Question

I’m working on a simple Java swing project. This is the code of the main class (name changed):

public class MainProg
{
    private static MainProg program; 

    //mainWin is a JFrame
    private MainWindow mainWin;

    //Event handler class which extends MouseAdapter
    private TrayManager trayMgr;


    public static void main(String[] args)
    {                
        program = new MainProg();
    }

    public MainProg()
    {
        mainWin = new MainWindow();
        trayMgr = new TrayManager();

        mainWin.startBtn.addMouseListener(trayMgr);

        mainWin.setVisible(true);
    }
}

As is clear, when the program starts, in main() it creates a new instance of the MainProg class, which then calls the constructor. In the constructor, it creates a new instance of the JFrame mainWin. It then attaches an event handler to a button on mainWin.

In the event handler class trayMgr, the only method is mouseClicked() which does nothing
except a System.out.println('Clicked');

The issue is, when I run this program in Netbeans, the JFrame is shown right away, but I seem to have to click the button 2-3 times before the message is printed in the console.

Is this just something specific to Netbeans, or do I have to change something to make the event handler be set before the window is made visible?

Answer 1

Your threading issue is not likely one that is causing your current problem, but there’s the theoretic potential for problems, and I’ve seen some real problems associated with some of the more touchy look and feels. Quite simply you should queue your code that starts your GUI onto the Swing event thread. You do this by doing:

public void main(String[] args) {
  SwingUtilities.invokeLater(new Runnable(
    public void run() {
      program = new MainProg();
    }
  ));
}

Someone else recommended using invokeAndWait(...) instead of invokeLater(...) but this can be risky especially if you inadvertently make this call from within the Swing event thread itself. For your situation you’re better off using invokeLater(...).

But again, I think the main problem with the code you have shown was inappropriate use of MouseListener where an ActionListener should have been used. Learning to code any GUI library can be quite tricky, and for that reason, you can’t assume anything. Check out the tutorials and learn from the experts. Also if you are considering coding Swing for the long haul, consider ditching the NetBean’s code-generation utilities and learn first to code Swing by hand. You won’t regret doing this.