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

class RoundedSelectionTree extends JTree {
  private static final Color SELECTED_COLOR = new Color(0xC8_00_78_D7, true);

  @Override protected void paintComponent(Graphics g) {
    int[] selectionRows = getSelectionRows();
    if (selectionRows != null) {
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                          RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(SELECTED_COLOR);
      Rectangle innerArea = SwingUtilities.calculateInnerArea(this, null);
      Area area = new Area();
      Arrays.stream(selectionRows)
          .mapToObj(this::getRowBounds)
          .map(r -> new Rectangle(innerArea.x, r.y, innerArea.width, r.height))
          .forEach(r -> area.add(new Area(r)));
      int arc = 10;
      for (Area a : singularization(area)) {
        Rectangle r = a.getBounds();
        g2.fillRoundRect(r.x, r.y, r.width - 1, r.height - 1, arc, arc);
      }
      g2.dispose();
    }
    super.paintComponent(g);
  }

  private static List<Area> singularization(Area rect) {
    List<Area> list = new ArrayList<>();
    Path2D path = new Path2D.Double();
    PathIterator pi = rect.getPathIterator(null);
    double[] coords = new double[6];
    while (!pi.isDone()) {
      int pathSegmentType = pi.currentSegment(coords);
      switch (pathSegmentType) {
        case PathIterator.SEG_MOVETO:
          path.moveTo(coords[0], coords[1]);
          break;
        case PathIterator.SEG_LINETO:
          path.lineTo(coords[0], coords[1]);
          break;
        case PathIterator.SEG_CLOSE:
          path.closePath();
          list.add(new Area(path));
          path.reset();
          break;
        default:
          break;
      }
      pi.next();
    }
    return list;
  }

  @Override public void updateUI() {
    super.updateUI();
    UIManager.put("Tree.repaintWholeRow", Boolean.TRUE);
    setCellRenderer(new TransparentTreeCellRenderer());
    setOpaque(false);
    setRowHeight(20);
    UIDefaults d = new UIDefaults();
    String key = "Tree:TreeCell[Enabled+Selected].backgroundPainter";
    d.put(key, new TransparentTreeCellPainter());
    putClientProperty("Nimbus.Overrides", d);
    putClientProperty("Nimbus.Overrides.InheritDefaults", false);
    addTreeSelectionListener(e -> repaint());
  }
}