super.addNotify()

I'm reading Killer Game Programming in Java, however I'm confused about something, and that is how addNotify works exactly.

In an effort to understand, I did this:

Code:

---

public class GamePanel extends JPanel implements Runnable
{

        // some variables .......
        public GamePanel()
        {
                System.out.println("Constructor started");
                setBackground(Color.white);
                setPreferredSize(new Dimension(PWIDTH, PHEIGHT));

                // create game components
                // ..
                System.out.println("Constructor ended");
        } // END OF GAMEPANEL

        @Override
        public void addNotify()
        {
                System.out.println("addNotify started");
                super.addNotify();
                startGame();
                System.out.println("addNotify ended");
        }
// some more stuff, not relevant
}

---

The JFrame:

Code:

---

public class Main extends JFrame
{
       
        public Main()
        {
                System.out.println("Constructor main started");
                setTitle("game");
                setSize(500, 400);
                setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                add(new GamePanel());
                try
                {
                        Thread.sleep(3000);
                        System.out.println("setVisible getting called");
                } catch (InterruptedException e)
                {
                        e.printStackTrace();
                }
                setVisible(true);
                try
                {
                        System.out.println("setVisible ended");
                        Thread.sleep(3000);
                } catch (InterruptedException e)
                {
                        e.printStackTrace();
                }
                System.out.println("Constructor main ended");       
        }
       
        public static void main(String[] args) {
                new Main();
        }

}

---

The result was this:

Constructor main started
Constructor started
Constructor ended
(*Imagine a 3 second delay here*)
setVisible getting called
addNotify started
addNotify ended
setVisible ended
(*Imagine another 3 second delay*)
Constructor main ended


So it is clear addNotify gets called only when setVisible() of the JFrame is called, and that is desired I suppose. But why? GamePanel isn't an subclass of Main, is it? Maybe I'm just tired, but I dont get how this works exactly

GamePanel is a subclass of JPanel. The JPanel is added to the JFrame, so the JPanel becomes visible when the JFrame becomes visible.