Lecture 12, part 2: Refactoring. NCCU Fall 2005 Dec. 13, 2005

Transcription

1 Lecture 12, part 2: Refactoring NCCU Fall 2005 Dec. 13,

2 Refactoring 什? 不 行 理 降 行 不 Unit Testing! Martin Fowler (and Kent Beck, John Brant, William Opdyke, Don Roberts), Refactoring- Improving the Design of Existing Code, Addison Wesley,

3 不 行 類 3

4 Refactoring Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing its observable behavior. Refactor (verb): to restructure software by applying a series of refactorings. 4

5 行? Bug Code Review bad smell 5

6 Bad smells 不 說 (bad smell) 易 Bad Smell Peer Review Bad Smell Bad Smell 6

7 Bad Smell Examples Duplicate Code Long Methods Large Classes Long Parameter Lists Too Many Case Statements 數 不良 Code Standard 7

8 Visible refactoring UML 8

9 Invisible refactoring Collection if String foundperson(string[] people){ for (int i = 0; i < people.length; i++) { if (people[i].equals ("Don")){ return "Don"; if (people[i].equals ("John")){ return "John"; if (people[i].equals ("Kent")){ return "Kent"; return ""; String foundperson(string[] people){ List candidates = Arrays.asList(new String[] {"Don", "John", "Kent"); for (int i=0; i<people.length; i++) if (candidates.contains(people[i])) return people[i]; return ""; UML 不 9

10 :Make changes as small as possible. Many small changes are easier than one big change. :Test after each change. 10

11 Test-Driven Start Choose task Y Done? N Write a new, small test Refactor run tests Implement just enough so that the test will fail y n code smells? y green? n implement just enough so that tests should pass 11

12 Refactoring Book Book has a catalogue of: 22 bad smells i.e., things to look out for, anti-patterns 72 refactorings i.e., what to do when you find them. As with GoF book on OODPs, there is overlap between the catalogue items, and the ideas start to blur. 12

13 Typical Refactorings Class Refactorings Method Refactorings Attribute Refactorings add (sub)class to hierarchy add method to class add variable to class rename class rename method rename variable remove class remove method remove variable push method down push method up add parameter to method move method to component push variable down push variable up create accessors abstract variable extract code in new method These simple refactorings can be combined to provide bigger restructurings such as the introduction of design patterns. 13

14 Some Frequently Used Refactorings Extract method Pull-up method Move method Move field Replace temp with query Replace conditional logic with polymorphism Replace type code with Strategy/State pattern Extract class 14

15 Extract method example //identify the problem methoda() { //code <Subroutine> Extract the subroutine: <Subroutine> Put it into a method: methodb() { //code <Subroutine> (similar to subroutine in methoda) //Replace extracted code methoda() { //code methodc(); methodc() { <Subroutine> methodb() { //code methodc(); 15

16 Farm template method Same code in two related classes. Farm template method: Push commonalities into closest mutual ancestor. Ancestor Class A Class B 16

17 Pull Up Method 17

18 Replace Type Code with State/Strategy 18

19 Refactoring, applied Straight from the book: a program to calculate and print a statement of a customer s charges at a video store...price depends on how long the movie is rented and the category of the movie...also compute frequent renter points 19

20 Refactoring: Movie Class diagram of the starting point classes. Movie PriceCode: int Rental 1 * * daysrented: int 1 Customer Statement() 20

21 Refactoring: Movie Class public class Movie { public static final int CHILDREN=2; public static final int REGULARS=0; public static final int NEW_RELEASE=1; private String _title; private int _pricecode; public void setpricecode(int arg) { _pricecode = arg; public String gettitle() { return _title; public Movie(String title, int pricecode) { _title=title; _pricecode = pricecode; public int getpricecode() { return _pricecode; 21

22 Refactoring: Rental Class public class Rental { private Movie _movie; private int _daysrented; public Rental(Movie movie, int daysrented) { _movie = movie; _daysrented = daysrented ; public int getdaysrented() { return _daysrented ; public Movie getmovie() { return _movie; 22

