Java - JPanel do not paint even though method is called

Question

I try to create a game. Game logic and painting work fine if I start the game directly. But there is empty JPanel after I start a game from a menu. Why?

Start game directly:

There is a "infinitive" loop called gameLoop() which stops after a game ends.

public class Window extends JFrame {
 public Window() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLocationRelativeTo(null);
    setResizable(false);
    setTitle("Shop-Lifter");
    setFocusable(true);
    setVisible(true);
 }

 public static void main(String[] args) {
    Window window = new Window();
    GameCore gameCore = new GameCore(level);

    window.setSize(gameCore.size);
    window.add(gameCore);
    gameCore.requestFocus();
    gameCore.gameLoop();
    window.dispose();
 }
}

There is a method "paint" in gameCore

@Override
public void paint(Graphics g) {
    super.paint(g);
    graphicsHendler.paint(g);
}

But I try to create a "menu" where you can select a level. So I create a Window object, add JPanel with a 2 buttons. After click on a button a game is supposed to start. But there is only grey JPanel in JFrame. GameLoop() is running but not painting. If I do not start a game by gameCore.gameLoop(); there is a paint() called once after object is created.

Create a game menu:

public class Window extends JFrame {

 public Window() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLocationRelativeTo(null);
    setResizable(false);
    setTitle("Shop-Lifter");
    setFocusable(true);
    setVisible(true);
 }

 public static void main(String[] args) {
    Window window = new Window();
    WelcomeScreen levelChooser = new WelcomeScreen();
    window.setSize(levelChooser.getPreferredSize());
    window.add(levelChooser);
 }
}

And button's method starting a game:

private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
    Window w = new Window();
    GameCore gc = new GameCore(LevelLoader.loadLevel("ll1.lvl"));

    w.setPreferredSize(gc.size);
    w.add(gc);
    w.requestFocus();
    w.pack();
    gc.gameLoop();   
}

Answer 1

Most probably gc.gameLoop() method is a blocking call, which blocks the Event Dispatcher Thread (EDT). If this is the case, you should do it in a separate thread. Another way to do it is to use active rendering.

See the following links:

• Passive vs Active Rendering
• stackoverflow : Handling GUI with SWING with multithreading when the GUI does not update informations