import java.net.*;public class UDPClient { public static void main(String[] args) throws Exception { DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("localhost"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; while (true) { String sentence = "hello server"; 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("Server: " + modifiedSentence); } }}

这是一个Java基于UDP协议的客户端程序，它会向指定IP地址和端口发送消息，并接收来自服务端的响应消息。在这个程序中，客户端会不断发送消息，并等待响应消息，然后将响应消息输出到控制台。

要测试这个程序，你需要先运行一个UDP服务端程序，然后再运行两个客户端程序，分别作为两个用户进行通信。下面是一个简单的UDP服务端程序示例：

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 sentence = new String(receivePacket.getData());
            InetAddress IPAddress = receivePacket.getAddress();
            int port = receivePacket.getPort();
            System.out.println("Client: " + sentence);
            String capitalizedSentence = sentence.toUpperCase();
            sendData = capitalizedSentence.getBytes();
            DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);
            serverSocket.send(sendPacket);
        }
    }
}

你可以将上面的服务端程序保存到一个Java文件中，然后用javac命令进行编译。然后再运行两个客户端程序，分别作为两个用户进行通信即可。