Exception in thread "main" java.lang.NullPointerException error when calling a method to write a string into Excel sheet in JAVA

Exception in thread "main" java.lang.NullPointerException error when calling a method to write a string into Excel sheet in JAVA. Error is as below

Exception in thread "main" java.lang.NullPointerException at ManualVolteCxCreation.ExcelDataReader.writeExcelTestResult(ExcelDataReader.java:51)

Calling writeExcelTestResult method from the main class and it is as below.

excelDataReader.writeExcelTestResult(startRowIndex, "There are already connections with");

Excel Data Reader class has the constructor in which initialize the excel file, excel workbook and the excel sheet and two methods to read excel data and write to the 7th column of the excel file. Excel Data Reader class is as below.

import java.io.File;
import java.io.FileInputStream;
import org.apache.poi.xssf.usermodel.XSSFRow;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class ExcelDataReader {
	
	ConfigReader configReader = new ConfigReader();
	File testFilePath;
	FileInputStream fis;
	XSSFWorkbook volteConnections;
	XSSFSheet testDataFile;
	XSSFRow rowTestData; 
	
	public ExcelDataReader() {
		
	try {
		
		testFilePath = new File(configReader.readPropertyValue("excel")); 
		fis = new FileInputStream(testFilePath); 
		
		// Finds the workbook instance for XLSX file 
		volteConnections = new XSSFWorkbook(fis);	
		
		// Return first sheet from the XLSX workbook 
		testDataFile = volteConnections.getSheetAt(0);
		
	} catch (Exception e) {
		
		e.printStackTrace();
		
	} 

	}	
	
    public XSSFRow readExcelRowTestData(int rowIndex) {
    	
    	rowTestData = testDataFile.getRow(rowIndex);
    	return rowTestData;
    		
    }
    
    public void writeExcelTestResult(int rowIndex, String testResult) {
    	
    	testDataFile.getRow(rowIndex).getCell(7).setCellValue(testResult);
    	
   }
		
			
}

I debug and tried to see where the problem, but found every variable was initialize as shown in below image.

So any suggestion how can I sort this issue?

You just need to create the cell first and then call the setCellValue method.

In order to save the file you need FileOutputStream object as well.

So you can modify the writeExcelTestResult method as below.

public void writeExcelTestResult(int rowIndex, String testResult) {
    	
    	  if (cellTestData == null) {
    		  
    		  cellTestData = testDataFile.getRow(rowIndex).createCell(7);
    		  
    	  }
          
    	  cellTestData.setCellValue(testResult);
    	  try {
			fis.close();
			fos = new FileOutputStream(testFilePath);
			volteConnections.write(fos);
			fos.close();
		} catch (IOException e) {
			
			e.printStackTrace();
		}    	  
    	
   }