Java初学者，请各位帮帮忙

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

public class MouseDemo2 extends JFrame implements MouseListener, MouseMotionListener
{
int flag;
int x = 0;
int y = 0;
int startx, starty, endx, endy;

public MouseDemo2()
{
Container contentPane = getContentPane();
contentPane.addMouseListener(this);
contentPane.addMouseMotionListener(this);
setSize(300, 300);
setVisible(true);
addWindowListener(new WindowAdapter() {
public void widowClosing(WindowEvent e)
{
System.exit(0);
}});
}

public void mousePressed(MouseEvent e)
{
startx = e.getX();
starty = e.getY();
}

public void mouseReleased(MouseEvent e)
{
endx = e.getX();
endy = e.getY();
}

public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public void mouseClicked(MouseEvent e) {}

public void mouseMoved(MouseEvent e)
{
flag = 1;
x = e.getX();
y = e.getY();
repaint();
}

public void mouseDragged(MouseEvent e)
{
flag = 2;
x = e.getX();
y = e.getY();
repaint();
}

public void update(Graphics g)
{
g.setColor(this.getBackground());
g.fillRect(0, 0, getWidth(), getHeight());
paint(g);
}

public void paint(Graphics g)
{
g.setColor(Color.black);
if(flag == 1)
{
g.drawString("鼠标坐标：("+x+", "+y+")", 10, 50);
g.drawLine(startx, starty, endx, endy);
}
if(flag == 2)
{
g.drawString("拖拽鼠标坐标: ("+x+", "+y+")", 10, 50);
g.drawLine(startx, starty, x, y);
}
}

public static void main(String[] args)
{
new MouseDemo2();
}
}
运行老是出问题，直线画不出来，请高手指教啊！