The document discusses refactoring techniques to replace type codes with other object-oriented constructs. It describes replacing magic numbers with symbolic constants to make code more readable. It also covers replacing type codes with classes, subclasses, or state/strategy objects to encapsulate behavior associated with different types and allow polymorphism. This improves extensibility and maintains type safety compared to using raw integer type codes.

1. Hubert ChanRefactoring – chap 8-2

2. 8.9 Replace Magic Number with Symbolic ConstantMagic Number 特性具有特殊意義，卻又不能明確表現出其意義。可讀性差 / 不明確 / 牽一髮動全身解決方案以 Constant 取代Magic Numberdouble potentialEngery(double mass, double height) { return mass * 9.81 * height}double potentialEngery(double mass, double height) { return mass * GRAVITATIONAL_CONSTANT * height}static final double GRAVITATIONAL_CONTSTANT = 9.81;

3. 8.9 Replace Magic Number with Symbolic ConstantRefactoring 時機Magic Number 是 Type CodeReplace Type Code With Class陣列長度使用 Array.length()關鍵字conststaticfinal

4. 8.10 Encapsulate Fieldclass A { public String _name;}class A {private String _name; public String getName() {return _name;} public void setName(String arg) {_name = arg;}}

5. 8.10 Encapsulate Field動機封裝 Encapsulation資料隱藏 Data Hidingpublic 欄位：資料可以被修改、存取，但是擁有的物件卻無法控制其行為改變存取資料為（Objective-C 2.0 property）readonlyreadwriteassigncopy

6. 8.11 Encapsulate Collection目的傳回 Collection 的一個 readonly view提供 add/remove 元素的 function

7. 8.11 Encapsulate Collection動機Getter 不該傳回 collection 本身暴露過多實作細節使用者可以存取 getter 拿到的 collection使用 add/remove 函式添加元素使用者不用管底層使用哪種 Collection（List, Array, Vector）

8. 8.11 Encapsulate Collectionclass Course { public Course (String name, booleanisAdvanced) {} public booleanisAdvanced() {}}Class Person { public Set getCourses() { return _courses; } public void setCourses(Set arg) { _courses = arg; } private Set _courses;}//使用者直接操作 data//直接 assign，非複製

9. 8.11 Encapsulate CollectionPerson kent = new Person();Set s = new HashSet();s.add(new Course("Smalltalk Programming", false));s.add(new Course("Appreciating Single Malts", true));kent.setCourses(s);Course refact = new Course("Refactoring", true);kent.getCourses().add(refact);kent.getCourses().remove(refact);//直接 assign，非複製//使用者直接操作 data//使用者直接操作 data

10. 8.11 Encapsulate CollectionRefactoring for JAVA 1.2 – 使用 add/remove 函式Class Person { public void addCourse(Course arg) { _courses.add(arg); } public void removeCourse(Course arg) _courses.remove(arg); } public void initializeCourses(Set arg) {Assert.isTrue(_courses.isEmpty());_courses.addAll(arg); }}//內部的 _courses != arg

11. 8.11 Encapsulate CollectionRefactoring for JAVA 1.2 – 使用 add/remove 函式Person kent = new Person();Set s = new HashSet();s.add(new Course("Smalltalk Programming", false));s.add(new Course("Appreciating Single Malts", true));kent.setCourses(s);Person kent = new Person();kent.addCourse(new Course("Smalltalk Programming", false));kent.addCourse(new Course("Appreciating Single Malts", true));

12. 8.11 Encapsulate CollectionRefactoring for JAVA 1.2 – 不該讓使用者透過 getter 修改物件內的 collectionGetter 回傳 unmodified setkent.getCourses().add(new Course("C Programming", false));Class Person { public Set getCourse() {return Collections.unmodifiableSet(_courses);}}//回傳 unmodifiable的 Set

