Read / Write Excel file (.xls or .xlsx) using Apache POI

ReadWriteExcelFile.java

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;

import org.apache.poi.hssf.usermodel.HSSFCell;
import org.apache.poi.hssf.usermodel.HSSFRow;
import org.apache.poi.hssf.usermodel.HSSFSheet;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;

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 ReadWriteExcelFile {

	public static void readXLSFile() throws IOException
	{
		InputStream ExcelFileToRead = new FileInputStream("C:/Test.xls");
		HSSFWorkbook wb = new HSSFWorkbook(ExcelFileToRead);

		HSSFSheet sheet=wb.getSheetAt(0);
		HSSFRow row; 
		HSSFCell cell;

		Iterator rows = sheet.rowIterator();

		while (rows.hasNext())
		{
			row=(HSSFRow) rows.next();
			Iterator cells = row.cellIterator();
			
			while (cells.hasNext())
			{
				cell=(HSSFCell) cells.next();
		
				if (cell.getCellType() == HSSFCell.CELL_TYPE_STRING)
				{
					System.out.print(cell.getStringCellValue()+" ");
				}
				else if(cell.getCellType() == HSSFCell.CELL_TYPE_NUMERIC)
				{
					System.out.print(cell.getNumericCellValue()+" ");
				}
				else
				{
					//U Can Handel Boolean, Formula, Errors
				}
			}
			System.out.println();
		}
	
	}
	
	public static void writeXLSFile() throws IOException {
		
		String excelFileName = "C:/Test.xls";//name of excel file

		String sheetName = "Sheet1";//name of sheet

		HSSFWorkbook wb = new HSSFWorkbook();
		HSSFSheet sheet = wb.createSheet(sheetName) ;

		//iterating r number of rows
		for (int r=0;r < 5; r++ )
		{
			HSSFRow row = sheet.createRow(r);
	
			//iterating c number of columns
			for (int c=0;c < 5; c++ )
			{
				HSSFCell cell = row.createCell(c);
				
				cell.setCellValue("Cell "+r+" "+c);
			}
		}
		
		FileOutputStream fileOut = new FileOutputStream(excelFileName);
		
		//write this workbook to an Outputstream.
		wb.write(fileOut);
		fileOut.flush();
		fileOut.close();
	}
	
	public static void readXLSXFile() throws IOException
	{
		InputStream ExcelFileToRead = new FileInputStream("C:/Test.xlsx");
		XSSFWorkbook  wb = new XSSFWorkbook(ExcelFileToRead);
		
		XSSFWorkbook test = new XSSFWorkbook(); 
		
		XSSFSheet sheet = wb.getSheetAt(0);
		XSSFRow row; 
		XSSFCell cell;

		Iterator rows = sheet.rowIterator();

		while (rows.hasNext())
		{
			row=(XSSFRow) rows.next();
			Iterator cells = row.cellIterator();
			while (cells.hasNext())
			{
				cell=(XSSFCell) cells.next();
		
				if (cell.getCellType() == XSSFCell.CELL_TYPE_STRING)
				{
					System.out.print(cell.getStringCellValue()+" ");
				}
				else if(cell.getCellType() == XSSFCell.CELL_TYPE_NUMERIC)
				{
					System.out.print(cell.getNumericCellValue()+" ");
				}
				else
				{
					//U Can Handel Boolean, Formula, Errors
				}
			}
			System.out.println();
		}
	
	}
	
	public static void writeXLSXFile() throws IOException {
		
		String excelFileName = "C:/Test.xlsx";//name of excel file

		String sheetName = "Sheet1";//name of sheet

		XSSFWorkbook wb = new XSSFWorkbook();
		XSSFSheet sheet = wb.createSheet(sheetName) ;

		//iterating r number of rows
		for (int r=0;r < 5; r++ )
		{
			XSSFRow row = sheet.createRow(r);

			//iterating c number of columns
			for (int c=0;c < 5; c++ )
			{
				XSSFCell cell = row.createCell(c);
	
				cell.setCellValue("Cell "+r+" "+c);
			}
		}

		FileOutputStream fileOut = new FileOutputStream(excelFileName);

		//write this workbook to an Outputstream.
		wb.write(fileOut);
		fileOut.flush();
		fileOut.close();
	}

	public static void main(String[] args) throws IOException {
		
		writeXLSFile();
		readXLSFile();
		
		writeXLSXFile();
		readXLSXFile();

	}

}

If flush() used right after write(f) no data is been updated to the file. Why so?