fis.close();
String[] functionNames = { "calculatePayment" } ;
FreeRefFunction[] functionImpls = { new CalculateMortgage() } ;
UDFFinder udfToolpack = new DefaultUDFFinder( functionNames, functionImpls ) ;
// register the user-defined function in the workbook
workbook.addToolPack(udfToolpack);
FormulaEvaluator evaluator = workbook.getCreationHelper().createFormulaEvaluator();