JComboBoxのアイテムとして表示したJCheckBoxを複数選択する

class CheckedComboBox<E extends CheckableItem> extends JComboBox<E> {
  private boolean keepOpen;
  private transient ActionListener listener;

  protected CheckedComboBox() {
    super();
  }

  protected CheckedComboBox(ComboBoxModel<E> aModel) {
    super(aModel);
  }

  protected CheckedComboBox(E[] m) {
    super(m);
  }

  @Override public Dimension getPreferredSize() {
    return new Dimension(200, 20);
  }

  @Override public void updateUI() {
    setRenderer(null);
    removeActionListener(listener);
    super.updateUI();
    listener = e -> {
      if ((e.getModifiers() & AWTEvent.MOUSE_EVENT_MASK) != 0) {
        updateItem(getSelectedIndex());
        keepOpen = true;
      }
    };
    setRenderer(new CheckBoxCellRenderer<CheckableItem>());
    addActionListener(listener);
    getInputMap(JComponent.WHEN_FOCUSED).put(
        KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0), "checkbox-select");
    getActionMap().put("checkbox-select", new AbstractAction() {
      @Override public void actionPerformed(ActionEvent e) {
        Accessible a = getAccessibleContext().getAccessibleChild(0);
        if (a instanceof BasicComboPopup) {
          BasicComboPopup pop = (BasicComboPopup) a;
          updateItem(pop.getList().getSelectedIndex());
        }
      }
    });
  }

  private void updateItem(int index) {
    if (isPopupVisible()) {
      E item = getItemAt(index);
      item.selected ^= true;
      setSelectedIndex(-1);
      setSelectedItem(item);
    }
  }

  @Override public void setPopupVisible(boolean v) {
    if (keepOpen) {
      keepOpen = false;
    } else {
      super.setPopupVisible(v);
    }
  }

  public static <E extends CheckableItem> String getCheckedItemString(
        ListModel<E> model) {
    return IntStream.range(0, model.getSize())
        .mapToObj(model::getElementAt)
        .filter(CheckableItem::isSelected)
        .map(Objects::toString)
        .sorted()
        .collect(Collectors.joining(", "));
  }
}