MetalLookAndFeelでJFileChooserの下部にコンポーネントを追加する

class EncodingFileChooserUI extends MetalFileChooserUI {
  public final JComboBox<String> combo = new JComboBox<>(
      new String[] {"UTF-8", "UTF-16", "Shift_JIS", "EUC-JP"});
  protected EncodingFileChooserUI(JFileChooser filechooser) {
    super(filechooser);
  }

  @Override public void installComponents(JFileChooser fc) {
    super.installComponents(fc);
    JPanel bottomPanel = getBottomPanel();

    JLabel label = new JLabel("Encoding:") {
      @Override public Dimension getPreferredSize() {
        return SwingUtils.stream(bottomPanel)
          .filter(JLabel.class::isInstance).map(JLabel.class::cast)
          .findFirst()
          .map(JLabel::getPreferredSize)
          .orElse(super.getPreferredSize());
      }
    };
    label.setDisplayedMnemonic('E');
    label.setLabelFor(combo);

    JPanel panel = new JPanel();
    panel.setLayout(new BoxLayout(panel, BoxLayout.LINE_AXIS));
    panel.add(label);
    panel.add(combo);

    // 0: fileNamePanel
    // 1: RigidArea
    // 2: filesOfTypePanel
    bottomPanel.add(Box.createRigidArea(new Dimension(1, 5)), 3);
    bottomPanel.add(panel, 4);

    SwingUtils.stream(bottomPanel)
      .filter(JLabel.class::isInstance).map(JLabel.class::cast)
      .forEach(l -> {
        l.setHorizontalAlignment(SwingConstants.RIGHT);
        l.setBorder(BorderFactory.createEmptyBorder(0, 0, 0, 5));
      });
  }
}