Fire event from one class to another.

Question

I have a Login JFrame class that allows users to enter username and password. I then have another JFrame class which will monitor when someone logs in. I am trying to get the username and password to appear on the monitor login frame text area when the user presses enter on the login frame.
I can get it ot work by passing the Monitor class into the Login classes constructor but I want to be able to open the classes separately.When I try to open separatley at present I get java.lang.NullPointerException
     at project.LoginGUI.actionPerformed(LoginGUI.java:70) which is referring to the following code:      
if(listen.equals("OK")){
     GymMonitor.username.setText(username.getText());     
Both classes are in the same package. What I want to know is how to fire an event from one class to another? when the class you are firing to is constructed separately.
I hope this question is not too verbose.
Thanks

Answer 1

Generally for something like this you would use a listener.
Your login window is its own entity--it has a user interface, and it gets some information which someone else could ask it for. It could even generate its own events, such as when the user presses "OK". You would first define an interface for anyone who wants to know when someone logs in:
public interface ILoginListener extends java.util.EventListener
  public void login(LoginEvent event);
}You would then define the LoginEvent class to contain information like what the user entered for username and password. You could give your login dialog a couple of methods:
  private Vector myListeners = new Vector();
  public void addLoginListener(ILoginListener listener) {
    myListeners.add(listener);
  public void removeLoginListener(ILoginListener listener) {
    myListeners.remove(listener);
  protected void fireLogin(LoginEvent event) {
    for (Iterator it = myListeners.iterator(); it.hasNext(); ) {
      ILoginListener listener = (ILoginListener)it.next();
      listener.login(event);
  }You'd have your login dialog call fireLogin every time the user logged in.
Then, you could implement ILoginListener in your monitor window:
  public void login(LoginEvent event) {
    // now do something with the event you just got.
  }All the code I put in here is really generic stuff. You'll write this kind of stuff hundreds of times probably during your career. I haven't tested it though.
Hope this helps. :)