在JFrame中更新进度条

嗨，我的进度栏有问题。我有一个Start类，它是一个JFrame类，一个Excel类是一个普通的java类。 Excel类对其他类进行一些调用。现在代码本身可以正常工作，现在我尝试添加进度条。但是，当我尝试更新进度条时，它会更具有Start功能：

if (excelFileLocation.getText().equals("")) {
            JOptionPane.showMessageDialog(null, "U heeft geen excel bestand gekozen, kies eerst een excel bestand.", "Geen excel bestand gekozen.", JOptionPane.ERROR_MESSAGE);
        } else if (folderLocation.getText().equals("")) {
            JOptionPane.showMessageDialog(null, "U heeft geen eindmap gekozen voor de gemaakte flyers, kies eerst een eindmap.", "Geen map gekozen.", JOptionPane.ERROR_MESSAGE);
        } else if (logoFolderLocation.getText().equals("")) {
            JOptionPane.showMessageDialog(null, "U heeft map gekozen met de schoollogo's, kies eerst een map.", "Geen map gekozen.", JOptionPane.ERROR_MESSAGE);
        } else {
            try {
                makeFlyers.setBackground(Color.red);
                makeFlyers.setEnabled(false);
                JOptionPane.showMessageDialog(null, "", "Flyers worden gemaakt!", JOptionPane.INFORMATION_MESSAGE);
                Excel excel = new Excel();
                excel.readExcel(new File(excelFileLocation.getText()), new File(this.folderLocation.getText()), new File(logoFolderLocation.getText()), this);
                System.out.println("Klaar met flyers maken!");
                JOptionPane.showMessageDialog(null, "Alle flyers zijn succesvol gemaakt!", "Succes!", JOptionPane.PLAIN_MESSAGE);
                resetButton(makeFlyers);
            } catch (IIOException ex) {
                JOptionPane.showMessageDialog(null, "Het logo kan niet worden geplaatst, kijk of er correct word verwezen naar de logo.", "Fout bij plaatsen van logo", JOptionPane.ERROR_MESSAGE);
                handleException(ex);
                resetButton(makeFlyers);
            } catch (InvalidFormatException | ParseException ex) {
                handleException(ex);
                resetButton(makeFlyers);
            } catch (IndexOutOfBoundsException | NullPointerException ex) {
                System.out.println("Klaar met flyers maken!");
                JOptionPane.showMessageDialog(null, "Alle flyers zijn succesvol gemaakt!", "Succes!", JOptionPane.PLAIN_MESSAGE);
                resetButton(makeFlyers);
            } catch (IOException ex) {
                handleException(ex);
                JOptionPane.showMessageDialog(null, "Het excel bestand wat u wilt gebruiken kan niet worden gevonden, of staat nog open in een ander venster.", "Fout bij uitlezen van Excel bestand", JOptionPane.ERROR_MESSAGE);
                resetButton(makeFlyers);
            }
        }

我使用此功能之外的功能更新进度条：

public void updateProgressBar(int value) {
progressBar.setValue(value);
}

这是我的Excel函数：

public class Excel {

    Start start;
    File outputFolder;
    File logoFolder;

    public void readExcel(File excelFile, File output, File logoLocation, Start start) throws IOException, InvalidFormatException, ParseException {
        this.outputFolder = output;
        this.logoFolder = logoLocation;
        // Retrieving the number of sheets in the Workbook
        try ( // Creating a Workbook from an Excel file (.xls or .xlsx)
                Workbook workbook = WorkbookFactory.create(excelFile)) {
            for (Sheet sheet : workbook) {
                if (sheet.getSheetName().equals("secundair flyer")) {
                    createFlyersFromExcel(sheet);
                }
            }
        }
    }

    public void handleException(Exception ex) {
        StringWriter sw = new StringWriter();
        PrintWriter pw = new PrintWriter(sw);
        ex.printStackTrace(pw);
        System.out.println(sw.toString());
    }

    public void createFlyersFromExcel(Sheet sheet) throws IOException, ParseException {
        DataFormatter dataFormatter = new DataFormatter();
        boolean firstRow = false;
        int currentRow = 0;
        int totalRows = sheet.getPhysicalNumberOfRows();
        for (Row row : sheet) {
            currentRow++;
            updateProgressBarFromCurrentRow(currentRow, totalRows);
            if (firstRow == false) {
                firstRow = true;
            } else {
                if (!dataFormatter.formatCellValue(row.getCell(0)).equals("0")) {
                    List<String> values = new ArrayList<>();
                    for (Cell cell : row) {
                        cell.setCellFormula(null);
                        String cellValue = dataFormatter.formatCellValue(cell);
                        if (!cellValue.equals("0")) {
                            values.add(cellValue);
                        } else {
                            values.add(null);
                        }
                    }
                    if (values.get(1) == null) {
                    } else {
                        School s = new School(values, logoFolder);
                        Flyer flyer = new Flyer(s, outputFolder);
                    }
                }
            }
        }
    }

    public void updateProgressBarFromCurrentRow(int current, int totalRows) {
        start.updateProgressBar(current/totalRows*100);
    }
}

更新进度条后，它会立即回到“开始”功能并执行最后几行代码。我做错什么了吗？我没有看到错误。