How to append to existing excel file using Java HSSF

Question

I am still relatively new to Java and have cobbled together enough code that allows me to write my data out to a new excel file. However, I want it to write (append to the end) to the existing file. prepareDataToWriteToExcel() gets some data to write 3 columns of data.

public List writeDataToExcelFile(String fileName) throws IOException {
    Map excelData = prepareDataToWriteToExcel();
    List receiversList=new  ArrayList();
    HSSFWorkbook myWorkBook = new HSSFWorkbook();
    HSSFSheet mySheet = myWorkBook.createSheet();
    HSSFRow myRow = null;
    HSSFCell myCell = null;
    Iterator it=excelData.entrySet().iterator(); 
    int rowNum=0;
    while (it.hasNext()) {
        myRow = mySheet.createRow(rowNum);
        Map.Entry pairs = (Map.Entry)it.next();
        String[]arr= (String[]) pairs.getValue();
        for (int cellNum = 0; cellNum < arr.length ; cellNum++){
            myCell = myRow.createCell((short) cellNum);
            myCell.setCellValue(arr[cellNum]);     
        }
        receiversList.add(arr[2]);
        rowNum++;
    }
    try{
        FileOutputStream out = new FileOutputStream(fileName);
        myWorkBook.write(out);
        System.out.println("WRITING TO EXCEL COMPLETED");
        out.close();
    }catch(Exception e){}  
    return receiversList;
}

Answer 1

I found this to be a very good example of updating an existing excel worksheet. As you'll notice, you create your workbook with the file you want to modify. This will allow you to find where you can create your new column and write your data too.

try {
    FileInputStream file = new FileInputStream(new File("C:\\update.xls"));

    HSSFWorkbook workbook = new HSSFWorkbook(file);
    HSSFSheet sheet = workbook.getSheetAt(0);
    Cell cell = null;

    // Find empty cell     

    // Update the value of cell

    file.close();

    FileOutputStream outFile = new FileOutputStream(new File("C:\\update.xls"));
    workbook.write(outFile);
    outFile.close();

} catch (FileNotFoundException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}