尝试从另一个类修改JFrame时出现空指针异常？

Question

EDITED CODE:

public static void main(String[] args){

    JFrame frame = new JFrame();

    frame.setLayout(new FlowLayout(FlowLayout.CENTER, 0, 0));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    frame.add(new StartPanel());
    frame.add(new InstructionsPanel());
    frame.add(new GamePanel());

    frame.getContentPane().getComponent(1).setVisible(false);
    frame.getContentPane().getComponent(2).setVisible(false);

    frame.setPreferredSize(new Dimension(500, 500));
    frame.pack();
    frame.setVisible(true);

}

No matter what outside class I try to modify the frame from (any of the 3 panel classes above), I get a null pointer exception pointing to the line I am modifying something in the frame.

Answer 1

Your Panel classes are created before the JFrame is created so the JFrame will be null in the Panel class constructors. But as per my comment, you should bring your code into the instance world by getting rid of those static modifiers. Your whole program design, to put it mildly, smells. Your main method should instead look something like:

private static void createAndShowGui() {
  // Model model = new MyModel();
  View view = new View();
  // Control control = new MyControl(model, view);

  JFrame frame = new JFrame("My GUI");
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.getContentPane().add(view.getMainComponent());
  frame.pack();
  frame.setLocationByPlatform(true);
  frame.setVisible(true);
}

public static void main(String[] args) {
  SwingUtilities.invokeLater(new Runnable() {
     public void run() {
        createAndShowGui();
     }
  });
}

And View could look like:

public class View
  private StartPanel s = new StartPanel();
  private InstructionsPanel i = new InstructionsPanel();
  private GamePanel g = new GamePanel();
  private JPanel mainComponent = new JPanel();

  public View() {
    // create your GUI here
    // add components to mainComponent...
  }

  public JComponent getMainComponent() {
    return mainComponent;
  }
}