Swing/AccordionPanel のバックアップ(No.20)

abstract class ExpansionPanel extends JPanel{
  abstract public JPanel makePanel();
  private final String title;
  private final JLabel label;
  private final JPanel panel;

  public ExpansionPanel(String title) {
    super(new BorderLayout());
    this.title = title;
    label = new JLabel("↓ "+title) {
      @Override protected void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D)g;
        //Insets ins = getInsets();
        g2.setPaint(new GradientPaint(50, 0, Color.WHITE,
            getWidth(), getHeight(), new Color(200,200,255)));
        g2.fillRect(0, 0, getWidth(), getHeight());
        super.paintComponent(g);
      }
    };
    label.addMouseListener(new MouseAdapter() {
      @Override public void mousePressed(MouseEvent evt) {
        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 b1 = BorderFactory.createMatteBorder(0,2,2,2,Color.WHITE);
    Border b2 = BorderFactory.createEmptyBorder(10,10,10,10);
    Border b3 = BorderFactory.createCompoundBorder(b1, b2);
    panel.setBorder(b3);
    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((panel.isVisible()?"↑ ":"↓ ")+title);
    revalidate();
    EventQueue.invokeLater(new Runnable() {
      @Override public void run() {
        panel.scrollRectToVisible(panel.getBounds());
      }
    });
  }
}