Swing/ColorComboBox のバックアップ(No.34)

class AlternateRowColorComboBox<E> extends JComboBox<E> {
  private static final Color EVEN_BGC = new Color(0xE1_FF_E1);
  private static final Color ODD_BGC = Color.WHITE;
  private transient ItemListener itemColorListener;

  protected AlternateRowColorComboBox(ComboBoxModel<E> model) {
    super(model);
  }

  @Override public void setEditable(boolean flag) {
    super.setEditable(flag);
    Component field = getEditor().getEditorComponent();
    if (flag && field instanceof JComponent) {
      ((JComponent) field).setOpaque(true);
      field.setBackground(getAlternateRowColor(getSelectedIndex()));
    }
  }

  @Override public void updateUI() {
    removeItemListener(itemColorListener);
    super.updateUI();
    ListCellRenderer<? super E> r = getRenderer();
    setRenderer((list, value, index, isSelected, cellHasFocus) -> {
      Component c = r.getListCellRendererComponent(list, value, index, isSelected, cellHasFocus);
      if (index >= 0 && c instanceof JComponent) {
        ((JComponent) c).setOpaque(true);
      }
      if (!isSelected) {
        c.setBackground(getAlternateRowColor(index));
      }
      return c;
    });
    itemColorListener = e -> {
      if (e.getStateChange() != ItemEvent.SELECTED) {
        return;
      }
      JComboBox<?> cb = (JComboBox<?>) e.getItemSelectable();
      Color rc = getAlternateRowColor(cb.getSelectedIndex());
      if (cb.isEditable()) {
        cb.getEditor().getEditorComponent().setBackground(rc);
      } else {
        cb.setBackground(rc);
      }
    };
    addItemListener(itemColorListener);
    EventQueue.invokeLater(() -> {
      Component c = getEditor().getEditorComponent();
      c.setBackground(getAlternateRowColor(getSelectedIndex()));
      if (c instanceof JComponent) {
        ((JComponent) c).setOpaque(true);
      }
    });
  }

  protected static Color getAlternateRowColor(int index) {
    return index % 2 == 0 ? EVEN_BGC : ODD_BGC;
  }
}