《重构:改善既有代码的设计》读书笔记4 简化条件表达式
(1)Decompose Conditional 分解条件表达式
你有一个复杂的条件(if-then-else)语句。从if、else、else三个段落中分别提炼出独立函数。
1 if(data.before(SUMMER_START)||data.after(SUMMER_END)) 2 charge=quantity*_winterRate+_winterServiceCharge; 3 else charge=quantity*_summerRate; 4 5 --> 6 7 if(notSummer(date)) 8 charge=winterCharge(quantity); 9 else charge=summerCharge(quantity);
复杂的条件逻辑是导致复杂度上升的地点之一,编写代码来检查不同的条件分支、昨不同的事会得到一个很长的函数。大型函数自身就会使代码的可读性下降,而条件逻辑会使代码更难阅读, 将每条分支分解成新函数,可以突出条件逻辑。
(2)Consolidate Conditional Expression 合并条件表达式
一些列测试条件得到相同的结果,将这些测试合并为一个表达式。
double disabillityAmount(){ if(seniority<2) return 0; if(isPartTime) return 0; } --> double disabillityAmount(){ if(seniority<2||isPartTime) return 0; } --> double double disabillityAmount(){ if(function()) return 0 }
(3)Consolidate Duplicate Conditional Fragments(合并重复的条件片段)
在条件表达式每个分支上有着相同的一段代码,将这段重复代码搬移到条件表达式之外。如果try 和catah 有重复代码 移到final区域。
if(judge()){ total = price * 0.5; send(); } else{ total = price * 0.8; send(); } ==> if(judge()){ total = price * 0.5; } else{ total = price * 0.8; } send()
(4)Remove Control Flag 移除控制标记
在一系列布尔表达式中,某个变量带有控制标记的作用,以break或return语句取代控制标记。
void 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; } } } } --》 void checkSecurity(String[] people) { boolean found = false; for (int i = 0; i < people.length; i++) { if (! found) { if (people[i].equals ("Don")){ sendAlert(); break; } if (people[i].equals ("John")){ sendAlert(); found = true; } } } }
-->
void 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; } } }
例二 在这里,变量found 做了两件事:它既是控制标记,也是运算结果。遇到这种情况,我喜欢先把计算found 变量的代码提炼到一个独立函数中:
void checkSecurity(String[] people) {
String found = "";
for (int i = 0; i < people.length; i++) {
if (found.equals("")) {
if (people[i].equals ("Don")){
sendAlert();
found = "Don";
}
if (people[i].equals ("John")){
sendAlert();
found = "John";
}
}
}
someLaterCode(found);
}
-->
void checkSecurity(String[] people) {
String found = foundMiscreant(people);
someLaterCode(found);
}
String foundMiscreant(String[] people){
String found = "";
for (int i = 0; i < people.length; i++) {
if (found.equals("")) {
if (people[i].equals ("Don")){
sendAlert();
return "Don";
}
if (people[i].equals ("John")){
sendAlert();
found = "John";
}
}
}
return found;
}
即使不需要返回某值,你也可以使用语句来取代控制标记。这时候你只需 要一个空的return 语句就行了。
(5)replace nested conditional with guard clauses 以卫语句取代嵌套条件表达式
卫语句就是把复杂的条件表达式拆分成多个条件表达式,比如一个很复杂的表达式,嵌套了好几层的if - then-else语句,转换为多个if语句,实现它的逻辑,这多条的if语句就是卫语句.
