JTableのセルを横方向に連結する

class ColumnSpanningCellRenderer extends JPanel implements TableCellRenderer {
  private static final int TARGET_COLIDX = 0;
  private final JTextArea textArea = new JTextArea(2, 999999);
  private final JLabel label = new JLabel();
  private final JLabel iconLabel = new JLabel();
  private final JScrollPane scroll = new JScrollPane(textArea);

  protected ColumnSpanningCellRenderer() {
    super(new BorderLayout());

    scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_NEVER);
    scroll.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
    scroll.setBorder(BorderFactory.createEmptyBorder());
    scroll.setViewportBorder(BorderFactory.createEmptyBorder());
    scroll.setOpaque(false);
    scroll.getViewport().setOpaque(false);

    textArea.setBorder(BorderFactory.createEmptyBorder());
    textArea.setMargin(new Insets(0, 0, 0, 0));
    textArea.setForeground(Color.RED);
    textArea.setEditable(false);
    textArea.setFocusable(false);
    textArea.setOpaque(false);

    iconLabel.setBorder(BorderFactory.createEmptyBorder(0, 4, 0, 4));
    iconLabel.setOpaque(false);

    Border b1 = BorderFactory.createEmptyBorder(2, 2, 2, 2);
    Border b2 = BorderFactory.createMatteBorder(0, 0, 1, 1, Color.GRAY);
    label.setBorder(BorderFactory.createCompoundBorder(b2, b1));

    setBackground(textArea.getBackground());
    setOpaque(true);
    add(label, BorderLayout.NORTH);
    add(scroll);
  }

  @Override public Component getTableCellRendererComponent(
      JTable table, Object value, boolean isSelected,
      boolean hasFocus, int row, int column) {
    OptionPaneDescription d;
    if (value instanceof OptionPaneDescription) {
      d = (OptionPaneDescription) value;
      add(iconLabel, BorderLayout.WEST);
    } else {
      String title = Objects.toString(value, "");
      int mrow = table.convertRowIndexToModel(row);
      Object o = table.getModel().getValueAt(mrow, 0);
      if (o instanceof OptionPaneDescription) {
        OptionPaneDescription t = (OptionPaneDescription) o;
        d = new OptionPaneDescription(title, t.icon, t.text);
      } else {
        d = new OptionPaneDescription(title, null, "");
      }
      remove(iconLabel);
    }
    label.setText(d.title);
    textArea.setText(d.text);
    iconLabel.setIcon(d.icon);

    Rectangle cr = table.getCellRect(row, column, false);
    if (column != TARGET_COLIDX) {
      cr.x -= iconLabel.getPreferredSize().width;
    }
    scroll.getViewport().setViewPosition(cr.getLocation());

    if (isSelected) {
      setBackground(Color.ORANGE);
    } else {
      setBackground(Color.WHITE);
    }
    return this;
  }
}

class OptionPaneDescription {
  public final String title;
  public final Icon icon;
  public final String text;
  protected OptionPaneDescription(String title, Icon icon, String text) {
    this.title = title;
    this.icon  = icon;
    this.text  = text;
  }
}