使用java读取excel时将null更改为空白

Question

如果我使用具有空值的excel表，则此代码会出现“java.lang.NullPointerException”错误。有没有办法将空值更改为空白？谢谢。

public class GradesDB {

public static void main (String[] args) throws Exception {

    //initiating workbook
    File excel = new File("Users/Database.xlsx");  //file location
    FileInputStream fis= new FileInputStream(excel);        
    XSSFWorkbook wb = new XSSFWorkbook(fis);         
    XSSFSheet ws = wb.getSheet("Students");  //sheet with null value


        int rowNum = ws.getLastRowNum() +1;
        int colNum = ws.getRow(0).getLastCellNum();
        String[][] data = new String[rowNum][colNum];  //storing row and col

 //iteration
 for (int i =0; i <rowNum; i++) {
     XSSFRow row = ws.getRow(i);
        for (int j = 0; j <colNum; j++){
            XSSFCell cell = row.getCell(j);
            String value = cellToString(cell);
            data[i][j] = value;
            System.out.println(value);
        }
 }
 //System.out.println(data[4][3]);
}
public static String cellToString(XSSFCell cell) {

    int type;
    Object result = null;
    type = cell.getCellType();

    switch (type) {
    case 0 :
        result = cell.getNumericCellValue();  //numeric value
        break;
    case 1 :
        result = cell.getStringCellValue();   //string value
        break;
    case 2 :
        result = cell.getBooleanCellValue();
        break;

    }
    return result.toString();
}
}

我不确定问题出在哪里。