使用Apache POI在Java中更新excel文件

Question

我正在尝试更新我的excel文件，但出于某种原因，我在第二次尝试时从单元格中获取null。

public class ExcelWriter {


    XSSFSheet sheet;

    public ExcelWriter(List<String> nurses, List<String> therapist)  {


        FileInputStream file;
        try {

            file = new FileInputStream("Schedule.xlsx");
            XSSFWorkbook workbook = new XSSFWorkbook(file);
            sheet = workbook.getSheetAt(0);

            for (int i=0;i<nurses.size();i++)
            {
                writeToCell(i+1,0,nurses.get(i));
            }

           for (int i=0;i<therapist.size();i++)
            {
                writeToCell(i+10,0,nurses.get(i)); //Getting null here
            }


            file.close();

            FileOutputStream outFile =new FileOutputStream(new File("Schedule.xlsx"));
            workbook.write(outFile);
            outFile.close();

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        }

    private void writeToCell(int row, int col, String string) {

        Cell cell = null;

        XSSFRow sheetrow = sheet.getRow(row);

        cell = sheetrow.getCell(col); //Getting null here
        cell = sheetrow.createCell(col); 
        cell.setCellValue(string);

    }



}

日志：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.michlind.cheduler.ExcelWriter.writeToCell(ExcelWriter.java:66)
at com.michlind.cheduler.ExcelWriter.<init>(ExcelWriter.java:40)
at com.michlind.cheduler.MainClass$2.actionPerformed(MainClass.java:343)