JButtonにJLayerを設定して展開・折り畳み可能な分割ボタンを作成する

class SplitButtonLayerUI extends LayerUI<ExpandableSplitButton> {
  @Override public void installUI(JComponent c) {
    super.installUI(c);
    ((JLayer<?>) c).setLayerEventMask(
        AWTEvent.MOUSE_EVENT_MASK | AWTEvent.MOUSE_MOTION_EVENT_MASK);
  }

  @Override public void uninstallUI(JComponent c) {
    ((JLayer<?>) c).setLayerEventMask(0);
    super.uninstallUI(c);
  }

  @Override protected void processMouseEvent(
      MouseEvent e, JLayer<? extends ExpandableSplitButton> layer) {
    ExpandableSplitButton button = layer.getView();
    switch (e.getID()) {
      case MouseEvent.MOUSE_ENTERED:
        button.getAnim().expand();
        button.setForeground(new Color(
            UIManager.getColor("List.selectionForeground").getRGB()));
        break;

      case MouseEvent.MOUSE_EXITED:
        button.setMouseOnArrow(false);
        if (!button.isPopupOpen()) {
          button.getAnim().collapse();
        }
        button.setForeground(UIManager.getColor("List.foreground"));
        break;

      case MouseEvent.MOUSE_PRESSED:
      case MouseEvent.MOUSE_CLICKED:
      case MouseEvent.MOUSE_RELEASED:
        if (SwingUtilities.isRightMouseButton(e)) {
          // Right click is consumed to suppress automatic display of JPopupMenu
          e.consume();
        } else {
          // Left click: Popup for arrow area, Action for title area
          if (button.isOnArrowArea(e.getPoint())) {
            button.showPopup();
            e.consume();
          }
        }
        // Clicking on the title area does not consume
        //   → JButton executes the Action
        break;

      default:
        break;
    }
  }

  @Override protected void processMouseMotionEvent(
      MouseEvent e, JLayer<? extends ExpandableSplitButton> layer) {
    if (e.getID() == MouseEvent.MOUSE_MOVED) {
      ExpandableSplitButton btn = layer.getView();
      boolean onArrow = btn.isOnArrowArea(e.getPoint());
      if (onArrow != btn.isMouseOnArrow()) {
        btn.setMouseOnArrow(onArrow);
        btn.repaint();
      }
    }
  }
}