package com.wuming.chat;

public class TalkStudent {
    public static void main(String[] args) {
        //开启两个线程
        new Thread(new TalkSend(7777,"localhost",9999)).start();
        new Thread(new TalkReceive(8888,"老师")).start();
    }
}

==============

package com.wuming.chat;

public class TalkTeacher {
    public static void main(String[] args) {
        new Thread(new TalkSend(5555,"localhost",8888)).start();//5555端口是随便写的；8888对应学生端口
        new Thread(new TalkReceive(9999,"学生")).start();//9999端口对应学生端口
    }
}

=============

package com.wuming.chat;

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

public class TalkReceive implements Runnable{
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    DatagramSocket socket=null;
    private int port;
    private String msgFrom;

    public TalkReceive(int port,String msgFrom) {
        this.port = port;
        this.msgFrom = msgFrom;
        try {
            socket = new DatagramSocket(port);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        while (true) {
            try {
                //准备接受包裹
                byte[] container = new byte[1024];
                DatagramPacket packet = new DatagramPacket(container, 0, container.length);
                socket.receive(packet);//阻塞式接受包裹
                //断开连接 bye
                byte[] data = packet.getData();
                String receiveData = new String(data, 0, packet.getLength());
                System.out.println(msgFrom + ":" + receiveData);
                if (receiveData.equals("bye")) {
                    break;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}
=======================

package com.wuming.chat;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetSocketAddress;
import java.net.SocketException;

public class TalkSend implements Runnable{
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    DatagramSocket socket=null;
    BufferedReader reader=null;
    private int fromPort;
    private String toIp;
    private int toPort;

    public TalkSend(int fromPort, String toIp, int toPort) {
        this.fromPort = fromPort;
        this.toIp = toIp;
        this.toPort = toPort;
        try {
             socket = new DatagramSocket(fromPort);
            //准备数据：控制台读取System.in
            BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        //准备数据：控制台读取System.in
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        while (true){
            try{
                String data = reader.readLine();
                byte[] datas = data.getBytes();
                DatagramPacket packet = new DatagramPacket(datas, 0, data.length(), new InetSocketAddress(this.toIp,this.toPort));
                socket.send(packet);
                if (data.equals("bye")){
                    break;
                }
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }
}

===============

控制台输入：

打开下面这个结果为