VB2008´ÓÈëÃŵ½¾«Í¨(PDF¸ñʽӢÎÄ°æ)-µÚ52²¿·Ö
°´¼üÅÌÉÏ·½Ïò¼ü ¡û »ò ¡ú ¿É¿ìËÙÉÏÏ·ҳ£¬°´¼üÅÌÉ쵀 Enter ¼ü¿É»Øµ½±¾ÊéĿ¼ҳ£¬°´¼üÅÌÉÏ·½Ïò¼ü ¡ü ¿É»Øµ½±¾Ò³¶¥²¿£¡
¡ª¡ª¡ª¡ªÎ´ÔĶÁÍꣿ¼ÓÈëÊéÇ©ÒѱãÏ´μÌÐøÔĶÁ£¡
¡¡¡¡¡¡¡¡Function¡¡CreateIne£¨ByVal¡¡amount¡¡As¡¡Double£©¡¡As¡¡ITaxIne¡¡
¡¡¡¡¡¡¡¡Function¡¡CreateTaxAccount£¨£©¡¡As¡¡ITaxAccount¡¡
End¡¡Interface¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Page¡¡196¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡
174¡¡¡¡¡¡¡¡¡¡¡¡¡¡CH¡¡AP¡¡T¡¡E¡¡R¡¡¡¡¡¡7¡¡¡¡¡¡¡ö¡¡¡¡¡¡¡¡L¡¡E¡¡A¡¡R¡¡N¡¡IN¡¡G¡¡¡¡¡¡AB¡¡OU¡¡T¡¡¡¡¡¡CO¡¡M¡¡P¡¡O¡¡N¡¡E¡¡N¡¡TS¡¡¡¡¡¡AN¡¡D¡¡¡¡C¡¡L¡¡AS¡¡S¡¡¡¡H¡¡I¡¡E¡¡R¡¡AR¡¡C¡¡H¡¡IE¡¡S¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡We¡¡have¡¡four¡¡interfaces¡¡here£º¡¡¡¡ITaxIne£»¡¡ITaxDeduction£»¡¡ITaxAccount£»¡¡and¡¡ITaxEngine¡£¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡ITaxIne¡¡and¡¡ITaxDeduction¡¡are¡¡pure¡¡behavioral¡¡interfaces¡£¡¡Pure¡¡behavioral¡¡means¡¡the¡¡inter
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡face¡¡does¡¡one¡¡thing£»¡¡but¡¡it¡¡might¡¡be¡¡implemented¡¡in¡¡conjunction¡¡with¡¡other¡¡interfaces¡£¡¡ITaxEngine¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡and¡¡ITaxAccount¡¡are¡¡behavioral¡¡functional¡¡interfaces¡£¡¡Behavioral¡¡functional¡¡means¡¡that¡¡the¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡interfaces¡¡are¡¡typically¡¡implemented¡¡by¡¡themselves£»¡¡and¡¡usually¡¡not¡¡with¡¡another¡¡interface¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡For¡¡example£»¡¡you¡¡might¡¡implement¡¡a¡¡Swiss¡¡tax¡¡system¡¡where¡¡there¡¡are¡¡two¡¡classes£»¡¡defined¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡as¡¡follows£º¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Class¡¡SwissTaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Class¡¡SwissTaxAccount¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxAccount¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡And¡¡if¡¡you¡¡wish¡¡to¡¡implement¡¡an¡¡American¡¡tax¡¡system£»¡¡the¡¡two¡¡classes¡¡would¡¡be¡¡defined¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡as¡¡follows£º¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Class¡¡AmericanTaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Class¡¡AmericanTaxAccount¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxAccount¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡The¡¡user¡¡of¡¡either¡¡the¡¡American¡¡or¡¡Swiss¡¡tax¡¡system¡¡will¡¡not¡¡know¡¡the¡¡specific¡¡details¡¡of¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡those¡¡tax¡¡systems¡£¡¡Initially£»¡¡users¡¡would¡¡need¡¡to¡¡determine¡¡which¡¡tax¡¡system¡¡they¡¡wished¡¡to¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡use¡£¡¡This¡¡decision¡¡is¡¡made¡¡using¡¡something¡¡called¡¡a¡¡factory£»¡¡as¡¡explained¡¡in¡¡the¡¡uping¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡°Abstracting¡¡Instantiations¡¡with¡¡Factories¡±¡¡section¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implementing¡¡a¡¡Base¡¡Class¡¡Tax¡¡Engine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Whenever¡¡you¡¡define¡¡interfaces£»¡¡you¡¡will¡¡need¡¡to¡¡implement¡¡them¡£¡¡In¡¡most¡¡cases£»¡¡you¡¡will¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡create¡¡a¡¡MustInherit¡¡base¡¡class¡¡that¡¡provides¡¡a¡¡certain¡¡amount¡¡of¡¡default¡¡functionality¡£¡¡The¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡MustInherit¡¡base¡¡class¡¡serves¡¡the¡¡same¡¡purpose¡¡as¡¡outlined¡¡in¡¡the¡¡previous¡¡chapter£º¡¡to¡¡provide¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡a¡¡certain¡¡amount¡¡of¡¡basic¡¡functionality¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡In¡¡the¡¡case¡¡of¡¡the¡¡tax¡¡engine£»¡¡we¡¡need¡¡to¡¡implement¡¡the¡¡ITaxEngine¡¡interface¡¡and¡¡provide¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡default¡¡implementations¡¡for¡¡some¡¡methods¡£¡¡The¡¡following¡¡is¡¡the¡¡plete¡¡base¡¡class¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡implementation¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Public¡¡MustInherit¡¡Class¡¡BaseTaxEngine¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Protected¡¡_calculatedTaxable¡¡As¡¡Double¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Public¡¡Overridable¡¡Function¡¡CalculateTaxToPay£¨ByVal¡¡account¡¡As¡¡ITaxAccount£©¡¡_¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡As¡¡Double¡¡Implements¡¡ITaxEngine¡£CalculateTaxToPay¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Page¡¡197¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡CH¡¡AP¡¡T¡¡E¡¡R¡¡¡¡¡¡7¡¡¡¡¡¡¡ö¡¡¡¡¡¡¡¡L¡¡E¡¡AR¡¡N¡¡IN¡¡G¡¡¡¡¡¡AB¡¡O¡¡U¡¡T¡¡¡¡¡¡CO¡¡M¡¡P¡¡O¡¡N¡¡E¡¡N¡¡TS¡¡¡¡¡¡AN¡¡D¡¡¡¡¡¡C¡¡L¡¡AS¡¡S¡¡¡¡H¡¡I¡¡E¡¡R¡¡AR¡¡C¡¡HI¡¡E¡¡S¡¡175¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡_calculatedTaxable¡¡=¡¡0¡£0¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡For¡¡Each¡¡ine¡¡As¡¡ITaxIne¡¡In¡¡account¡£Ine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡If¡¡ine¡¡IsNot¡¡Nothing¡¡Then¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡_calculatedTaxable¡¡=¡¡_calculatedTaxable¡¡£«¡¡ine¡£TaxableAmount¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡If¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Next¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡For¡¡Each¡¡deduction¡¡As¡¡ITaxDeduction¡¡In¡¡account¡£Deductions¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡If¡¡¡¡deduction¡¡IsNot¡¡Nothing¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡_calculatedTaxable¡¡=¡¡_calculatedTaxable¡¡deduction¡£Amount¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡If¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Next¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Return¡¡account¡£GetTaxRate£¨_calculatedTaxable£©¡¡*¡¡_calculatedTaxable¡¡
¡¡¡¡¡¡¡¡End¡¡Function¡¡
¡¡¡¡¡¡¡¡Public¡¡Overridable¡¡Function¡¡CreateDeduction£¨ByVal¡¡amount¡¡As¡¡Double£©¡¡_¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡As¡¡ITaxDeduction¡¡Implements¡¡ITaxEngine¡£CreateDeduction¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Return¡¡New¡¡TaxDeduction£¨amount£©¡¡
¡¡¡¡¡¡¡¡End¡¡Function¡¡