23 Refactoring: Customer Class public class Customer { private String _name; private Vector _rentals =new Vector(); public Customer(String name) { _name = name; public void addrental(rental arg) { _rentals.addelement(arg); public String getname() { return _name; 23

24 Refactoring: Customer Class public class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { double thisamount = 0; Rental each = (Rental) rentals.nextelement(); // determine amounts for each line switch (each.getmovie().getpricecode()) { case Movie.REGULAR: thisamount += 2; if (each.getdaysrented() > 2) thisamount+=(each.getdaysrented()-2) * 1.5; break; 24

25 Refactoring: Customer Class public class Customer public String statement()... case Movie.NEW_RELEASE: thisamount += each.getdaysrented() * 3; break; case Movie.CHILDRENS: thisamount += 1.5; if (each.getdaysrented() > 3) thisamount+=(each.getdaysrented()-3) * 1.5; break; // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; 25

26 Refactoring: Customer Class public class Customer public String statement()... //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(thisAmount) + \n ; totalamount += thisamount; // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 26

27 Refactoring Sequence diagram for the statement method. 27

28 Refactoring: problem statement Add a htmlstatment method which returns a customer statement string containing html tags....and there will be some changes to the way movies are classified...affecting frequent renter points and charging. 28

29 Refactoring: step 1 Write a test suite! Refactoring should not affect the outcome of tests. The test suite must exercise the published interface of the classes. Obviously, refactoring should not affect the published interface. So, avoid publishing interfaces too early. 29

30 Refactoring: step 2 statement() is overly long, apply the Extract Method refactoring public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { double thisamount = 0; Rental each = (Rental) rentals.nextelement(); // determine amounts for each line switch (each.getmovie().getpricecode()) { case Movie.REGULAR: thisamount += 2; if (each.getdaysrented() > 2) thisamount+=(each.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: thisamount += each.getdaysrented() * 3; break; case Movie.CHILDRENS: thisamount += 1.5; if (each.getdaysrented() > 3) thisamount+=(each.getdaysrented()-3) * 1.5; break; 30

31 Refactoring: step 2 public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { double thisamount = 0; Rental each = (Rental) rentals.nextelement(); thisamount = amountfor(each); // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(thisAmount) + \n ; totalamount += thisamount; // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 31

32 Refactoring: step 2 public int amountfor(rental each) { int thisamount = 0; switch (each.getmovie().getpricecode()) { case Movie.REGULAR: thisamount += 2; if (each.getdaysrented() > 2) thisamount+=(each.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: thisamount += each.getdaysrented() * 3; break; case Movie.CHILDRENS: thisamount += 1.5; if (each.getdaysrented() > 3) thisamount+=(each.getdaysrented()-3) * 1.5; break; return thisamount; 32

33 Refactoring: step 3 TEST 33

34 Refactoring: step 4 oops, (double) -> (int) bug! public double amountfor(rental each) { double thisamount = 0; switch (each.getmovie().getpricecode()) { case Movie.REGULAR: thisamount += 2; if (each.getdaysrented() > 2) thisamount+=(each.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: thisamount += each.getdaysrented() * 3; break; case Movie.CHILDRENS: thisamount += 1.5; if (each.getdaysrented() > 3) thisamount+=(each.getdaysrented()-3) * 1.5; break; return thisamount; 34

35 Refactoring: step 5 Variable names not helpful public double amountfor(rental each) { double thisamount = 0; switch (each.getmovie().getpricecode()) { case Movie.REGULAR: thisamount += 2; if (each.getdaysrented() > 2) thisamount+=(each.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: thisamount += each.getdaysrented() * 3; break; case Movie.CHILDRENS: return thisamount; thisamount += 1.5; if (each.getdaysrented() > 3) thisamount+=(each.getdaysrented()-3) * 1.5; break; 35

36 Refactoring: step 5 public double amountfor(rental arental) { double result = 0; switch (arental.getmovie().getpricecode()) { case Movie.REGULAR: result += 2; if (arental.getdaysrented() > 2) result +=(arental.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: result += arental.getdaysrented() * 3; break; case Movie.CHILDRENS: result += 1.5; if (arental.getdaysrented() > 3) result +=(arental.getdaysrented()-3) * 1.5; break; return result ; 36

