DocumentFilterで先頭文字を大文字に変換する

class FirstCharToUpperCaseDocumentFilter extends DocumentFilter {
  private final JTextComponent textField;
  protected FirstCharToUpperCaseDocumentFilter(JTextComponent textField) {
    super();
    this.textField = textField;
  }

  @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(length, 1,
                 doc.getText(length, 1).toUpperCase(Locale.ENGLISH), null);
      textField.setCaretPosition(offset);
    }
    fb.remove(offset, length);
  }

  @Override public void replace(
        DocumentFilter.FilterBypass fb, int offset, int length,
        String text, AttributeSet attrs) throws BadLocationException {
    String str = text;
    if (offset == 0 && Objects.nonNull(text) && !text.isEmpty()) {
      str = text.substring(0, 1).toUpperCase(Locale.ENGLISH) + text.substring(1);
    }
    fb.replace(offset, length, str, attrs);
  }
}