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InputStream inp = new FileInputStream("workbook.xls");   //InputStream inp = new FileInputStream("workbook.xlsx");     Workbook wb = WorkbookFactory.create(inp);   Sheet sheet = wb.getSheetAt(0);   Row row = sheet.getRow(2);   Cell cell = row.getCell(3);   if (cell == null)       cell = row.createCell(3);   cell.setCellType(Cell.CELL_TYPE_STRING);   cell.setCellValue("a test");     // Write the output to a file   FileOutputStream fileOut = new FileOutputStream("workbook.xls");   wb.write(fileOut);   fileOut.close();