Wrong client and server UDP connection in Java - server

Question

I must to do chat server for my subject.
Where is my problem ?
I need to write UDP server class which should send and receive messages from users and transfer it to GUI
Second server should have methods for collect public keys of any user, changing owns ect. Additionally he should store this these keys
What do I have?
I have some code from first server, two Threads for sending and receiving messages and some code in client , but it isn't synchronized. And I don't know how to do it
This is some code from client main method: tfServer --> text field for getting this from user
InetAddress ia = InetAddress.getByName(tfServer.getText());
SenderThread sender = new SenderThread(ia,Integer.valueOf(tfPort.getText()));
sender.start();
ReceiverThread receiver = new ReceiverThread(sender.getSocket());
receiver.start();
First server code :
import java.net.* ;
public class Server {
int port;
private final static int PACKETSIZE = 100 ;
private boolean isStopped = false;
public Server(){
}
public Server(int port) {
this.port = port;
}
public void stop() {
this.isStopped = true;
}
public void start() {
try
{
DatagramSocket socket = new DatagramSocket(this.port) ;
System.out.println( "Serwer gotowy..." ) ;
if(!this.isStopped){
for( ;; ){
DatagramPacket packet = new DatagramPacket( new byte[PACKETSIZE], PACKETSIZE ) ;
socket.receive( packet ) ;
System.out.println( packet.getAddress() + " " + packet.getPort() + ": " + new String(packet.getData()) ) ;
socket.send( packet ) ;
}
}
}
catch( Exception e )
{
System.out.println( e ) ;
}
}
}
And class from first server to receive :
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
public class ReceiverThread extends Thread {
private DatagramSocket udpClientSocket;
private boolean stopped = false;
public ReceiverThread(DatagramSocket ds) throws SocketException {
this.udpClientSocket = ds;
}
public void halt() {
this.stopped = true;
}
public void run() {
byte[] receiveData = new byte[1024];
while (true) {
if (stopped)
return;
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
try {
udpClientSocket.receive(receivePacket);
String serverReply = new String(receivePacket.getData(), 0, receivePacket.getLength());
System.out.println("UDPClient: Response from Server: \"" + serverReply + "\"\n");
Thread.yield();
}
catch (IOException ex) {
System.err.println(ex);
}
}
}
}
Second class from first server to send messages :
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
public class ReceiverThread extends Thread {
private DatagramSocket udpClientSocket;
private boolean stopped = false;
public ReceiverThread(DatagramSocket ds) throws SocketException {
this.udpClientSocket = ds;
}
public void halt() {
this.stopped = true;
}
public void run() {
byte[] receiveData = new byte[1024];
while (true) {
if (stopped)
return;
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
try {
udpClientSocket.receive(receivePacket);
String serverReply = new String(receivePacket.getData(), 0, receivePacket.getLength());
System.out.println("UDPClient: Response from Server: \"" + serverReply + "\"\n");
Thread.yield();
}
catch (IOException ex) {
System.err.println(ex);
}
}
}
}
And server PKI :
public class ServerPKI {
}