waits for messages from clients. When a message is received, the server converts it to uppercase and sends it back to the client. The client sends a message to the server, waits for a response, and then prints the response. Note that UDP is connectionless, so there is no need to establish a connection before sending or receiving messages.