JFileChooserに画像プレビューを追加

fileChooser = new JFileChooser();
fileChooser.setAccessory(new ImagePreview(fileChooser));
// ...

class ImagePreview extends JComponent implements PropertyChangeListener {
  private static final int PREVIEW_WIDTH = 90;
  private static final int PREVIEW_MARGIN = 5;
  private ImageIcon thumbnail;
  private File file;

  public ImagePreview(JFileChooser fc) {
    super();
    setPreferredSize(new Dimension(PREVIEW_WIDTH + PREVIEW_MARGIN * 2, 50));
    fc.addPropertyChangeListener(this);
  }

  private void loadImage() {
    if (file == null) {
      thumbnail = null;
      return;
    }
    ImageIcon tmpIcon = new ImageIcon(file.getPath());
    if (tmpIcon.getIconWidth() > PREVIEW_WIDTH) {
      float scale = PREVIEW_WIDTH / (float) tmpIcon.getIconWidth();
      int newW = (int) (tmpIcon.getIconWidth()  * scale);
      int newH = (int) (tmpIcon.getIconHeight() * scale);
      BufferedImage img = new BufferedImage(newW, newH, BufferedImage.TYPE_INT_ARGB);
      Graphics2D g2 = img.createGraphics();
      g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                          RenderingHints.VALUE_INTERPOLATION_BILINEAR);
      g2.drawImage(tmpIcon.getImage(), 0, 0, newW, newH, null);
      g2.dispose();
      thumbnail = new ImageIcon(img);
    } else {
      thumbnail = tmpIcon;
    }
  }

  @Override public void propertyChange(PropertyChangeEvent e) {
    boolean update = false;
    String prop = e.getPropertyName();
    if (JFileChooser.DIRECTORY_CHANGED_PROPERTY.equals(prop)) {
      file = null;
      update = true;
    } else if (JFileChooser.SELECTED_FILE_CHANGED_PROPERTY.equals(prop)) {
      file = (File) e.getNewValue();
      update = true;
    }
    if (update) {
      thumbnail = null;
      if (isShowing()) {
        loadImage();
        repaint();
      }
    }
  }

  @Override protected void paintComponent(Graphics g) {
    if (thumbnail == null) {
      loadImage();
    }
    if (thumbnail != null) {
      int x = getWidth()  / 2 - thumbnail.getIconWidth()  / 2;
      int y = getHeight() / 2 - thumbnail.getIconHeight() / 2;
      if (y < 0) {
        y = 0;
      }
      if (x < PREVIEW_MARGIN) {
        x = PREVIEW_MARGIN;
      }
      thumbnail.paintIcon(this, g, x, y);
    }
  }
}