java how to use apache poi code example

Example: apache poi

I used Apache POI libraries to read and write from
excel file, I add the Apache poi dependencies to my pom.xml file.
In order to connect I use following classes

-FileInputStream from Java. It is used to create connection
to the file. We pass the file path as constructor to it.

-WorkBook is a class that represents the excel file. We create
workbook is a class from Apache POI that represents the excel
file. We create Workbook object using the FileInputStream
object.

-Sheet represents a single sheet from the excel file. We create
sheet using Workbook object. We can create worksheet using
the 0 based index.

public String readExcel(String path, String sheetName, int rowNum, int colNum){
        try {
            FileInputStream file = new FileInputStream(path);
            Workbook book = WorkbookFactory.create(file);
            Sheet sheet = book.getSheet(sheetName);
            Row row = sheet.getRow(rowNum);
	    	Cell cell = row.getCell(colNum);
            String cellData = cell.toString();
            return cellData;
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

How to get row and column numbers:
    int rowCount = workSheet.getLastRowNum()+1; ==> why we add '+1'? Because 
    row num starts from 0.
    int colCount = workSheet.getRow(0).getLastCellNum();
    String sheetName = workSheet.getSheetName();

The data we get from excel can be converted to different formats such as set, 
list, map

-------------------EXCEL WRITING AND SAVING---------------------------------

The first creation part is same then go to a cell where you want to write.

Row row = sheet.getRow(0)
Cell resultCell = row.getCell(2)

Let's imagine you have values in index 0 and index 1.
Now you want to create a cell on index 2. First, check if it is null to 
avoid problems.

if(resultCell==null){
   resultCell = row.createCell(2);
}
resultCell.setValue("Germany");

in order to save:
// class is used to open file and write to it
FileOutputStream fileOutputStream = new FileOutputStream("src/test/resources/
Countries.xlsx");
// write the changes to the file and save
workbook.write(fileOutputStream);