boolean finished = false;
    
    public void paint(Graphics g)
    {
        if (finished)
            g.drawImage(screen, 0, 0, this);
    }

    // this overide avoid the unnecessary clear on each paint()
    public void update(Graphics g)
    {
        paint(g);
    }

    public boolean mouseDown(Event e, int x, int y)
    {
        renderPixel(x, y);
        repaint();
        return true;
    }

    public boolean mouseDrag(Event e, int x, int y)
    {
        renderPixel(x, y);
        repaint();
        return true;
    }

    public boolean mouseUp(Event e, int x, int y)
    {
        renderPixel(x, y);
        repaint();
        return true;
    }