public OperatorIF buildOperator(String act)throws BadOperatorException{
        if ("add".equalsIgnoreCase(act)) {
            return new AddOperatorImpl();
        }else if ("sub".equalsIgnoreCase(act)) {
            return new SubOperatorImpl();
        }else if ("div".equalsIgnoreCase(act)) {
            return new DivOperatorImpl();
        }else if ("mul".equalsIgnoreCase(act)) {
            return new MulOperatorImpl();
        }else {
            throw new BadOperatorException("Bad Operator!!");
        }
    }