¡¡¡¡¡¡¡¡Public¡¡Overridable¡¡Function¡¡CreateIne£¨ByVal¡¡amount¡¡As¡¡Double£©¡¡As¡¡ITaxIne¡¡_¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxEngine¡£CreateIne¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡'¡¡Second¡¡argument¡¡is¡¡the¡¡tax¡¡rate¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Return¡¡New¡¡TaxIne£¨amount£»¡¡1¡£0£©¡¡
¡¡¡¡¡¡¡¡End¡¡Function¡¡
¡¡¡¡¡¡¡¡Public¡¡MustOverride¡¡Function¡¡CreateTaxAccount£¨£©¡¡As¡¡ITaxAccount¡¡_¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Implements¡¡ITaxEngine¡£CreateTaxAccount¡¡
End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡The¡¡base¡¡class¡¡must¡¡implement¡¡all¡¡interface¡¡methods£»¡¡regardless¡¡of¡¡whether¡¡or¡¡not¡¡the¡¡method¡¡¡¡
has¡¡an¡¡implementation¡£¡¡The¡¡CalculateTaxToPay£¨£©£»¡¡CreateDeduction£¨£©£»¡¡and¡¡CreateIne£¨£©¡¡¡¡
methods¡¡have¡¡implementations¡£¡¡The¡¡CreateTaxAccount£¨£©¡¡method¡¡does¡¡not¡¡have¡¡an¡¡implemen
tation¡¡and¡¡is¡¡declared¡¡as¡¡MustOverride¡£¡¡The¡¡methods¡¡with¡¡implementations¡¡have¡¡an¡¡Overridable¡¡¡¡
keyword¡¡associated¡¡with¡¡them£»¡¡indicating¡¡any¡¡class¡¡derived¡¡from¡¡BaseTaxEngine¡¡can¡¡override¡¡¡¡
the¡¡functionality¡¡if¡¡they¡¡don¡¯t¡¡like¡¡the¡¡default¡¡functionality¡£¡¡¡¡
¡¡¡¡¡¡¡¡¡¡In¡¡the¡¡implementation¡¡of¡¡CalculateTaxToPay£¨£©£»¡¡the¡¡ine¡¡£¨account¡£Ine£©¡¡is¡¡added¡¡¡¡
together¡¡and¡¡deductions¡¡£¨account¡£Deductions£©¡¡are¡¡subtracted¡¡from¡¡the¡¡ine¡£¡¡The¡¡resulting¡¡¡¡
total¡¡is¡¡used¡¡as¡¡a¡¡query¡¡amount¡¡£¨account¡£GetTaxRate£¨£©£©¡¡to¡¡retrieve¡¡the¡¡actual¡¡tax¡¡rate¡¡used¡¡to¡¡¡¡
calculate¡¡against¡¡the¡¡payable¡¡tax¡£¡¡
¡öNote¡¡¡¡The¡¡implementation¡¡of¡¡CalculateTaxToPay£¨£©¡¡is¡¡a¡¡shared¡¡functionality£»¡¡which¡¡implies¡¡that¡¡there¡¡¡¡
cannot¡¡be¡¡any¡¡code¡¡specific¡¡to¡¡a¡¡derived¡¡type¡£¡¡All¡¡of¡¡the¡¡calculations¡¡and¡¡data¡¡manipulations¡¡are¡¡executed¡¡¡¡
against¡¡an¡¡interface£»¡¡making¡¡it¡¡possible¡¡to¡¡generalize¡¡operations¡£¡¡Whenever¡¡you¡¡implement¡¡base¡¡class¡¡methods¡¡or¡¡¡¡
shared¡¡pieces¡¡of¡¡code£»¡¡you¡¡should¡¡try¡¡to¡¡keep¡¡the¡¡source¡¡code¡¡derived¡¡class¡agnostic¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Page¡¡198¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡
176¡¡¡¡¡¡¡¡¡¡¡¡¡¡CH¡¡AP¡¡T¡¡E¡¡R¡¡¡¡¡¡7¡¡¡¡¡¡¡ö¡¡¡¡¡¡¡¡L¡¡E¡¡A¡¡R¡¡N¡¡IN¡¡G¡¡¡¡¡¡AB¡¡OU¡¡T¡¡¡¡¡¡CO¡¡M¡¡P¡¡O¡¡N¡¡E¡¡N¡¡TS¡¡¡¡¡¡AN¡¡D¡¡¡¡C¡¡L¡¡AS¡¡S¡¡¡¡H¡¡I¡¡E¡¡R¡¡AR¡¡C¡¡H¡¡IE¡¡S¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Overriding¡¡for¡¡Specialized¡¡Functionality¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡In¡¡the¡¡base¡¡class¡¡implementation£»¡¡the¡¡data¡¡member¡¡_calculatedTaxable¡¡is¡¡declared¡¡as¡¡Protected¡£¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡As¡¡you¡¡learned¡¡in¡¡the¡¡previous¡¡chapter£»¡¡this¡¡means¡¡that¡¡_calculatedTaxable¡¡can¡¡be¡¡manipu
