I am new to Java and I am trying to read an excel file. 
   jars included: log4j-1.2.17,poi-3.9,poi-ooxml-3.5-beta5,poi-ooxml-3.7-20101029,poi-ooxml-schemas-3.7-beta1,xmlbeans-2.5.0,xmlbeans-xmlpublic-2.4.0
package net.codejava.excel;
import java.io.*;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.usermodel.WorkbookFactory;
import org.apache.poi.xssf.usermodel.XSSFCell;
import org.apache.poi.xssf.usermodel.XSSFRow;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
public class SPACE_CreateThroughExcel{
    public static void main(String[] args) throws Exception{
        SPACE_CreateThroughExcel create = new SPACE_CreateThroughExcel();
        create.read();
    }
    public void read(){
        try {           
            String excelFilePath = "C:/Users/Rachana/workspace/SPACEOM/WebContent/Data/Data.xlsx";
            InputStream is = new FileInputStream(new File(excelFilePath));
            XSSFWorkbook wb = (XSSFWorkbook) WorkbookFactory.create(is);
            XSSFSheet ws = wb.getSheetAt(0);
            int rowNum = ws.getLastRowNum() + 1;
            System.out.println(rowNum);
            int colNum = ws.getRow(0).getLastCellNum();
        String[][] data = new String[rowNum][colNum];
        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);
                }
        }
        } catch (Exception e) { 
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    public static String cellToString(XSSFCell cell){
        int type;
        Object result;
        type = cell.getCellType();
        switch(type){
            case 0:
                result = cell.getNumericCellValue();
                break;
            case 1:
                result = cell.getStringCellValue();
                break;
            default:
                throw new RuntimeException("No support");
        }
        return result.toString();
    }
}
This is my code. I am trying to read the cells of excel file into a 2D array which I want to later use in my jsp file. When I run it it shows the following errors:
at net.codejava.excel.SPACE_CreateThroughExcel.cellToString(SPACE_CreateThroughExcel.java:46)
at net.codejava.excel.SPACE_CreateThroughExcel.read(SPACE_CreateThroughExcel.java:30)
at net.codejava.excel.SPACE_CreateThroughExcel.main(SPACE_CreateThroughExcel.java:64)
My excel file is something like this:
 SPLD_DeviceID_Mfg  SPLD_DeviceID_ModelNo   SPLD_DeviceID_SrNo                                                                              
                                                    4  
                                  apl               3         
Some of the fields in my excel file are empty. I need to store the empty value in my array for later use. I am checking only the types for integer and String. What to do in case of null?
 
     
     
    