howto set specific amounts of COLUMNS with POI

For purpose of reading Excels by using Apache POI XSSFWorkbook (XSSFSheet and XSSFRow), codes like this:

FileInputStream fileInputStream = new FileInputStream(path);
XSSFWorkbook workbook = new XSSFWorkbook(fileInputStream);
XSSFSheet sheet = workbook.getSheet(sheetName);
XSSFRow row = sheet.getRow(i);

i indicates difference rows, where the columns of each row are different.

My question is: how can I assign the columns of the row, even some of the cells are blank.