37 Refactoring: step 6 Moving amount computation (does not use info from Customer, only Rental) class Customer... public double amountfor(rental arental) { double result = 0; switch (arental.getmovie().getpricecode()) { case Movie.REGULAR: result += 2; if (arental.getdaysrented() > 2) result +=(arental.getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: result += arental.getdaysrented() * 3; break; case Movie.CHILDRENS: result += 1.5; if (arental.getdaysrented() > 3) result +=(arental.getdaysrented()-3) * 1.5; break; return result ; 37

38 Refactoring: step 6 class Rental... public double getcharge() { double result = 0; switch (getmovie().getpricecode()) { case Movie.REGULAR: result += 2; if (getdaysrented() > 2) result +=(getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: result += getdaysrented() * 3; break; case Movie.CHILDRENS: return result ; result += 1.5; if (getdaysrented() > 3) result +=(getdaysrented()-3) * 1.5; break; 38

39 Refactoring: step 6 class Customer... public double amountfor(rental arental) { return arental.getcharge(); 39

40 Refactoring: step 7 class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { double thisamount = 0; Rental each = (Rental) rentals.nextelement(); thisamount = amountfor(each); // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(thisAmount) + \n ; totalamount += thisamount; // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 40

41 Refactoring: step 7 class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { double thisamount = 0; Rental each = (Rental) rentals.nextelement(); thisamount = each.getcharge(); // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(thisAmount) + \n ; totalamount += thisamount; // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 41

42 Refactoring State of classes after moving the charge method. amountfor has been deleted. Movie Rental 1 * Customer PriceCode: int * daysrented: int 1 getcharge() Statement() 42

43 Refactoring: step 8 Replace Temp with Query (thisamount is redundant) class Customer... class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; totalamount += each.getcharge(); // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 43

44 Refactoring: step 9 Extract Method (frequent renter computation) class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); // add frequent renter points frequentrenterpoints ++; // add bonus for a two day new release rental if ((each.getmovie().getpricecode() == Movie.NEW_RELEASE)&& each.getdaysrented() > 1) frequentrenterpoints++; //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; totalamount += each.getcharge(); // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 44

45 Refactoring: step 9 class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); frequentrenterpoints += each.getfrequentrenterpoints(); //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; totalamount += each.getcharge(); // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 45

46 Refactoring: step 9 class Rental... public int getfrequentrenterpoints() { if ((getmovie().getpricecode() == Movie.NEW_RELEASE) && getdaysrented() > 1) return 2; else return 1; 46

47 Refactoring Sequence diagram before extraction and movement of the frequent renter points calculation 47

48 Refactoring Sequence diagram after extraction and movement of the frequent renter points calculation getfrequentrenterpoints() 48

49 Refactoring: step 10 Replace Temp with Query (the temporaries make the method complex and force code duplication) class Customer... public String statement() { double totalamount = 0; int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); frequentrenterpoints += each.getfrequentrenterpoints(); //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; totalamount += each.getcharge(); // add footer lines result += Amount owed is +Sting.valueOf(totalAmount) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints) + frequent renter points\n ; return result; 49

50 Refactoring: step 10 class Customer... public String statement() { int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); frequentrenterpoints += each.getfrequentrenterpoints(); //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; // add footer lines result += Amount owed is +Sting.valueOf(getTotalCharge() gettotalcharge()) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints)+ frequent renter points\n ; return result; 50

51 Refactoring: step 10 class Customer... private double gettotalcharge() { double result = 0; Enumeration rentals = _rentals.elements(); while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); result += each.getcharge(); return result; 51

52 Refactoring: step 11 Replace Temp with Query class Customer... public String statement() { int frequentrenterpoints = 0; Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); frequentrenterpoints += each.getfrequentrenterpoints(); //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; // add footer lines result += Amount owed is +Sting.valueOf(getTotalCharge()) + \n ; result += You earned +Sting.valueOf(frequentRenterPoints)+ frequent renter points\n ; return result; 52

