Swing/FirstCharToUpperCase のバックアップ(No.11)

class FirstCharToUpperCaseDocumentFilter extends DocumentFilter {
  private final JTextComponent textArea;
  public FirstCharToUpperCaseDocumentFilter(JTextComponent textArea) {
    super();
    this.textArea = textArea;
  }
  @Override public void insertString(
      DocumentFilter.FilterBypass fb, int offset, String text, AttributeSet attrs)
      throws BadLocationException {
    if (text == null) {
      return;
    }
    replace(fb, offset, 0, text, attrs);
  }
  @Override public void remove(
      DocumentFilter.FilterBypass fb, int offset, int length)
      throws BadLocationException {
    Document doc = fb.getDocument();
    if (offset == 0 && doc.getLength() - length > 0) {
      fb.replace(0, length + 1, doc.getText(length, 1).toUpperCase(), null);
      textArea.setCaretPosition(0);
    } else {
      fb.remove(offset, length);
    }
  }
  @Override public void replace(
      DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attrs)
      throws BadLocationException {
    if (offset == 0 && text != null && text.length() > 0) {
      text = text.substring(0, 1).toUpperCase() + text.substring(1);
    }
    fb.replace(offset, length, text, attrs);
  }
}