double getPayAmount() { double result; if (_isDead) result = deadAmount(); else { if (_isSeparated) result = separatedAmount(); else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); }; } return result; }; => double getPayAmount() { if (_isDead) return deadAmount(); if (_isSeparated) return separatedAmount(); if (_isRetired) return retiredAmount(); return normalPayAmount(); };
(6) replace conditional with ploymorphism 以多态取代条件表达式
你手上有个条件式,它根据对象型别的不同而选择不同的行为。
将这个条件式的每个分支放进一个subclass 内的覆写函数中,然后将原始函数声明为抽象函数(abstract method)
double getSpeed() { switch (_type) { case EUROPEAN: return getBaseSpeed(); case AFRICAN: return getBaseSpeed() - getLoadFactor() * _numberOfCoconuts; case NORWEGIAN_BLUE: return (_isNailed) ? 0 : getBaseSpeed(_voltage); } throw new RuntimeException ("Should be unreachable"); }
多态(polymorphism)最根本的好处就是:如果你需要根据对象的不同型别而采取不同的行为,多态使你不必编写明显的条件式(explicit conditional )。
正因为有了多态,所以你会发现:「针对type code(型别码)而写的switch 语句」 以及「针对type string (型别名称字符串)而写的if-then-else 语句」在面向对象程序中很少出现。
abstract class EmployeeType...
abstract int getTypeCode();
class Engineer extends EmployeeType...
int getTypeCode() {
return Employee.ENGINEER;
}
class Engineer... int payAmount(Employee emp) { return emp.getMonthlySalary(); }
class Salesman...
int payAmount(Employee emp) {
return emp.getMonthlySalary() + emp.getCommission();
}
class Manager...
int payAmount(Employee emp) {
return emp.getMonthlySalary() + emp.getBonus();
}
class EmployeeType... int payAmount(Employee emp) { switch (getTypeCode()) { case ENGINEER: return emp.getMonthlySalary(); case SALESMAN: return emp.getMonthlySalary() + emp.getCommission(); case MANAGER: return emp.getMonthlySalary() + emp.getBonus(); default: throw new RuntimeException("Incorrect Employee"); } }
--》
class Employee...
int payAmount() {
return _type.payAmount(this);
}
class EmployeeType... abstract int payAmount(Employee emp);
abstract int getTypeCode()
(9)Introduce null object 引入null 对象
class NullCustomer extends Customer { public boolean isNull() { return true; } } class Customer... public boolean isNull() { return false; }
或者
interface Nullable { boolean isNull(); } class Customer implements Nullable
class Customer... static Customer newNull() { return new NullCustomer(); }
class Site... Customer getCustomer() { return (_customer == null) ? Customer.newNull(): _customer; }
Customer customer = site.getCustomer(); BillingPlan plan; if (customer == null) plan = BillingPlan.basic(); else plan = customer.getPlan(); ... String customerName; if (customer == null) customerName = "occupant"; else customerName = customer.getName(); ... int weeksDelinquent; if (customer == null) weeksDelinquent = 0; else weeksDelinquent = customer.getHistory().getWeeksDelinquentInLastYear();
--》
Customer customer = site.getCustomer();
BillingPlan plan;
if (customer.isNull()) plan = BillingPlan.basic();
else plan = customer.getPlan();
...
String customerName;
if (customer.isNull()) customerName = "occupant";
else customerName = customer.getName();
...
int weeksDelinquent;
if (customer.isNull()) weeksDelinquent = 0;
else weeksDelinquent = customer.getHistory().getWeeksDelinquentInLastYear();
(9) 引入断言
某一段代码需要对程序状态(state)做出某种假设。
以assertion(断言)明确表现这种假设。
double getExpenseLimit() {
// should have either expense limit or a primary project
return (_expenseLimit != NULL_EXPENSE) ?
_expenseLimit:
_primaryProject.getMemberExpenseLimit();
}
=>
double getExpenseLimit() { Assert.isTrue (_expenseLimit != NULL_EXPENSE || _primaryProject != null); return (_expenseLimit != NULL_EXPENSE) ? _expenseLimit: _primaryProject.getMemberExpenseLimit(); }
这段代码包含了一个明显假设:任何员工要不就参与某个项目,要不就有个人开支限额。我们可以使用assertion 在代码中更明确地指出这一点