Question

Simple code from java.sun:

public class BasicApp implements Runnable {

    JFrame mainFrame;
    JLabel label;

    public void run() {
        mainFrame = new JFrame("BasicApp");
        label = new JLabel("Hello, world!");
        label.setFont(new Font("SansSerif", Font.PLAIN, 22));
        mainFrame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        mainFrame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                mainFrame.setVisible(false);
                // Perform any other operations you might need
                // before exit.
                System.exit(0);
            }
        });
        mainFrame.add(label);
        mainFrame.pack();
        mainFrame.setVisible(true);
    }

    public static void main(String[] args) {
        Runnable app = new BasicApp();
        try {
            SwingUtilities.invokeAndWait(app);
        } catch (InvocationTargetException ex) {
            ex.printStackTrace();
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
    }
}

I can put all of this method into main(), but why do I need a separate run method that also implements the runnable to execute it? What is the idea behind this concept? Thanks.

Answer 1

From Oracle SDN: Threads and Swing

Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.

The gist of it is that the code needs to be run when Swing is good and ready to run it. Not necessarily right when you call it.