13. 8.11 Encapsulate Collection將行為移至 class 內Move Method更好維護class Person {intnumberOfAdvancedCourses(Person person) { Iterator iter = person.getCourses().iterator();int count = 0; while (iter.hasNext()) { Course each = (Course) iter.next(); if (each.isAdvanced()) count++; } return count; }}

14. 8.12 Replace Record with Data Class手法把傳統的 record structure 轉為 data class使用 getter/setterclass Person { String getName() { return _name; } String setName(String arg) { _name = arg; } private String _name;}class PersonRecord { public char[] name;}

15. 8.13 Replace Type Code with Class目的Type Code 沒有型別檢驗Type Code => Class，factory class 可以做型別檢驗

16. 8.13 Replace Type Code with Class目標是 Person 中的 Type Code class Person { public static final int O = 0; public static final int A = 1; public static final int B = 2; public static final int AB = 3; private int _bloodGroup; public Person (intbloodGroup) { _bloodGroup = bloodGroup; } public void setBloodGroup(intarg) { _bloodGroup = arg; } public intgetBloodGroup() { return _bloodGroup; } }//型別是 int，不是特別的型別

17. 8.13 Replace Type Code with Class把 Person 中 Type Code 換成 BloodGroup Class class BloodGroup { public static final BloodGroup O = new BloodGroup(0); public static final BloodGroup A = new BloodGroup(1); public static final BloodGroup B = new BloodGroup(2); public static final BloodGroup AB = new BloodGroup(3); private static final BloodGroup[] _values = {O, A, B, AB}; private finalint _code;privateBloodGroup (int code ) { _code = code; } public intgetCode() { return _code; } public static BloodGroup code(intarg) { return _values[arg]; } }//class instance 而非 Type Code//constructor 是 private

18. 8.13 Replace Type Code with Class使用 Class 並維持原本的對外介面 class Person { public static final int O = BloodGroup.O.getCode(); public static final int A = BloodGroup.A.getCode(); public static final int B = BloodGroup.B.getCode(); public static final int AB = BloodGroup.AB.getCode();private BloodGroup _bloodGroup; public Person (intbloodGroup) { _bloodGroup = BloodGroup.code(bloodGroup); } public intgetBloodGroup() { return _bloodGroup.getCode(); } public void setBloodGroup(intarg) { _bloodGroup = BloodGroup.code (arg); } }

19. 8.13 Replace Type Code with Class在 Constructor 使用 Class 做參數為新的介面加入 setter/getter class Person { public Person (BloodGroupbloodGroup) { _bloodGroup = bloodGroup; } public void setBloodGroup(BloodGrouparg) { _bloodGroup = arg;} public BloodGroupgetBloodGroup() { return _bloodGroup; } }//使用 BloodGroup// Setter// Getter

20. 8.13 Replace Type Code with Class使用者端的改變Person thePerson = new Person(PersonA);thePerson.getBloodGroupCode();thePerson.setBloodGroup(Person.AB);Person thePerson = new Person(BloodGroup.A);thePerson.getBloodGroup();thePerson.setBloodGroup(BloodGroup.AB);

21. 8.14 Replace Type Code with Subclasses使用時機有一個 type code 會影響 class 的行為為了 Replace Conditional with Polymorphism 鋪路彰顯「特定類別的行為」

22. 8.14 Replace Type Code with Subclasses目標 class具有會影響行為的 type code class Employee{ private int _type; static final int ENGINEER = 0; static final int SALESMAN = 1; static final int MANAGER = 2; Employee (int type) { _type = type;}}//Type Code

23. 8.14 Replace Type Code with Subclasses在 Base Class 將行為的 function 抽出來getType() class Employee{abstractintgetType();} class Engineer extends Employee {intgetType() { return Employee.ENGINEER; }}//abstract 強制 subclass 需要 implement// implement

24. 8.14 Replace Type Code with Subclasses修改 factory class class Employee { static Employee create(int type) {switch (type) { case ENGINEER: return new Engineer(); case SALESMAN: return new Salesman(); case MANAGER: return new Manager(); default: ... } }//getType邏輯在各類別中

