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");
    }
}
}