Chat implementation on UDP protocol

ChatClient.java

package network_tech;

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


public class ChatClient {

   public static void main(String args[]) throws Exception{
       String host = null;
       if (args.length < 1){
           System.out.println("Usage: java ChatClient <server_hostname>");
           System.exit(0);
       } else {
           host = args[0];
       }
       DatagramSocket socket = new DatagramSocket();
       MessageReceiver receiver = new MessageReceiver(socket);
       MessageSender sender = new MessageSender(socket, host);
       Thread rt = new Thread(receiver);
       Thread st = new Thread(sender);
       rt.start();
       st.start();
   }
}

ChatServer.java

package network_tech;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;


public class ChatServer extends Thread{
    public final static int PORT = 7331;
    private final static int BUFFER = 1024;

    private DatagramSocket socket;
    private ArrayList<InetAddress> clientAddresses;
    private HashSet<String> existingClients;
    private ArrayList<Integer> clientPorts;
    public ChatServer() throws IOException {
        socket = new DatagramSocket(PORT);
        clientAddresses = new ArrayList<>();
        clientPorts = new ArrayList<>();
        existingClients = new HashSet<>();
    }

    public void run(){
        byte[] buf = new byte[BUFFER];
        while(true){
            try{
                Arrays.fill(buf, (byte)0);
                DatagramPacket packet = new DatagramPacket(buf, buf.length);
                socket.receive(packet);

                String content = new String(buf, 0, buf.length);

                InetAddress clientAddress = packet.getAddress();
                int clientPort = packet.getPort();

                String id = clientAddress.toString() + "," + clientPort;
                if (!existingClients.contains(id)){
                    existingClients.add(id);
                    clientPorts.add(clientPort);
                    clientAddresses.add(clientAddress);
                }

                System.out.println(id + " : " + content);
                byte[] data = (id + " : "+ content).getBytes();
                for (int i = 0; i < clientAddresses.size(); i++){
                    InetAddress cl = clientAddresses.get(i);
                    int cp = clientPorts.get(i);
                    packet = new DatagramPacket(data, data.length, cl, cp);
                    socket.send(packet);
                }
            } catch (IOException e){
                System.err.println(e);

            }
        }
    }
    public static void main(String[] args){
        try {
            ChatServer s = new ChatServer();
            s.start();
        } catch(IOException e){
            e.printStackTrace();
        }
    }
}

MessageReceiver.java

package network_tech;

import java.net.DatagramPacket;
import java.net.DatagramSocket;


class MessageReceiver implements Runnable{
    DatagramSocket socket;
    byte buf[];
    MessageReceiver(DatagramSocket s){
        socket = s;
        buf = new byte[1024];
    }
    public void run(){
        while(true){
            try{
                DatagramPacket packet = new DatagramPacket(buf, buf.length);
                socket.receive(packet);
                String received = new String(packet.getData(), 0, packet.getLength());
                System.out.println(received);
            } catch (Exception e){
                System.err.println(e);
            }
        }
    }
}

MessageSender.java

package network_tech;

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


class MessageSender implements Runnable {
    public final static int PORT = 7331;
    private DatagramSocket socket;
    private String hostname;
    MessageSender(DatagramSocket s, String h){
        socket = s;
        hostname = h;
    }
    private void sendMessage(String s) throws Exception{
        byte buf[] = s.getBytes();
        InetAddress address = InetAddress.getByName(hostname);
        DatagramPacket packet = new DatagramPacket(buf, buf.length, address, PORT);
        socket.send(packet);
    }
    public void run(){
        boolean connected = false;
        do {
            try{
                sendMessage("GREETINGS");
                connected = true;
            } catch (Exception e){
                e.printStackTrace();
            }
        } while (!connected);
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        while(true){
            try{
                while (!in.ready()){
                    Thread.sleep(100);
                }
                sendMessage(in.readLine());
            } catch(Exception e){
                System.err.println(e);
            }
        }
    }
}