repaint（）不更新JPanel

Question

So I made a program where there is a GUI where the user can enter the parameters for a fireworks launch, and there is a panel in the middle of the screen where the launch is supposed to be drawn when a button is pressed. In my program however the JPanel is not updating and nothing is being drawn. The code is below

//This is the method that build the JPanel in the center for the launch
public JPanel buildCenter() {
    JPanel center=new JPanel();
    center.setBackground(Color.black);
    center.setVisible(true);
    return center;
}


//This is the method the build the GUI, the buttons and such are in the other panels labeled top, west, east, etc.
public void buildGUI(){
    configureSliders();
    configureRadioButtons();
    JFrame frame=new JFrame();
    JPanel panel=new JPanel();
    JPanel top=new JPanel();
    panel.setLayout(new BorderLayout());
    Fireworks.setFont(new Font("Helvetica", Font.BOLD, 30));
    frame.setLayout(new BorderLayout());
    top.setLayout(new BoxLayout(top, BoxLayout.X_AXIS));
    top.add(Box.createHorizontalGlue());
    top.add(Fireworks);
    top.add(Box.createHorizontalGlue());
    frame.add(top, BorderLayout.NORTH);
    frame.add(panel, BorderLayout.CENTER);
    frame.setSize(1920,1080);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    panel.add(buildCenterTop(),BorderLayout.NORTH);
    panel.add(buildCenter(), BorderLayout.CENTER);
    panel.add(buildwest(), BorderLayout.WEST);
    panel.add(buildeast(),BorderLayout.EAST);
    panel.add(Launch, BorderLayout.SOUTH);
    Launch.addActionListener(this);
}
//this is the action performed method where repaint won't work. fire is my fireworks object with the paintcomponent method for the launch.
@Override
public void actionPerformed(ActionEvent e) {
    if (e.getSource().equals(Launch)) {
        setColor();
        setTime();
        setExplosion();
        fire.setVelocity(veloslider.getValue());
        fire.setTheta(thetaslider.getValue());
        buildCenter().add(fire);
        buildCenter().repaint();
        buildCenter().validate();

    }

Answer 1

All your buildCenter() method does is create an empty panel with a black background.

You then add this empty panel to the frame:

panel.add(buildCenter(), BorderLayout.CENTER);

Then in your ActionListener you do:

buildCenter().add(fire);
buildCenter().repaint();
buildCenter().validate();

All this does is create 3 more empty panels. You don't want to create 3 more panels. You want to add components to the existing panel.

What you need to do is create a single instance of your "center" panel and then keep a variable that references this panel so you can update the panel in the future.

So you need an instance variable to be defined in your class:

private JPanel centerPanel;

Then in the buildGui() method you create the panel:

//panel.add(buildCenter(), BorderLayout.CENTER);
centerPanel = buildCenter();
panel.add(buildCenter, BorderLayout.CENTER);

Then in your ActionListener you can add components to the center panel:

//buildCenter().add(fire);
//buildCenter().repaint();
//buildCenter().validate();
centerPanel.add( fire );
centerPanel.revalidate();
centerPanel.repaint();