53 Refactoring: step 11 Replace Temp with Query class Customer... public String statement() { Enumeration rentals = _rental.elements(); String result = Rental Record for + getname() + \n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); //show figures for this rental result += \t + each.getmovie().gettitle()+ \t + String.valueOf(each.getCharge()) + \n ; // add footer lines result += Amount owed is +Sting.valueOf(getTotalCharge()) + \n ; result += You earned +Sting.valueOf(getFrequentRenterPoints() getfrequentrenterpoints())+ frequent renter points\n ; return result; 53

54 Refactoring: step 11 class Customer... private double getfrequentrenterpoints() { double result = 0; Enumeration rentals = _rentals.elements(); while (rentals.hasmoreelements()) { return result; Rental each = (Rental) rentals.nextelement(); result += each.getfrequentrenterpoints(); 54

55 Diagrams after the Refactorings Movie PriceCode: int Rental 1 * daysrented: int * 1 getcharge() getfrequentrenterpoints() Customer Statement() acustomer arental amovie statement *[for all rentals] getcharge getpricecode getdaysrented getpricecode 55

56 Refactoring Remarks Most refactoring reduce code size, but this is not necessarily the case. The point is to make code easier to modify and more readable. Performance gets a hit by running the same loop three times, or does it? Profile the program and find the answer. 56

57 Software extension (Not Refactoring) The requested method can be added with minimal code duplication. (HTML string) class Customer... public String htmlstatement() { Enumeration rentals = _rental.elements(); String result = <H1>Rental Record for<em> + getname() + <EM></H1><P>\n ; while (rentals.hasmoreelements()) { Rental each = (Rental) rentals.nextelement(); //show figures for this rental result += each.getmovie().gettitle()+ : + String.valueOf(each.getCharge()) + <BR>\n ; // add footer lines result += <P>Amount owed is<em> +Sting.valueOf(getTotalCharge()) + </EM><P>\n ; result += You earned <EM> +Sting.valueOf(getFrequentRenterPoints() getfrequentrenterpoints())+ </EM> frequent renter points<p>\n ; return result; 57

58 A More General Approach Define a method object to encapsulate the statement method in the form of a new Statement class with TextStatement and HtmlStatement subclasses Rename statement method to value within the Statement classes and pass a Customer as argument Add getrentals method to return an Enumeration of current Rentals Relax visibility of gettotalcharge and gettotalfrequentrenterpoints 58

59 New functionality Getting ready to change the classification of the movies in the store. Perhaps new classification, perhaps modification to existing. Charging and frequent renting will be affected. 59

60 Refactoring: step 12 Replacing conditional logic (switch-case) on Price Code with OO polymorphism 60

61 Refactoring: step 12 Move getcharge from Rental to Movie class Rental... public double getcharge() { double result = 0; switch (getmovie().getpricecode()) { case Movie.REGULAR: result += 2; if (getdaysrented() > 2) result +=(getdaysrented()-2) * 1.5; break; case Movie.NEW_RELEASE: result += getdaysrented() * 3; break; case Movie.CHILDRENS: result += 1.5; if (getdaysrented() > 3) result +=(getdaysrented()-3) * 1.5; break; return result ; 61

62 Refactoring: step 12 class Movie... public double getcharge(int daysrented) { double result = 0; switch (getpricecode()) { case REGULAR: result += 2; if (getdaysrented() > 2) break; case NEW_RELEASE: result +=(getdaysrented()-2) * 1.5; result += getdaysrented() * 3; break; case CHILDRENS: return result ; result += 1.5; if (getdaysrented() > 3) break; result +=(getdaysrented()-3) * 1.5; 62

63 Refactoring: step 12 class Rental... public double getcharge() { return _movie.getcharge(_daysrented); 63

64 Refactoring: step 13 Move getfrequentrenterpoints() to Movie class Rental... public int getfrequentrenterpoints() { if ((getmovie().getpricecode() == Movie.NEW_RELEASE) else && getdaysrented() > 1) return 2; return 1; 64