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡lated¡¡in¡¡a¡¡derived¡¡class¡£¡¡However£»¡¡if¡¡you¡¡look¡¡at¡¡how¡¡the¡¡data¡¡member¡¡is¡¡used£»¡¡you¡¡will¡¡see¡¡that¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡only¡¡CalculateTaxToPay£¨£©¡¡assigns¡¡the¡¡data¡¡member¡£¡¡The¡¡purpose¡¡of¡¡the¡¡data¡¡member¡¡is¡¡to¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡provide¡¡information¡¡about¡¡the¡¡operation¡¡CalculateTaxToPay£¨£©¡¡without¡¡giving¡¡the¡¡exact¡¡details¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡of¡¡the¡¡operation¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡The¡¡idea¡¡behind¡¡_calculatedTaxable¡¡and¡¡the¡¡declaration¡¡of¡¡CalculateTaxToPay£¨£©¡¡is¡¡to¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡provide¡¡a¡¡mechanism¡¡where¡¡the¡¡derived¡¡class¡¡does¡¡not¡¡need¡¡to¡¡calculate¡¡things¡¡again¡£¡¡Consider¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡the¡¡example¡¡of¡¡a¡¡country¡¡where£»¡¡if¡¡your¡¡taxable¡¡ine¡¡is¡¡above¡¡400¡¡currency¡¡units£»¡¡a¡¡surtax¡¡of¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡10¡¡currency¡¡units¡¡is¡¡calculated¡£¡¡You¡¡don¡¯t¡¡know¡¡what¡¡your¡¡taxable¡¡ine¡¡is¡¡until¡¡the¡¡function¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡CalculateTaxToPay£¨£©¡¡is¡¡executed£»¡¡and¡¡that¡¡function¡¡returns¡¡only¡¡the¡¡total¡¡payable¡¡taxes¡£¡¡So¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡how¡¡do¡¡you¡¡know¡¡if¡¡you¡¡should¡¡apply¡¡the¡¡surtax¡¡in¡¡this¡¡situation£¿¡¡One¡¡solution¡¡is¡¡to¡¡reverse
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡calculate¡¡the¡¡payable¡¡taxes£»¡¡but¡¡that¡¡would¡¡involve¡¡quite¡¡a¡¡few¡¡additional¡¡steps¡£¡¡An¡¡easier¡¡solution¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡is¡¡to¡¡write¡¡some¡¡code¡¡in¡¡the¡¡base¡¡class¡¡method¡¡of¡¡CalculateTaxToPay£¨£©¡¡that¡¡stores¡¡the¡¡taxable¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡ine¡¡so¡¡the¡¡subclass¡¡has¡¡access¡¡to¡¡it¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡The¡¡original¡¡implementation¡¡of¡¡CalculateTaxToPay£¨£©¡¡does¡¡not¡¡consider¡¡a¡¡surtax£»¡¡so¡¡the¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡derived¡¡class¡¡must¡¡contain¡¡that¡¡functionality¡£¡¡Since¡¡CalculateTaxToPay£¨£©¡¡can¡¡be¡¡overridden¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡without¡¡the¡¡data¡¡member¡¡_calculatedTaxable£»¡¡the¡¡derived¡¡class¡¡would¡¡need¡¡to¡¡implement¡¡the¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡functionality¡¡in¡¡the¡¡base¡¡class¡¡to¡¡calculate¡¡whether¡¡or¡¡not¡¡the¡¡surtax¡¡applies¡£¡¡Following¡¡is¡¡an¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡sample¡¡derived¡¡class¡¡implementation¡¡of¡¡the¡¡tax¡¡engine¡¡for¡¡such¡¡a¡¡situation£»¡¡stored¡¡in¡¡a¡¡namespace¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡called¡¡Surtax£»¡¡to¡¡distinguish¡¡it¡¡from¡¡the¡¡base¡¡functionality¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Namespace¡¡Surtax¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Friend¡¡Class¡¡TaxEngine¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Inherits¡¡BaseTaxEngine¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Public¡¡Overrides¡¡Function¡¡CalculateTaxToPay£¨ByVal¡¡account¡¡As¡¡ITaxAccount£©¡¡_¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡As¡¡Double¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Dim¡¡taxToPay¡¡As¡¡Double¡¡=¡¡MyBase¡£CalculateTaxToPay£¨account£©¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡If¡¡_calculatedTaxable¡¡¡·¡¡400¡¡Then¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡taxToPay¡¡=¡¡taxToPay¡¡£«¡¡10¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡If¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Return¡¡taxToPay¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Function¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Public¡¡Overrides¡¡Function¡¡CreateTaxAccount£¨£©¡¡As¡¡ITaxAccount¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Throw¡¡New¡¡Exception£¨¡¨The¡¡method¡¡or¡¡operation¡¡is¡¡not¡¡implemented¡£¡¨£©¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Function¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Class¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡End¡¡Namespace¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡In¡¡the¡¡implementation¡¡of¡¡CalculateTaxToPay£¨£©£»¡¡we¡¡replace¡¡the¡¡Overridable¡¡keyword¡¡with¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Overrides£»¡¡implying¡¡that¡¡the¡¡functionality¡¡of¡¡TaxEngine¡¡replaces¡¡the¡¡functionality¡¡of¡¡BaseTaxEngine¡£¡¡¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Since¡¡our¡¡fictional¡¡country¡¡calculates¡¡the¡¡basic¡¡tax¡¡similarly¡¡to¡¡most¡¡countries£»¡¡the¡¡functionality¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡of¡¡BaseTaxEngine¡£CalculateTaxToPay£¨£©¡¡can¡¡be¡¡used¡£¡¡Thus£»¡¡the¡¡first¡¡line¡¡of¡¡TaxEngine¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡CalculateTaxToPay£¨£©¡¡is¡¡MyBase¡£CalculateTaxToPay£¨£©£»¡¡meaning¡¡the¡¡base¡¡class¡¡£¨BaseTaxEngine£©¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡method¡¡CalculateTaxToPay£¨£©¡¡is¡¡called¡£¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Page¡¡199¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡CH¡¡AP¡¡T¡¡E¡¡R¡¡¡¡¡¡7¡¡¡¡¡¡¡ö¡¡¡¡¡¡¡¡L¡¡E¡¡AR¡¡N¡¡IN¡¡G¡¡¡¡¡¡AB¡¡O¡¡U¡¡T¡¡¡¡¡¡CO¡¡M¡¡P¡¡O¡¡N¡¡E¡¡N¡¡TS¡¡¡¡¡¡AN¡¡D¡¡¡¡¡¡C¡¡L¡¡AS¡¡S¡¡¡¡H¡¡I¡¡E¡¡R¡¡AR¡¡C¡¡HI¡¡E¡¡S¡¡177¡¡
¡¡¡¡¡¡¡¡¡¡¡¡Calling¡¡the¡¡base¡¡class¡¡results¡¡in¡¡calculating¡¡a¡¡basic¡¡tax¡¡to¡¡pay¡¡amount¡£¡¡We¡¡need¡¡to¡¡figure¡¡out¡¡¡¡
if¡¡a¡¡surtax¡¡applies£»¡¡and¡¡that¡¡is¡¡where¡¡the¡¡protected¡¡data¡¡member¡¡_calculatedTaxable¡¡es¡¡into¡¡¡¡
play¡£¡¡Having¡¡called¡¡BaseTaxEngine¡£CalculateTaxToPay£¨£©£»¡¡the¡¡data¡¡member¡¡_calculatedTaxable¡¡is¡¡¡¡
assigned¡¡and¡¡contains¡¡the¡¡amount¡¡that¡¡is¡¡being¡¡taxed¡£¡¡Thus£»¡¡TaxEngine¡£CalculateTaxToPay£¨£©¡¡¡¡
can¡¡make¡¡a¡¡decision¡¡if¡¡more¡¡than¡¡400¡¡currency¡¡units¡¡have¡¡been¡¡earned¡£¡¡And¡¡if¡¡so£»¡¡then¡¡the¡¡vari
able¡¡taxToPay¡¡is¡¡incremented¡¡with¡¡another¡¡10¡¡currency¡¡units¡£¡¡Had¡¡_calculatedTaxable¡¡not¡¡¡¡
existed£»¡¡TaxEngine¡£CalculateTaxToPay£¨£©¡¡would¡¡have¡¡needed¡¡to¡¡call¡¡the¡¡base¡¡class¡¡functionality¡¡¡¡
to¡¡get¡¡the¡¡basic¡¡tax¡¡rate£»¡¡and¡¡the