Painter can be used to draw on components backgrounds. The use of this interface allows reuse of a background Painter for various components.
A simple example of a background painter is shown here to draw a circle background:

 
Painter p = new Painter(cmp) {
    public void paint(Graphics g, Rectangle rect) {
        boolean antiAliased = g.isAntiAliased();
        g.setAntiAliased(true);
        int r = Math.min(rect.getWidth(), rect.getHeight())/2;
        int x = rect.getX() + rect.getWidth()/2 - r;
        int y = rect.getY() + rect.getHeight()/2 - r;
        switch (style) {
            case CircleButtonStrokedDark:
            case CircleButtonStrokedLight: {
                if (cmp.getStyle().getBgTransparency() != 0) {
                    int alpha = cmp.getStyle().getBgTransparency();
                    if (alpha <0) {
                        alpha = 0xff;
                    }
                    g.setColor(cmp.getStyle().getBgColor());
                    g.setAlpha(alpha);
                    g.fillArc(x, y, 2*r-1, 2*r-1, 0, 360);
                    g.setAlpha(0xff);
                }
                g.setColor(cmp.getStyle().getFgColor());
                g.drawArc(x, y, 2*r-1, 2*r-1, 0, 360);
                break;
            }
            case CircleButtonFilledDark:
            case CircleButtonFilledLight:
            case CircleButtonTransparentDark:
            case CircleButtonTransparentLight: {
                int alpha = cmp.getStyle().getBgTransparency();
                if (alpha < 0) {
                    alpha = 0xff;
                }
                g.setAlpha(alpha);
                g.setColor(cmp.getStyle().getBgColor());
                g.fillArc(x, y, 2*r, 2*r, 0, 360);
                g.setAlpha(0xff);
                break;
            }
        }

        g.setAntiAliased(antiAliased);
    }
};
cmp.getAllStyles().setBgPainter(p);

Painters can also be used to draw the glasspane which is layered on top of the form. The example shows a glasspane running on top of a field to show a validation hint, notice that for real world usage you should probably look into Validator. Also notice that this example uses the shorter Java 8 lambda syntax to represent the glasspane.