65 Refactoring: step 13 class Movie... public int getfrequentrenterpoints(int daysrented) { if ((getpricecode() == Movie.NEW_RELEASE) && daysrented > 1) else return 2; return 1; class Rental... public int getfrequentrenterpoints() { return _movie.getfrequentrenterpoints(_daysrented); 65

66 Refactoring: Using Inheritance Inheritance among movie classes Won t work. A movie can change its classification during its lifetime. But an object cannot change its class! Movie getcharge getfrequent ChildrensMovie getcharge RegularMovie getcharge NewRelease getcharge getfrequent 66

67 Refactoring Using Patterns State/Strategy patterns A movie has a price object Movie PriceCode: int getcharge getfrequent 1 Price getcharge getfrequent ChildrensPrice getcharge RegularPrice getcharge NewReleasePrice getcharge getfrequent 67

68 Movie PriceCode: int getcharge getfrequent Replace Type Code with State/Strategy 1 Price getcharge getfrequent 1 ChildrensPrice RegularPrice getcharge getcharge NewReleasePrice getcharge getfrequent Rental daysrented: int getcharge() getfrequent * Customer Name: String Statement() htmlstatement() 68

69 Refactoring: step 14 Replace Type Code with State/Strategy class Movie... public Movie(String name, int pricecode) { _name = name; _pricecode = pricecode; 69

70 Refactoring: step 14 class Movie... public Movie(String name, int pricecode) { _name = name; setpricecode(pricecode); 70

71 Refactoring: step 14 abstract class Price { abstract int getpricecode(); class ChildrenPrice extends Price { int getpricecode(){ return MOVIE.CHILDREN; class NewReleasePrice extends Price { int getpricecode(){ return MOVIE.NEW_RELEASE; class RegularPrice extends Price { int getpricecode(){ return MOVIE.REGULAR; 71

72 Refactoring: step 15 class Movie... public int getpricecode() { return _pricecode; public void setpricecode(int arg) { _pricecode = arg; private int _pricecode; 72

73 Refactoring: step 15 class Movie... public int getpricecode() { return _price.getpricecode; public void setpricecode(int arg) { switch (arg) { case REGULAR: _price = new RegularPrice(); break; case CHILDREN: _price = new ChildrenPrice(); break; case NEW_RELEASE: _price = new NewReleasePrice(); break; default: throw new IllegalArgumentException( Incorrect Incorrect Price Code ); private Price _price; 73

74 Refactoring: step 16 Move Method: move switch-case to a method class Movie... public double getcharge(int daysrented) { double result = 0; switch (getpricecode()) { case REGULAR: result += 2; if (getdaysrented() > 2) result +=(getdaysrented()-2) * 1.5; break; case NEW_RELEASE: result += getdaysrented() * 3; break; case CHILDRENS: result += 1.5; if (getdaysrented() > 3) result +=(getdaysrented()-3) * 1.5; break; return result ; 74

75 Refactoring: step 16 class Movie... public double getcharge(int daysrented) { return _price.getcharge(daysrented); 75

76 Refactoring: step 16 Replace Conditional with Polymorphism class Price... double getcharge(int daysrented) { double result = 0; switch (getpricecode()) { case MOVIE.REGULAR: result += 2; if (getdaysrented() > 2) result +=(getdaysrented()-2) * 1.5; break; case MOVIE.NEW_RELEASE: result += getdaysrented() * 3; break; case MOVIE.CHILDRENS: result += 1.5; if (getdaysrented() > 3) result +=(getdaysrented()-3) * 1.5; break; return result ; 76

77 Refactoring: step 16 class RegularPrice... double getcharge(int daysrented) { double result = 2; if (getdaysrented() > 2) result +=(getdaysrented()-2) * 1.5; return result; class NewReleasePrice... double getcharge(int daysrented) { return daysrented * 3; class ChildrenPrice... double getcharge(int daysrented) { double result = 1.5; if (getdaysrented() > 3) result +=(getdaysrented()-3) * 1.5; return result ; class Price... abstract double getcharge(int daysrented); Three kinds of Prices 77

