How to update JFrame.

Hi all,

I am creating a program in which I have a JFrame that switches between different panels that are on it. I am using the remove() method of the JFrame to remove the current panel from the frame and then add the new one on. My question is, how can I get the Frame to update without having to hide the window and bring it back into focus? Or is there a different way I should be doing this? Here is some sample code that I have.

Java Code:

public static void main(String[] args) {
                Main main = new Main();
                
                JFrame frame = new JFrame("Gradebook Alpha 1.0");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setResizable(false);
                frame.setLayout(new BorderLayout());
                frame.add(main, BorderLayout.CENTER);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
        
                try {
                        Thread.sleep(1500);
                } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                }
                frame.remove(main);
                System.out.println("removed");
                
                frame.add(new JButton("A"));
        }

Thank you for your time.