Apache POI append data to excel sheet not working

Question

I am reading data from an arraylist and writing this to an excel sheet. The problem is my excel is getting overwritten each time. Here is my code. I can't figure out what is wrong here :( Can someone please help?

public static void main( String[] args ) throws Exception
{
        List<String> fileData = new ArrayList<String>();
        for(File file:files) {
            fileData.add(readFileContents(file.getAbsolutePath()));
        }
        for(String fileContent:fileData) {
            //do some stuff that in turn calls the writeDataToExcel method 

    }
}

private static void writeDataToExcel(String test,Map<String,String> dataMap,Object object) throws IOException {
    File file = new File("input/data.xls");
    Map<String,Object[]> data = new LinkedHashMap<String,Object[]>();       
    XSSFWorkbook workbook = null; 
    int count = 0;
    XSSFSheet sheet = null;
    if(file.exists()) {
        workbook = new XSSFWorkbook(new FileInputStream(file));
        sheet = workbook.getSheet("Data Sheet");
    }
    else {
        workbook = new XSSFWorkbook();
        sheet = workbook.createSheet("Data Sheet");
        //count = sheet.getLastRowNum();
    }

    data.put("1", new Object[]{"Id","Name","Field","Description","Value"});
    for(Map.Entry<String, String> dataMp:dataMap.entrySet()) {
        data.put(Integer.toString(count+2), new Object[]{id,object.getClass().getSimpleName(),dataMp.getKey(),dataMp.getValue(),"null"});
        count++;
    }
    Set<String> keyset = data.keySet();
    int rownum = 0;
    for (String key : keyset) {
        Row row = sheet.createRow(rownum++);
        Object [] objArr = data.get(key);
        int cellnum = 0;
        for (Object obj : objArr) {
            Cell cell = row.createCell(cellnum++);
             if(obj instanceof String)
                cell.setCellValue((String)obj);
        }
    }
    FileOutputStream fis = new FileOutputStream("input/data.xls");
    workbook.write(fis);
    if(fis!=null)
        fis.close();
}

Answer 1

I think problem is at line

int rownum = 0;

this will set rowNUm to zero each time and sheet will be written from zero row

You need to persist this rowNum value if you want to append data in the sheet