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;
    }