Problem using a GUI, a loop and a different thread for the loop

Question

Hi everyone!
First of all sorry if my post is in the wrong forum.
I'm designing a client-server project to allow users to comunicate with each other, and I have a problem in the client class. I'm using the UDP transport protocol, and I'm not allowed to use TCP. The thing is, each client is allowed to send and receive a message at any time, so by pushing a button the "sending event" triggers and sends it, and in order to receive i've launched a receiving process in another different thread, so the sending procces wouldn't be blocked.
This is the code of the client class, called Conversacion:
package mymessenger;
import javax.swing.*;
import java.net.*;
import java.io.*;
import java.lang.*;
import java.awt.*;
import java.util.*;
public class Conversacion extends javax.swing.JFrame {
    private InetAddress MaquinaServidor;
    private int PuertoServidor;
    private DatagramSocket MiSocket;
    private DatagramPacket PaqueteSalida;
    byte [] BufferEntrada= new byte[1024];
    private DatagramPacket PaqueteEntrada= new DatagramPacket (BufferEntrada,BufferEntrada.length);
    byte [] BufferSalida;
    int ack=0;
    int puerto_contrario;
    String direccion="192.168.1.102";
    String direccion_contraria;
    String destinatario_completo="";
    String nombre_destinatario="";
    String nombre_local="";
    String mensaje_recibido=""; // This is the variable I want to use every time a client receives a message
    boolean salir=false;
    public Conversacion()
        initComponents();
        esperar();
@SuppressWarnings("unchecked")
// Here would come the generated code by Netbeans, which is not relevant in this case
private void enviarActionPerformed(java.awt.event.ActionEvent evt) {                                     
   // This is the sending event that calls a function to send the message to the server, i'll post it in another message
    void enviar (String mens) 
        // Function used to send the message to the server, i'll post it in another message
    void esperar() // Here's the problem (I think)
        new Thread(new Runnable() // I launch it in a new thread, so I don't block "enviar"
            public void run() // At first, the client sends some message to the server, so the server knows some data of the client. Go to the while() command please
                String respuesta="";
                try
                    MiSocket= new DatagramSocket();
                    MaquinaServidor= InetAddress.getByName(direccion);
                    PuertoServidor=7777;
                    String comando="ENVIAME LOS DATOS AQUI, A LA CONVERSACION";
                    byte[] na={(byte)(ack%2)};
                    comando= (new String (na))+comando;
                    BufferSalida = comando.getBytes();
                    PaqueteSalida = new DatagramPacket(BufferSalida,BufferSalida.length,MaquinaServidor,PuertoServidor);
                    segsend(PaqueteSalida);
                    ack++;
                    System.out.println("Estoy esperando un mensaje del servidor");
                    segreceive(PaqueteEntrada);
                    String retorno = new String(BufferEntrada, 1,PaqueteEntrada.getLength()-1);
                    int pos= retorno.indexOf("(");
                    int pos2=retorno.indexOf("]");
                    int pos3=retorno.indexOf("Ð");
                    nombre_destinatario=retorno.substring(0,pos-1);
                    destinatario_completo=retorno.substring(0,pos2+1);
                    direccion_contraria=retorno.substring(pos2+1,pos3);
                    nombre_local=retorno.substring(pos3+1);
                    setTitle(destinatario_completo+" - CONVERSACIÓN");
                    segreceive(PaqueteEntrada);
                    respuesta = new String(BufferEntrada, 1,PaqueteEntrada.getLength()-1);
                    puerto_contrario = Integer.parseInt(respuesta);
                    while (!salir) // Here begins the infinite loop I use to receive messages over and over again
                        segreceive(PaqueteEntrada); // I use segreceive that is kind of the same as "receive". No big deal about this
                        mensaje_recibido = new String(BufferEntrada, 1,PaqueteEntrada.getLength()-1); // Every time a client receives a message, I want to store it in the variable "mensaje_recibido"
                        int pos4 = mensaje_recibido.indexOf(":");                     // This next 4 commands are not relevant
                        String auxiliar=mensaje_recibido.substring(pos4+2);
                        if (auxiliar.equals("FINALIZAR CONVERSACION"))
                            setVisible(false);
                        SwingUtilities.invokeLater(new Runnable() // Now I want to asign the value of mensaje_recibido (the message I just received) into texto_total, a Swing text area
                            public void run()
                                String auxi=texto_total.getText();
                                if (auxi.equals(""))
                                    texto_total.setText(mensaje_recibido); // if the text area was empty, then then i put into it the message
                                else
                                    texto_total.setText(auxi+"\n"+mensaje_recibido); // if it's not, then i put the message in the next line of the text area
                        Thread.sleep(1000);
                catch (Exception e3)
                    System.out.println(e3);
        }).start();
    }The communication starts and one user is able to send all the messages he wants to the other one, but when the other wants to send one to the first user, the first user receives a message with the same character over and over again that looks like an square ■. Again, this problem comes when a user that already sended a message, and wants to receive one for the first time (and next times). I searched in these forums all day and so far I couldn't fix it.
This problem happens in the receiver client, not in the sender client and not in the server. I already looked into that and those things are doing what they have to. I would be really really grateful if somebody could tell me which is the problem or how to fix this, because I'm becoming nuts.
Now I'm going to post the function "enviar" which sends the message, and the content of "enviarActionPerformed" which calls "enviar".
Again, excuse me if this is the wrong forum for this question. and sorry for my English too.

Answer 1

This is the rest of the code, though I don't think the problem is here, maybe by pushing the sending button it causes the receiving problem:
private void enviarActionPerformed(java.awt.event.ActionEvent evt) {                                      
        String mensaje=texto.getText();
        mensaje=nombre_local+" dice: "+mensaje;
        texto.setText("");
        String auxiliar=texto_total.getText();
        if (auxiliar.equals(""))
            texto_total.setText(mensaje);
        else
            texto_total.setText(auxiliar+"\n"+mensaje);
        enviar(mensaje);
    void enviar (String mens)
        String respuesta="";
        try
            MiSocket= new DatagramSocket();
            MaquinaServidor= InetAddress.getByName(direccion);
            PuertoServidor=7777;
            BufferedReader Teclado= new BufferedReader(new InputStreamReader(System.in));
            BufferEntrada = new byte[1024];
            PaqueteEntrada = new DatagramPacket (BufferEntrada,BufferEntrada.length);
            BufferSalida=new byte[1024];
            String comando="RETRANSMITIR MENSAJE";
            byte[] na={(byte)(ack%2)};
            comando= (new String (na))+comando;
            BufferSalida = comando.getBytes();
            PaqueteSalida = new DatagramPacket(BufferSalida,BufferSalida.length,MaquinaServidor,PuertoServidor);
            segsend(PaqueteSalida);
            ack++;
            String puerto= Integer.toString(puerto_contrario);
            int pos = mens.indexOf(":");
            String auxiliar=mens.substring(pos+2);
            comando=mens+"-"+direccion_contraria+"*"+puerto;
            byte[] nb={(byte)(ack%2)};
            comando= (new String (nb))+comando;
            BufferSalida = comando.getBytes();
            PaqueteSalida = new DatagramPacket(BufferSalida,BufferSalida.length,MaquinaServidor,PuertoServidor);
            segsend(PaqueteSalida);
            ack++;
            if (auxiliar.equals("FINALIZAR CONVERSACION"))
                setVisible(false);
        catch(Exception e)
            System.out.println(e);
    }I'm gonna keep working on it, if I manage to fix this I'll let you know. Thanks a lot!