JTextAreaに行番号を表示

class LineNumberView extends JComponent {
  private static final int MARGIN = 5;
  private final JTextArea textArea;
  private final FontMetrics fontMetrics;
  // private final int topInset;
  private final int fontAscent;
  private final int fontHeight;
  private final int fontDescent;
  private final int fontLeading;

  public LineNumberView(JTextArea textArea) {
    this.textArea = textArea;
    Font font = textArea.getFont();
    fontMetrics = getFontMetrics(font);
    fontHeight = fontMetrics.getHeight();
    fontAscent = fontMetrics.getAscent();
    fontDescent = fontMetrics.getDescent();
    fontLeading = fontMetrics.getLeading();
    // topInset = textArea.getInsets().top;

    textArea.getDocument().addDocumentListener(new DocumentListener() {
      @Override public void insertUpdate(DocumentEvent e) {
        repaint();
      }

      @Override public void removeUpdate(DocumentEvent e) {
        repaint();
      }

      @Override public void changedUpdate(DocumentEvent e) {
        // repaint();
      }
    });
    textArea.addComponentListener(new ComponentAdapter() {
      @Override public void componentResized(ComponentEvent e) {
        revalidate();
        repaint();
      }
    });
    Insets i = textArea.getInsets();
    setBorder(BorderFactory.createCompoundBorder(
      BorderFactory.createMatteBorder(0, 0, 0, 1, Color.GRAY),
      BorderFactory.createEmptyBorder(i.top, MARGIN, i.bottom, MARGIN - 1)));
    setOpaque(true);
    setBackground(Color.WHITE);
    setFont(font);
  }

  private int getComponentWidth() {
    Document doc = textArea.getDocument();
    Element root = doc.getDefaultRootElement();
    int lineCount = root.getElementIndex(doc.getLength());
    int maxDigits = Math.max(3, String.valueOf(lineCount).length());
    Insets i = getInsets();
    return maxDigits * fontMetrics.stringWidth("0") + i.left + i.right;
    // return 48;
  }

  private int getLineAtPoint(int y) {
    Element root = textArea.getDocument().getDefaultRootElement();
    int pos = textArea.viewToModel(new Point(0, y));
    return root.getElementIndex(pos);
  }

  @Override public Dimension getPreferredSize() {
    return new Dimension(getComponentWidth(), textArea.getHeight());
  }

  @Override protected void paintComponent(Graphics g) {
    g.setColor(getBackground());
    Rectangle clip = g.getClipBounds();
    g.fillRect(clip.x, clip.y, clip.width, clip.height);

    g.setColor(getForeground());
    int base = clip.y;
    int start = getLineAtPoint(base);
    int end = getLineAtPoint(base + clip.height);
    int y = start * fontHeight;
    int rmg = getInsets().right;
    for (int i = start; i <= end; i++) {
      String text = String.valueOf(i + 1);
      int x = getComponentWidth() - rmg - fontMetrics.stringWidth(text);
      y += fontAscent;
      g.drawString(text, x, y);
      y += fontDescent + fontLeading;
    }
  }
}