JPanelをアコーディオン風に展開

abstract class AbstractExpansionPanel extends JPanel {
  private final String title;
  private final JLabel label;
  private final JPanel panel;
  public abstract JPanel makePanel();
  public AbstractExpansionPanel(String title) {
    super(new BorderLayout());
    this.title = title;
    label = new JLabel("\u25BC " + title) {
      @Override protected void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g.create();
        // Insets ins = getInsets();
        g2.setPaint(new GradientPaint(
            50, 0, Color.WHITE, getWidth(), getHeight(),
            new Color(200, 200, 255)));
        g2.fillRect(0, 0, getWidth(), getHeight());
        g2.dispose();
        super.paintComponent(g);
      }
    };
    label.addMouseListener(new MouseAdapter() {
      @Override public void mousePressed(MouseEvent e) {
        initPanel();
      }
    });
    label.setForeground(Color.BLUE);
    label.setBorder(BorderFactory.createEmptyBorder(2, 5, 2, 2));
    add(label, BorderLayout.NORTH);
    panel = makePanel();
    panel.setVisible(false);
    panel.setOpaque(true);
    panel.setBackground(new Color(240, 240, 255));
    Border outside = BorderFactory.createMatteBorder(0, 2, 2, 2, Color.WHITE);
    Border inside = BorderFactory.createEmptyBorder(10, 10, 10, 10);
    Border border = BorderFactory.createCompoundBorder(outside, inside);
    panel.setBorder(border);
    add(panel);
  }

  @Override public Dimension getPreferredSize() {
    Dimension d = label.getPreferredSize();
    if (panel.isVisible()) {
      d.height += panel.getPreferredSize().height;
    }
    return d;
  }

  @Override public Dimension getMaximumSize() {
    Dimension d = getPreferredSize();
    d.width = Short.MAX_VALUE;
    return d;
  }

  protected void initPanel() {
    panel.setVisible(!panel.isVisible());
    label.setText(String.format(
        "%s %s", panel.isVisible() ? "\u25B3" : "\u25BC", title));
    revalidate();
    // fireExpansionEvent();
    EventQueue.invokeLater(new Runnable() {
      @Override public void run() {
        panel.scrollRectToVisible(panel.getBounds());
      }
    });
  }
}