JScrollPane上にマウスカーソルが存在する場合のみJScrollBarを表示する

class ScrollBarOnHoverLayerUI extends LayerUI<JScrollPane> {
  @Override public void installUI(JComponent c) {
    super.installUI(c);
    if (c instanceof JLayer) {
      ((JLayer) c).setLayerEventMask(AWTEvent.MOUSE_EVENT_MASK);
    }
  }

  @Override public void uninstallUI(JComponent c) {
    if (c instanceof JLayer) {
      ((JLayer) c).setLayerEventMask(0);
    }
    super.uninstallUI(c);
  }

  @Override protected void processMouseEvent(
      MouseEvent e, JLayer<? extends JScrollPane> l) {
    JScrollPane sp = l.getView();
    switch (e.getID()) {
      case MouseEvent.MOUSE_ENTERED:
        sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        break;
      case MouseEvent.MOUSE_EXITED:
        sp.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
        break;
      default:
        break;
    }
    // super.processMouseEvent(e, l);
  }
}