How to initialize cells when creating a new Excel file (Apache POI)

I am currently using Apache POI to create empty excel files (that I will later read and edit). The problem is that whenever I try to get the cell, I always get a null value. I have tried initializing the first few columns and rows (the cells are no longer null) but with this approach, I cannot insert new rows and columns. How can I be able to initialize all cells of a sheet without having to set the number of rows and columns? Thanks

EDIT: Hi this is my code in creating excel files. I could not use the iterator to initialize all my cells since there are no rows and columns for the spreadsheet.

FileOutputStream fileOut = new FileOutputStream(loc + formID +".xls");
        HSSFWorkbook workbook = new HSSFWorkbook();
        Sheet sheet = workbook.createSheet("Sheet1");
        for (Row row : sheet) {
            for (Cell cell : row) {
                CellStyle style 开发者_JAVA技巧= workbook.createCellStyle();
                style.setFillBackgroundColor(IndexedColors.BLACK.getIndex());
                cell.setCellValue("");
                cell.setCellStyle(style);
            }
        } 
        workbook.write(fileOut);
        fileOut.flush();
        fileOut.close(); 

You might find that MissingCellPolicy can help with your needs. When calling getCell on a row, you can specify a MissingCellPolicy to have something happen if no cell was there, eg

Row r = sheet.getRow(2);
Cell c = r.getCell(5, MissingCellPolicy.CREATE_NULL_AS_BLANK);
c.setCellValue("This will always work, c will never be null");

org.apache.poi.ss.util.CellUtil can also help too:

// Get the row, creating it if needed
Row r = CellUtil.getRow(4, sheet);
// Get the cell, creating it if needed
Cell c = CellUtil.getCell(2, r);