Painted JPanel won't show up in JFrame

Question

When I run my code, I expect to see a JPanel in my JFrame, but nothing shows up. I had a button in the frame, and it shows up. But the JPanel doesn't show up, I even colored it in red. Here is the code for my JPanel:

import java.awt.*;
import javax.swing.JPanel;
public class graphic extends JPanel {
    private static final long serialVersionUID = -3458717449092499931L;
    public Game game;
    public graphic(Game game){
    this.game = game;
    this.setPreferredSize(new Dimension(400,400));
    this.setBackground(Color.RED);
}
public void paintComponent(Graphics g){
    for (Line l:game.mirrors){
        g.setColor(Color.BLACK);
        g.drawLine(l.start.x, l.start.y, l.end.x, l.end.y);
    }
}
}

And my JFrame code:

import java.awt.Container;
import java.awt.event.*;
import java.util.Timer;
import java.util.TimerTask;

import javax.swing.*;
public class Viewer implements ActionListener {
public JFrame frame;
public JButton drawShoot;
public boolean draw;
public Game game;
public graphic graphic;
public TimerTask timert;
public Timer timer;
public Viewer(){
    draw = true;
    game = new Game();
}
public static void main(String args[]){
    Viewer v = new Viewer();
    v.setup();
}
public void setup(){
    frame = new JFrame("Laser Stimulator");
    drawShoot = new JButton("Edit Mode");
    graphic = new graphic(game);
    graphic.repaint();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(300, 300, 600, 600);
    Container contentPane = frame.getContentPane();
    SpringLayout layout = new SpringLayout();
    contentPane.setLayout(layout);
    drawShoot.addActionListener(this);
    timert = new TimerTask() {
        @Override
        public void run() {

        }
    };
    timer =new Timer();
    timer.scheduleAtFixedRate(timert, 0, 1000/30);
    contentPane.add(graphic);
    layout.putConstraint(SpringLayout.NORTH, graphic, 0, SpringLayout.NORTH, contentPane);
    layout.putConstraint(SpringLayout.WEST, graphic, 0, SpringLayout.WEST, contentPane);
    frame.setVisible(true);
}
@Override
public void actionPerformed(ActionEvent e) {
    if (e.getSource()==drawShoot){
        draw = !draw;
        drawShoot.setText((draw)?"Edit Mode":"Shoot Mode");
    }
}
}

Answer 1

So the basic problem is you failed to honor the paint chain. You must call super.paintXxx when ever you override on the paint methods...

This method must call super.paintComponent(g), because it's responsible for clearing the background ;)

public void paintComponent(Graphics g) {
    for (Line l : game.mirrors) {
        g.setColor(Color.BLACK);
        g.drawLine(l.start.x, l.start.y, l.end.x, l.end.y);
    }
}

public class QuickTestPaintPane {

    public static void main(String[] args) {
        new QuickTestPaintPane();
    }

    public QuickTestPaintPane() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException ex) {
                } catch (InstantiationException ex) {
                } catch (IllegalAccessException ex) {
                } catch (UnsupportedLookAndFeelException ex) {
                }

                Viewer v = new Viewer();
                v.setup();

            }
        });
    }

    public class Viewer implements ActionListener {

        public JFrame frame;
        public JButton drawShoot;
        public boolean draw;
//        public Game game;
        public graphic graphic;
        public TimerTask timert;
        public Timer timer;

        public Viewer() {
            draw = true;
//            game = new Game();
        }

        public void setup() {
            frame = new JFrame("Laser Stimulator");
            drawShoot = new JButton("Edit Mode");
            graphic = new graphic();
//            graphic.repaint();
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setBounds(300, 300, 600, 600);
//            Container contentPane = frame.getContentPane();
            SpringLayout layout = new SpringLayout();
            frame.setLayout(layout);
            drawShoot.addActionListener(this);
            timert = new TimerTask() {
                @Override
                public void run() {
                }
            };
            timer = new Timer();
            timer.scheduleAtFixedRate(timert, 0, 1000 / 30);
            frame.add(graphic);
            layout.putConstraint(SpringLayout.NORTH, graphic, 0, SpringLayout.NORTH, frame.getContentPane());
            layout.putConstraint(SpringLayout.WEST, graphic, 0, SpringLayout.WEST, frame.getContentPane());
            frame.setVisible(true);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            if (e.getSource() == drawShoot) {
                draw = !draw;
                drawShoot.setText((draw) ? "Edit Mode" : "Shoot Mode");
            }
        }
    }

    public class graphic extends JPanel {

        private static final long serialVersionUID = -3458717449092499931L;
//        public Game game;

//        public graphic(Game game) {
        public graphic() {
//            this.game = game;
            this.setPreferredSize(new Dimension(400, 400));
            this.setBackground(Color.RED);
        }

        public void paintComponent(Graphics g) {
            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D) g;
            g2d.setColor(Color.BLACK);
            g2d.drawLine(0, 0, getWidth(), getHeight());
//            for (Line l : game.mirrors) {
//                g.setColor(Color.BLACK);
//                g.drawLine(l.start.x, l.start.y, l.end.x, l.end.y);
//            }
        }
    }
}