重構—改善既有程式的設計（chapter 9）

RefactoringChapter 9Simplifying Conditional Expressions

Read thisand then read thisDecompose Conditionalif (date.before (SUMMER_START) || date.after(SUMMER_END)) charge = quantity * _winterRate + _winterServiceCharge;else charge = quantity * _summerRate;if (notSummer(date)) charge = winterCharge(quantity);else charge = summerCharge (quantity);

Consolidate Conditional ExpressionBefore:After:double disabilityAmount() { if (_seniority < 2) return 0; if (_monthsDisabled > 12) return 0; if (_isPartTime) return 0; // ...}double disabilityAmount() { if (isNotEligableForDisability()) return 0; // ...}

Beforeif (isSpecialDeal()) {doSomePreparation();doNonRelatedWork(); total = price * 0.95; send();doSomeCleanUp();}else {doSomePreparation(); total = price * 0.98; send();doNonRelatedWork();doSomeCleanUp();}Consolidate Duplicate Conditional Fragments

Consolidate Duplicate Conditional FragmentsAfterdoNonRelatedWork();doSomePreparation();if (isSpecialDeal()) total = price * 0.95;else total = price * 0.98;send();doSomeCleanUp();

Remove Control FlagTrace thisvoid checkSecurity(String[] people) {boolean found = false; for (int i = 0; i < people.length; i++) { if (! found) { if (people[i].equals ("Don")){sendAlert(); found = true; } if (people[i].equals ("John")){sendAlert(); found = true; } } }}

Remove Control Flag○use return, break, continue …✕set a flag and check somewhere later

Alternative 1:Remove Control Flagvoid checkSecurity(String[] people) { for (int i = 0; i < people.length; i++) { if (people[i].equals ("Don")){sendAlert(); break; } if (people[i].equals ("John")){sendAlert(); break; } }}

Alternative 2:Remove Control Flagvoid checkSecurity(String[] people) { String found = foundMiscreant(people);someLaterCode(found);}String foundMiscreant(String[] people) { for (int i = 0; i < people.length; i++) { if (people[i].equals ("Don")){sendAlert(); return "Don"; } if (people[i].equals ("John")){sendAlert(); return "John"; } } return "";}

BeforeReplace Nested Conditional with Guard Clausesdouble getPayAmount() { double result; if (_isDead) { result = deadAmount(); } else { if (_isSeparated) { result = separatedAmount(); } else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); } } return result;}

AfterReplace Nested Conditional with Guard Clausesdouble getPayAmount() { if (_isDead) return deadAmount(); if (_isSeparated) return separatedAmount(); if (_isRetired) return retiredAmount(); return normalPayAmount();};

Mind implicit semanticsGuard clause-> (somehow) exceptional conditionif else-> two equivalent normal conditionsReplace Nested Conditional with Guard Clauses

Replace Nested Conditional with Guard ClausesHowever, you might have seen this …if (aquireResource1()) { if (aquireResource2()) {if (aquireResource3()){ if (aquireResource4()) { doRealWork();releaseResource4(); } releaseResource3(); } releaseResource2(); }releaseResource1();}

Replace Nested Conditional with Guard Clausesor this …if (false == aquireResource1())return;if (false == aquireResource2()) {releaseResource1(); return;}if (false == aquireResource3()){releaseResource2();releaseResource1();return;}doRealWork();releaseResource3();releaseResource2();releaseResource1();return;

BeforeReplace Conditional with Polymorphismclass Employee... int payAmount(Employee) { switch (getType()) { case EmployeeType.ENGINEER: return _monthlySalary; case EmployeeType.SALESMAN: return _monthlySalary + _commission; case EmployeeType.MANAGER: return _monthlySalary + _bonus; default: throw new RuntimeException("Incorrect Employee"); } }

！○Polymorphism might be better, usuallyReplace Conditional with Polymorphismswitch caseclauses

class Employee...intpayAmount() { return _type.payAmount(this);}class Salesman...intpayAmount(Employee emp) { return emp.getMonthlySalary() + emp.getCommission(); }class Manager...intpayAmount(Employee emp) { return emp.getMonthlySalary() + emp.getBonus(); }AfterReplace Conditional with Polymorphism

Same with Replace Conditional with Polymorphism

deal with special cases extensively checked

Before:Introduce Null Objectclass Customer... public String getName() {...} public BillingPlangetPlan() {...} public PaymentHistorygetHistory() {...}if (customer == null) plan = BillingPlan.basic();else plan = customer.getPlan();

AfterIntroduce Null Objectplan = customer.getPlan();class NullCustomer... public BillingPlangetPlan(){ return BillingPlan.basic(); }

重構—改善既有程式的設計（chapter 9）

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重構—改善既有程式的設計（chapter 9）