JComboBoxのアイテムをBorderで修飾してグループ分け

JComboBox combobox = new JComboBox();
JSeparator sep = new JSeparator();
ListCellRenderer lcr = combobox.getRenderer();
combobox.setRenderer(new ListCellRenderer() {
  @Override public Component getListCellRendererComponent(
        JList list, Object value, int index,
        boolean isSelected, boolean cellHasFocus) {
    MyItem item = (MyItem) value;
    JLabel label = (JLabel) lcr.getListCellRendererComponent(
        list, item, index, isSelected, cellHasFocus);
    if (item.hasSeparator()) {
      label.setBorder(
          BorderFactory.createMatteBorder(1, 0, 0, 0, Color.GRAY));
    } else {
      label.setBorder(BorderFactory.createEmptyBorder());
    }
    return label;
  }
});
DefaultComboBoxModel model = new DefaultComboBoxModel();
model.addElement(new MyItem("aaaa"));
model.addElement(new MyItem("eeeeeeeee", true));
model.addElement(new MyItem("bbb12"));
combobox.setModel(model);
combobox.setEditable(true);
// ...

class MyItem {
  private final String  item;
  private final boolean flag;
  public MyItem(String str) {
    this(str, false);
  }

  public MyItem(String str, boolean flg) {
    item = str;
    flag = flg;
  }

  @Override public String toString() {
    return item;
  }

  public boolean hasSeparator() {
    return flag;
  }
}
// ...