How to add new row in Excel file?

I have a problem with adding a new row in the excel file. My code:

public static void setExcelFile(String Path, String SheetName) throws Exception {
        try {
            FileInputStream ExcelFile = new FileInputStream(Path);
            workBook = new XSSFWorkbook(ExcelFile);
            workSheet = workBook.getSheet(SheetName);
        } catch (Exception e) {

            throw (e);

        }
    }    

public static void setCellData(String path, String value,  int RowNum, int ColNum) throws Exception {
        try{
            row  = workSheet.getRow(RowNum);
            cell = row.getCell(ColNum);
            if (cell == null) {
                cell = row.createCell(ColNum);
                cell.setCellValue(value);
            } else {
                cell.setCellValue(value);
            }
            FileOutputStream fileOut = new FileOutputStream(path);
            workBook.write(fileOut);
            fileOut.flush();
            fileOut.close();
        }catch(Exception e){

            throw (e);

        }

And:

public static void main(String[] args) {
        String exPath = "C:\\Drivers\\excel\\test.xlsx";
        try {
            ExcelUtility.setExcelFile(exPath, "MainTable");
            ExcelUtility.setCellData(exPath, "Pizza", 4, 0);
            ExcelUtility.setCellData(exPath, "Thin", 4, 1);
            ExcelUtility.setCellData(exPath, "Peperoni", 4, 2);
            ExcelUtility.setCellData(exPath, "Katy", 4, 3);
            ExcelUtility.setCellData(exPath, "N/A", 4, 4);
            ExcelUtility.setCellData(exPath, "14.99", 4, 5);
        } catch (Exception e) {
            e.printStackTrace();
        }


    }

In my excel file I have 4 row. With the above code when I tried to update any existing row it works great. But when I tried to add new row it give me exeption:

java.lang.NullPointerException
    at ui_automation.utils.ExcelUtility.setCellData(ExcelUtility.java:65)
    at ui_automation.stepDefinition.ExcelTest.main(ExcelTest.java:18)

Any ideas what I'm doing wrong?

The null pointer exception happens in your own code. You haven't provided the full source code, nor the excel file (or described how many rows and columns it has from the start), so I can't say what exactly it is that is null. But you can just look at your code at line 65 of ExcelUtility.java.

But just looking at the javadoc, there are a few candidates. For one, the workSheet object might be null, since workBook.getSheet(SheetName) returns null if there is no sheet named "MainTable". Otherwise, workSheet.getRow(RowNum) can still result in a null value, if there is no such row. And remember that the row is zero-based, so when you ask for row "4" it is actually the 5th row you are asking for. Does your excel file have five rows in this sheet?

If this is not the problem, then try to use the debugger of the IDE you are using, and put a breakpoint at the first line inside the method "setCellData" (ie, put a breakpoint on row = workSheet.getRow(RowNum);) then use the debugger to see the actual value of all the variables. Find the object that is causing the Null Pointer Exception, then trace the code backwards until you find why it is null. If you can't get your debugger to work, you can just add some simple System.out.println("variable: " + variable") at different places in your code, so that you can see the values when the code runs.

If nothing of this helps you find the problem, please post the full source code (including imports), with line numbers matching the one in your code. And a screenshot of your excel file, with the "MainTable" sheet open would help too.