Painting in JPanel

Question

I have a JFrame in which there is a JToolBar and a JPanel. I have to draw lines interactively in the panel by capturing mouse clicks with MouseListener. The mouse clicks are captured and the paintComponent method is even executed (I tested these using dialog boxes) but nothing is drawn in the panel. I think there is a problem with repaint() (called in mousePressed()) or the method paintComponent() but I don't know what it is. What is wrong with the code?

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Line2D;
import javax.swing.*;

public class Clipping extends JPanel implements MouseListener, ActionListener
{
    static JFrame frame;
    static JComboBox cboDraw;
    static JButton btnClear;
    static JButton btnClip;

    double x1, y1, x2, y2;
    boolean FirstPoint;

    public static void main(String[] args) 
    {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                CreateFrame();
            }
        });
    }

    private static void CreateFrame()
    {
        frame = new JFrame("Clipping");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(new Clipping());
        frame.setSize(500,500);
        frame.setVisible(true);
    }

    public Clipping()
    {
        setLayout(new BorderLayout());

        JToolBar toolbar = new JToolBar(JToolBar.VERTICAL);
        PopulateToolBar(toolbar);
        add(toolbar, BorderLayout.WEST);

        addMouseListener(this);
        cboDraw.addMouseListener(this);
        btnClip.addActionListener(this);
        btnClear.addActionListener(this);

    }

    private static void PopulateToolBar(JToolBar toolbar)
    {
        String[] cboList = {"Line", "Polygon"};
        cboDraw = new JComboBox(cboList);
        cboDraw.setMaximumSize(new Dimension(70,30));
        btnClip = new JButton("Set clip area");
        btnClear = new JButton("Clear");

        toolbar.add(cboDraw);
        toolbar.addSeparator();
        toolbar.add(btnClip);
        toolbar.addSeparator();
        toolbar.add(btnClear);

        cboDraw.setAlignmentX(Component.CENTER_ALIGNMENT);
        btnClip.setAlignmentX(Component.CENTER_ALIGNMENT);
        btnClear.setAlignmentX(Component.CENTER_ALIGNMENT);

        toolbar.setMargin(new Insets(10,10,10,10));
        toolbar.setFloatable(false);
        toolbar.setBackground(Color.black);
    }

    public void paintComponent(Graphics g)
    {
        Graphics2D g2 = (Graphics2D) g;
        if (cboDraw.getSelectedIndex() == 0) //draw line
        {
            g2.draw(new Line2D.Double(x1, y1, x2, y2));
        }
    }

    public void mousePressed(MouseEvent e)
    {
        if (e.getSource() != cboDraw) //to prevent line coordinates from being saved when selecting from combobox
        {
            if (cboDraw.getSelectedIndex() == 0) //user wants to draw line
            {
                if (FirstPoint == false) //first coordinates
                {
                    x1 = e.getX();
                    y1 = e.getY();
                    FirstPoint = true;
                }
                else //second coordinates
                {
                    x2 = e.getX();
                    y2 = e.getY();
                    repaint();
                    FirstPoint = false;
                }
            }
        }
    }

    public void actionPerformed(ActionEvent e) {}
    public void mouseClicked(MouseEvent e) {}
    public void mouseReleased(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}
}