Android->PC UDP communication

Dear All, I spent the day try to make work the connection between android and pc via UDP. The code that I put here is what I hope "closer-to-work". Still I can't send data from PC to the Android, while the reception of PC from Android works.

::::::::::::::::::::::::: PC side code::::::::::::::::::::::::::

import hypermedia.net.*;

int portRX=50000;
int portTX=51000;
String ip="224.0.0.2";
int rx_buffer_size=100;
String rx_string="Waiting for Data";
byte[] rx_byte=new byte[rx_buffer_size];
int rx_byte_count=0;
String message="";

UDP udpTX,udpRX;
void setup() {
  size(300,300);
  udpTX=new UDP(this,portTX,ip);
  udpTX.log(true);
  udpTX.setBuffer(5);
  udpTX.loopback(false);

  udpRX=new UDP(this,portRX,ip);
  udpRX.log(true);
  udpRX.listen(true);
}

void draw(){
  background(255);
  fill(0);
  text("Is TX mulitcast: "+udpTX.isMulticast(), 20,20);
  text("Has TX joined multicast: "+udpTX.isJoined(), 20,40);
  text("Is loopback enabled on TX: "+udpTX.isLoopback(), 20,60);

  //confirm if rx is multicast
  text("Is RX mulitcast: "+udpRX.isMulticast(), 20,80);
  text("Has RX joined multicast: "+udpRX.isJoined(), 20, 100);
  text(rx_string, 20, 120);
  text(message, 20, 140);
}

void keyReleased() {
  udpsender(key+"ciao buonasera");
}

//------------------------------------------------- RECEIVER
void receive( byte[] data ) {
 message= bytes2string(data);
}

//------------------------------------------------- SENDER
void udpsender(String whatIsend){
  byte[] data = new byte[4];
  data=string2bytes(whatIsend);
  udpTX.send( data );
}

//------------------------------------------------- STRING CONVERSIONS
byte[] string2bytes(String chaine) {
  byte[] bytes = new byte[0];
  for (int i = 0; i < chaine.length(); i++) {
    bytes = append(bytes, byte(chaine.charAt(i)));
  }
  return bytes;
}

String bytes2string(byte[] bytes){
  String chaine = str(char(bytes[0]));
  for (int i = 1; i < bytes.length; i++) {
    chaine = chaine+str(char(bytes[i]));
  }
  return chaine;
}

::::::::::::::::::::::::: Android side::::::::::::::::::::::::::

import hypermedia.net.*;

UDP udpTX,udpRX;
color uno=#9C5052;
color due=#00A881;

color backgro;
int stage=0;

String receivedFromUDP = "";
String ip="224.0.0.2";
int portRX=51000;
int portTX=50000;
int rx_buffer_size=100;
String rx_string="Waiting for Data";
byte[] rx_byte=new byte[rx_buffer_size];
int rx_byte_count=0;

void setup() {
  backgro=0;
  orientation(PORTRAIT);
  smooth();

  udpTX=new UDP(this,portTX,ip);
  udpTX.setBuffer(5);
  udpTX.loopback(false);

  udpRX=new UDP(this,portRX,ip);
  udpRX.listen(true);
  super.start();
}

void draw() {
  background(backgro);
  textAlign(CENTER, CENTER);

  if(stage==0) {
    fill(uno);
    rect(0, 0, width, height/2);

    fill(due);
    rect(0, height/2, width, height/2);

    fill(255);
    textSize(36);

    text("Player 1",width/2,height/4);
    text("Player 2",width/2,height/4*3);
  }

  textSize(12);
  if(udpRX.isListening()) text("OK", width/2,height/2);

  text("Is TX mulitcast: "+udpTX.isMulticast(), width/2,20);
  text("Has TX joined multicast: "+udpTX.isJoined(), width/2,40);
  text("Is loopback enabled on TX: "+udpTX.isLoopback(), width/2,60);

  //confirm if rx is multicast
  text("Is RX mulitcast: "+udpRX.isMulticast(), width/2,80);
  text("Has RX joined multicast: "+udpRX.isJoined(), width/2, 100);
  text(rx_string, width/2, 120);
  text("message:"+receivedFromUDP, width/2,height/3);
}

//------------------------------------------------- SENDER
void udpsender(String whatIsend){
  byte[] data = new byte[4];
  data=string2bytes(whatIsend);
  udpTX.send( data );
}

//------------------------------------------------- RECEIVER
void receive( byte[] data ) {
 receivedFromUDP= bytes2string(data);
}

//------------------------------------------------- BUTTONS
boolean rectOver = false;

void mousePressed() {
  if (overRect(0, 0, width, height/2)) {
    stage=1; backgro=uno;
  }
  else {stage=2; backgro=due;}
  udpsender("s"+stage);
}

boolean overRect(int x, int y, int widthRect, int heightRect) {
  if (mouseX >= x && mouseX <= x+widthRect &&
    mouseY >= y && mouseY <= y+heightRect) {
    return true;
  }
  else {return false;}
}

//------------------------------------------------- STRING CONVERSIONS
byte[] string2bytes(String chaine) {
  byte[] bytes = new byte[0];
  for (int i = 0; i < chaine.length(); i++) {
    bytes = append(bytes, byte(chaine.charAt(i)));
  }
  return bytes;
}

String bytes2string(byte[] bytes){
  String chaine = str(char(bytes[0]));
  for (int i = 1; i < bytes.length; i++) {
    chaine = chaine+str(char(bytes[i]));
  }
  return chaine;
}

::::::::::::::::::::::::: ::::::::::::::::::::::::: I saw others having similar issues, but still could not figure out what is going wrong with mine. Thank you