a端

import java.io.*;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

/**
 * a端发送“今天星期几”给b端
 */
public class UDPa {
    public static void main(String[] args) throws IOException {
        //a端绑定9999端口————a端从9999接收消息
        DatagramSocket dgSocket = new DatagramSocket(9999);
        byte[] bytes = "今天星期几?".getBytes();
        // public DatagramPacket(byte buf[], int offset, int length,
        //                          InetAddress address, int port)
        DatagramPacket packet = new DatagramPacket(bytes, 0, bytes.length,
                InetAddress.getLocalHost(), 9998);
        dgSocket.send(packet);

        byte[] bytes2 = new byte[1024];
        //public DatagramPacket(byte buf[], int length)
        packet = new DatagramPacket(bytes2,bytes2.length);
        dgSocket.receive(packet);
        System.out.println("a接收到的信息："+new String(packet.getData()));

        dgSocket.close(); // 记得关闭流！
    }
}

b端

import java.io.*;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

/**
 * b端接收a端发来的消息，并打印
 *  b再发送给a：“星期一”
 */
public class UDPb {
    public static void main(String[] args) throws IOException {
        //b端绑定9998端口————b端从9998接收消息
        DatagramSocket dgSocket = new DatagramSocket(9998);
        byte[] bytes = new byte[1024];
        //public DatagramPacket(byte buf[], int length)
        DatagramPacket packet = new DatagramPacket(bytes,bytes.length);
        dgSocket.receive(packet);
        System.out.println("b接收到的信息："+new String(packet.getData()));

        byte[] bytes2 = "今天星期一。".getBytes();
        // public DatagramPacket(byte buf[], int offset, int length,
        //                          InetAddress address, int port)
        packet = new DatagramPacket(bytes2, 0, bytes2.length,
                InetAddress.getLocalHost(), 9999);
        dgSocket.send(packet);

        dgSocket.close();
    }
}

运行结果

参考资料

https://www.bilibili.com/video/BV1fh411y7R8?p=678