单击JButton后，JPanel不会出现

Question

My JPanel doesn't show up when my JButton is clicked. It does shows up when I add the JPanel in my go() method. However, once I tried to execute it by click the JButton, it doesn't work. The program does goes into the loop of the actionPeformed() method of the listener though.

public class MyShape 
{
    JFrame frame;
    JPanel panel;
    JButton drawButton;

    public static void main (String[] args)
    {
        MyShape test = new MyRandomShape();
        test.go();
    }

    public void go()
    {
        drawButton = new JButton("Draw Shape!");
        drawButton.addActionListener(new DrawListener());

        frame = new JFrame();
        frame.add(drawButton, BorderLayout.NORTH);
        frame.setSize(500,500);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }    

    private class DrawListener implements ActionListener
    {
        public void actionPerformed(ActionEvent event)
        {
            if(empty)
            {
                System.out.print("IN");
                panel = new DrawPanel();
                frame.add(panel, BorderLayout.CENTER);
            }
        }
    }     

    private class DrawPanel extends JPanel
    {
        public void paintComponent(Graphics g)
        {
            super.paintComponents(g);

            int randNo = (int)(Math.random() * 3);

            int width = (int)(Math.random() * getWidth());
            int height = (int)(Math.random() * getHeight());
            int xpos = getWidth()/2-width/2;
            int ypos = getHeight()/2-height/2;

            int v1 = (int)(Math.random() * 256);
            int v2 = (int)(Math.random() * 256);
            int v3 = (int)(Math.random() * 256);

            g.setColor(new Color(v1, v2, v3));

            if(randNo == 0)
            {   
                g.fillOval(xpos, ypos, width, height);
            }
            else if(randNo == 1)
            {
                g.fillRect(xpos, ypos, width, height);
            }
            else
            {
                int startAngle = (int)(Math.random() * 360);
                int arcAngle = (int)(Math.random() * 360);
                g.fillArc(xpos, ypos, width, height, startAngle, arcAngle);
            }
        }
    }    
}

How do I get the JPanel to show up once the button is clicked?

Answer 1

You have to call parentComponent.revalidate() every time you do one or more parentComponent.add(childComponent) (or change its children in other ways, such as reorder or remove them).

In your case, your code should be

private class DrawListener implements ActionListener {
    public void actionPerformed(ActionEvent event)
    {
        if(empty)
        {
            System.out.print("IN");
            panel = new DrawPanel();
            frame.add(panel, BorderLayout.CENTER);
            frame.revalidate(); // <---------- important
        }
    }
}

Answer 2

Few changes, check this

 public static void main(String[] args) {
    MyShape test = new MyShape();
    test.go();
}

public void go() {
    drawButton = new JButton("Draw Shape!");
    drawButton.addActionListener(new DrawListener());

    frame = new JFrame();
    frame.getContentPane().add(drawButton, BorderLayout.NORTH);
    panel = new DrawPanel();
    frame.getContentPane().add(panel, BorderLayout.CENTER);
    frame.setSize(500, 500);
    frame.setVisible(true);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

private class DrawListener implements ActionListener {

    public void actionPerformed(ActionEvent event) {

            frame.getContentPane().remove(1);
            panel = new DrawPanel();
            frame.getContentPane().add(panel, BorderLayout.CENTER);
            frame.repaint();
            frame.validate();

    }
}