Swing/RoundGradientBorder のバックアップ(No.1)

class RoundGradientBorder extends EmptyBorder {
  private final float[] fractions = {0f, .25f, .5f, .75f, 1f};
  private final Color[] colors = {
      new Color(0xD3_03_02),
      new Color(0xFF_51_56),
      new Color(0xFF_DB_4E),
      new Color(0x00_FE_9B),
      new Color(0x2D_D9_FE)
  };

  protected RoundGradientBorder(int top, int left, int bottom, int right) {
    super(top, left, bottom, right);
  }

  @Override public void paintBorder(
        Component c, Graphics g, int x, int y, int width, int height) {
    Graphics2D g2 = (Graphics2D) g.create();
    g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                        RenderingHints.VALUE_ANTIALIAS_ON);
    Point2D start = new Point2D.Float(0f, 0f);
    Point2D end = new Point2D.Float((float) width, 0f);
    g2.setPaint(new LinearGradientPaint(start, end, fractions, colors));
    float stroke = 2f;
    float arc = 12f;
    Shape outer = new RoundRectangle2D.Float(
        x, y, width - 1f, height - 1f, arc, arc);
    Shape inner = new RoundRectangle2D.Float(
        x + stroke,
        y + stroke,
        width - stroke - stroke - 1f,
        height - stroke - stroke - 1f,
        arc - stroke - stroke,
        arc - stroke - stroke
    );
    Area rr = new Area(outer);
    rr.subtract(new Area(inner));
    g2.fill(rr);
    g2.dispose();
  }
}