Workbook wb = new HSSFWorkbook(); // or new XSSFWorkbook();
Sheet sheet1 = wb.createSheet("new sheet");
Sheet sheet2 = wb.createSheet("second sheet");

// Note that sheet name is Excel must not exceed 31 characters
// and must not contain any of the any of the following characters:
// 0x0000
// 0x0003
// colon (:)
// backslash (\)
// asterisk (*)
// question mark (?)
// forward slash (/)
// opening square bracket ([)
// closing square bracket (])

// You can use org.apache.poi.ss.util.WorkbookUtil#createSafeSheetName(String nameProposal)}
// for a safe way to create valid names, this utility replaces invalid characters with a space (' ')
String safeName = WorkbookUtil.createSafeSheetName("[O'Brien's sales*?]");
Sheet sheet3 = wb.createSheet(safeName);

FileOutputStream fileOut = new FileOutputStream("newWorksheet.xls");
wb.write(fileOut);
fileOut.close();

//Instantiating a Workbook object
Workbook workbook = new Workbook();

//Adding a new worksheet to the Workbook object
WorksheetCollection worksheets = workbook.getWorksheets();
Worksheet worksheet = worksheets.add("My Worksheet");

//Saving the Excel file
workbook.save("newWorksheet.xls");