25. 8.14 Replace Type Code with SubclassesRefactoring 之後把只與各個類別相關的函式放在類別中Push Down MethodPush Down Field

26. 8.15 Replace Type Code with State/Strategy使用時機有一個 type code 他會影響 class 行為無法使用 subclassing，例如 type 可變手法以 state object 取代 type code比較 Replace Type Code with Class

27. 8.15 Replace Type Code with State/Strategy目標：將 type code 以 state object 取代 class Employee{ private int _type; static final int ENGINEER = 0; static final int SALESMAN = 1; static final int MANAGER = 2; Employee (int type) { _type = type;}}//Type Code

28. 8.15 Replace Type Code with State/Strategy觀察：如何使用 type code class Employee{intpayAmount() { switch (_type) { case ENGINEER: return _monthlySalary; case SALESMAN: return _monthlySalary + _commission; case MANAGER: return _monthlySalary + _bonus; default: throw new RuntimeException("Incorrect Employee"); }}}//依照 Type Code 進行運算

29. 8.15 Replace Type Code with State/Strategy設計 state object / 共有 state 的部份使用 abstract class / abstract function abstract class EmployeeType { abstract intgetTypeCode(); } class Engineer extends EmployeeType {intgetTypeCode () { return Employee.ENGINEER; } }//abstract function//subclass implementation

30. 8.15 Replace Type Code with State/Strategy改變 state 時，改變 state object 的 class class Employee... private EmployeeType _type; void setType(intarg) { switch (arg) { case ENGINEER: _type = new Engineer(); break; case SALESMAN: _type = new Salesman(); break; case MANAGER: _type = new Manager(); break; default: ... } }//很像 factory pattern//搬出去？

31. 8.15 Replace Type Code with State/Strategy將 factory method 放到 state object class Employee... void setType(intarg) { _type = EmployeeType.newType(arg); }class EmployeeType... static EmployeeTypenewType(int code) { switch (code) { case ENGINEER: return new Engineer(); case SALESMAN: return new Salesman(); case MANAGER: return new Manager(); default: ... } } static final int ENGINEER = 0; static final int SALESMAN = 1; static final int MANAGER = 2;//setter 使用 state class 的 factory method// state class 的 factory method

32. 8.15 Replace Type Code with State/Strategy以 state object 的 state 進行運算接著可以用 Replace Conditional with Polymorphism class Employee...intpayAmount() { switch (getType()) { case EmployeeType.ENGINEER: return _monthlySalary; case EmployeeType.SALESMAN: return _monthlySalary + _commission; case EmployeeType.MANAGER: return _monthlySalary + _bonus; default:... } }

33. 8.16 Replace Subclass with Fields使用時機Subclass 的 function 是 constant method可以透過回傳一個 field 來取代 constant method

34. 8.16 Replace Subclass with Fields觀察abstract method 都只是回傳 hard-code 資料 abstract class Person { abstract booleanisMale(); abstract char getCode(); ... class Male extends Person {booleanisMale() { return true; } char getCode() { return 'M'; } }//return hard-code//return hard-code

35. 8.16 Replace Subclass with Fields以 Factory Method 隱藏 subclass 類別設定 subclass 的 constructor 為 private未來可以移除 subclass class Person... static Person createMale(){ return new Male(); } static Person createFemale() { return new Female(); }

36. 8.16 Replace Subclass with Fields為 superclass 加上想要取代的 field class Person... protected Person (booleanisMale, char code) { _isMale = isMale; _code = code;}booleanisMale() { return _isMale;}class Male... Male() { super (true, 'M');}//以 Field 取代 subclass 的 function

37. 8.16 Replace Subclass with Fields移除無用的 subclass class Person static Person createMale(){ return new Person(true, 'M'); }//以 factory pattern 抽換類別