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public class Test extends JFrame {
    private JPanel panel;
    private JLabel label;
    private JButton bot;
    
    Test() {
        this.setLayout(new GridLayout(1,1));
        panel = new JPanel();
        label = new JLabel("label");
        bot = new JButton("button");
        bot.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                pressedButton(evt);
            }
        });
        panel.setLayout(new GridLayout(2,1));
        panel.setAlignmentY(panel.CENTER_ALIGNMENT);
        this.add(panel);
        panel.add(label);
        panel.add(bot);
        this.pack();
    }
    
    public void pressedButton(ActionEvent e) {
        Pnb pnbInst = new Pnb(); //instanciate the class that will provide the new JPanel
        this.remove(panel); //remove the old panel from thr JFrame
        panel = pnbInst.getPanel(); //set the panel to the new panel (from pnbInst)
        panel.revalidate(); //revalidate?
        this.add(panel); //add the new panel to the JFrame
        
    }
    
    public static void main(String[] args) {
        new Test().setVisible(true);
    }
}