JComboBoxのItemを左右にクリップして配置

class MultiColumnCellRenderer extends JPanel implements ListCellRenderer {
  private final JLabel leftLabel = new JLabel();
  private final JLabel rightLabel;

  public MultiColumnCellRenderer(int rightWidth) {
    super(new BorderLayout());
    this.setBorder(BorderFactory.createEmptyBorder(1, 1, 1, 1));

    leftLabel.setOpaque(false);
    leftLabel.setBorder(BorderFactory.createEmptyBorder(0, 2, 0, 0));

    final Dimension dim = new Dimension(rightWidth, 0);
    rightLabel = new JLabel() {
      @Override public Dimension getPreferredSize() {
        return dim;
      }
    };
    rightLabel.setOpaque(false);
    rightLabel.setBorder(BorderFactory.createEmptyBorder(0, 2, 0, 2));
    rightLabel.setForeground(Color.GRAY);
    rightLabel.setHorizontalAlignment(SwingConstants.RIGHT);

    this.add(leftLabel);
    this.add(rightLabel, BorderLayout.EAST);
  }

  @Override public Component getListCellRendererComponent(
      JList list, Object value, int index,
      boolean isSelected, boolean cellHasFocus) {
    LRItem item = (LRItem) value;
    leftLabel.setText(item.getLeftText());
    rightLabel.setText(item.getRightText());

    leftLabel.setFont(list.getFont());
    rightLabel.setFont(list.getFont());

    if (index < 0) {
      leftLabel.setForeground(list.getForeground());
      this.setOpaque(false);
    } else {
      leftLabel.setForeground(
          isSelected ? list.getSelectionForeground() : list.getForeground());
      this.setBackground(
          isSelected ? list.getSelectionBackground() : list.getBackground());
      this.setOpaque(true);
    }
    return this;
  }

  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    return new Dimension(0, d.height);
  }

  @Override public void updateUI() {
    super.updateUI();
    this.setName("List.cellRenderer");
  }
}