Java

how write in file (.xls) without delete the older data

HSSFWorkbook wb = new HSSFWorkbook();
 HSSFSheet sheet = wb.createSheet("ma feuille");

 HSSFRow row = sheet.createRow(0);
 HSSFCell cell = null;

 cell = row.createCell((short) 0);
 cell.setCellValue(1234);

 sheet.addMergedRegion(new Region(0,(short)0,0,(short)3));

 FileOutputStream fileOut;
 try {
   fileOut = new FileOutputStream("monfichier.xls");
   wb.write(fileOut);
   fileOut.close();
 } catch (FileNotFoundException e) {
   e.printStackTrace();}

s.getRow(0).getCell(0).getRichStringCellValue().toString()

Cannot get a NUMERIC value from a STRING cell

i need to get data from a file excel in java and to display the data on a jTable please help me to resolve this problem tnx
i was try this code but i think is incorrect

public Object getCellValue(Sheet sheet, int rowNum, int colNum) {
 
    Row row = sheet.getRow(rowNum);
    if ( row==null ) {
         return null;
    }
    else {
         Cell cell = row.getCell(colNum);
         if ( cell==null ) {
            return null;
         }
         else {
             return getCellValue(cell);
         }
    }
 
}
public Object getCellValue(Cell cell) {
switch (cell.getCellTypeEnum()) {
case CellType.STRING:
    return cell.getRichStringCellValue().getString();
case CellType.NUMERIC:
    if (DateUtil.isCellDateFormatted(cell)) {
        return cell.getDateCellValue();
    } else {
        return cell.getNumericCellValue();
    }
case CellType.BOOLEAN:
    return cell.getBooleanCellValue();
case CellType.FORMULA:
    return evalCell(cell); 
case CellType.BLANK:
return null;
default:
throw new IllegalStateException("Unknown cell type");
}
 
private Object evalCell(Cell cell) {
 
FormulaEvaluator evaluator = cell.getSheet().getWorkbook().getCreationHelper().createFormulaEvaluator();
 
CellValue cellValue = evaluator.evaluate(cell);
 
switch (cellValue.getCellType()) {
    case Cell.CELL_TYPE_BOOLEAN:
        return cellValue.getBooleanValue();
    case Cell.CELL_TYPE_NUMERIC:
        return cellValue.getNumberValue();
    case Cell.CELL_TYPE_STRING:
        return cellValue.getStringValue();
    case Cell.CELL_TYPE_BLANK:
        return null;
    case Cell.CELL_TYPE_ERROR:
        throw new RuntimeException("Excel formula error");
    default:
    throw new IllegalStateException("Unknown cell type");
}				
 
 
}