Why does lines disappear on maximizing/minimizing applet?

public class xy extends Applet implements MouseListener,ActionListener
{	Button b;
	int x1, y1,x2,y2;
	public void init()
	{	b=new Button("Draw Line");	
		add(b);		
		x1=y1=x2=y2=0;
		b.addActionListener(this);
	}
	public void actionPerformed(ActionEvent ae)
{
	if(ae.getSource()==b){
	addMouseListener(this);}
}
	public void mouseClicked(MouseEvent me)
	{
		showStatus( "(" + x1 + "," + y1+ ")" );	
	}
	public void mousePressed(MouseEvent me)
	{
	x1=me.getX();
	y1=me.getY();
	}
	public void mouseReleased(MouseEvent me)
	{
	x2=me.getX();
	y2=me.getY();
	Graphics g = getGraphics();
	me.consume();	
	repaint();
	
	}
	public void mouseExited(MouseEvent me)
	{}
	public void mouseEntered(MouseEvent me)
	{}
	public void paint(Graphics g)
	{
		update(g);
	}
	public void update(Graphics g)
	{
		g.drawLine( x1,y1,x2,y2);
	}

}