How to append msg from second thread to main GUI thread?

Question

I am facing a problem which I cannot seem to solve.
It involves Swing and threads. I have a main GUI running on swing. And it retrieves database information and displays it. On the same GUI I have a JTextArea box which I need another thread to update as and when that second thread recieves message via a multicast socket.
I managed to create a runnable class of the second thread and successfully ran the second thread with the main thread in tandem. However when I use
display.append(msgreceived);
I get a nullPointerException. It works fine when i use System.out.println(msgreceived);.
What should I have done instead?

Answer 1

Here are part of the codes for my admin module.
public class Admin extends JFrame implements ActionListener {
//declares all the global variables
private JTextArea display;
public Admin() {
super("BX Online - Admin Module");
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
AdminGUI();
AdminGUI() {
contentPane = getContentPane();
contentPane.setLayout(new BorderLayout());
JPanel newsbox = new JPanel();
display = new JTextArea(2, 40);
display.setEditable(false);
newsbox.add(display);
contentPane.add(newsbox);
contentPane.add(tabpaneO);
setBounds(70, 50, 300, 500);
pack();
setVisible(true);
// all the methods activated by buttons goes here
static public void main(String[] argv) {
Admin a = new Admin();
News b = new News("test");
The following are part of the codes for my runnable class.
public class News implements Runnable {
protected boolean again = true;
private String name, recieve; //the global variables
private Thread t;
public JTextArea display;
public BXnews(String threadname) {
name = threadname;
t = new Thread(this, name);
System.out.println("New Thread: " + t); //visual check to make sure thread is started.
t.start();
private void msg() {
String recieve = ("testing");
System.out.println(recieve);
display.append(recieve);
public void run() {
while (again) //creates a loop so thread does not close
{msg();}
Well thats abt it. I'm not sure if you can compile this but basically the part I left out was the setup link for the News class which I don't see the need to add as what I primarily intend is for the message testing in News class to appear in Admin class thread in JTextArea "display".