78 Refactoring: step 17 Replace Conditional with Polymorphism class Rental... int getfrequentrenterpoints(int daysrented) { if ((getpricecode() == Movie.NEW_RELEASE) && daysrented > 1) else return 2; return 1; 78

79 Refactoring: step 17 class Movie... int getfrequentrenterpoints(int daysrented) { return _price.getfrequentrenterpoints(daysrented); class Price... int getfrequentrenterpoints(int daysrented) { return 1; class NewReleasePrice.. int getfrequentrenterpoints(int daysrented) { //override return (daysrented > 1)? 2:1; 79

80 Refactoring Principles Why do we refactor? To improve the design of software To make software easier to understand To help you find bugs To make you program faster When should we refactor? 1. Refactor when you add functionality 2. Refactor when you need to fix a bug 3. Refactor as you do code reviews Refactor when the code starts to smell. What about performance? What about performance? Worry about performance only when you have identified a performance problem 80

81 Effective refactoring Use unit testing Test -> Refactor -> Test Use automated refactoring tools (e.g., Eclipse) Don t twiddle manually Plan your refactoring ahead of time Complex software requires complex refacotring planning 81

82 Eclipse & Refactoring 82

83 Some Bad Code Smells 83

84 Bad Smells in Code If it stinks, change it. --Grandma Beck on child rearing Duplicated Code (stench 10) If the same code structure is repeated Extract Method- gather duplicated code Pull Up Field - move to a common parent Form Template Method - gather similar parts, leaving holes Substitute Algorithm - choose the clearer algorithm Extract class - for unrelated classes, create a new class with functionality 84

85 Duplication: Example #1 public boolean save() throws IOException{ if (outputfile == null) {return false; FileWriter write = new FileWriter(outputFile); movies.writeto(writer); writer.close(); return true; public boolean saveas() throws IOException{ outputfile = view.getfile(); if (outputfile == null) {return false; FileWriter write = new FileWriter(outputFile); movies.writeto(writer); writer.close(); return true; 85

86 Duplication: Example #2 public class MovieList { private int numberofmovies = 0; private Collection movies = new ArrayList(); public size() { return movies.size(); public void add(movie movietoadd){ movies.add(movietoadd); numberofmovies++; 86

87 Duplication: Example #2 - cont public class MovieList { private int numberofmovies = 0; // delete private Collection movies = new ArrayList(); public size() { return movies.size(); public void add(movie movietoadd){ movies.add(movietoadd); numberofmovies++; //delete 87

88 Bad Smells in Code Long Method (stench 7) If the body of a method is over a page (choose your page size) Extract Method - extract related behavior Replace Temp with Query - remove temporaries when they obscure meaning Introduce Parameter Object - slim down parameter lists by making them into objects Replace Method with Method Object - still too many parameters Decompose Conditionals - conditional and loops can be moved to their own methods 88

89 Bad Smells in Code Large Class (stench 7) If a class has either too many variables or too many methods Extract Class- to bundle variables/methods 89

90 Bad Smells in Code Long Parameter List (stench 6) A method does not need many parameter, only enough to be able to retrieve what it needs Replace Parameter with Method - turn a parameter into a message Introduce Parameter Object - turn several parameters into an object Example: Replace public void marry(string name, int age, boolean gender, String name2, int age2, boolean gender2) {... with public void marry(person person1, Person person2) {... 90

91 Bad Smells in Code Divergent Change (stench 5) If you find yourself repeatedly changing the same class then there is probably something wrong with it Extract Class - group functionality commonly changed into a class 91

92 Bad Smells in Code Switch Statements (stench 5) Switch statements lead to duplication and inhibit change Extract method -to remove the switch Move method - to get the method where polymorphism can apply Replace Type Code with State/Strategy -set up inheritance Replace Conditional with Polymorphism - get rid of the switch 92

93 Bad Smells in Code Feature Envy (stench 6) If a method seems more interested in a class other than the class it actually is in Move Method - move the method to the desired class Extract Method - if only part of the method shows the symptoms 93

94 Bad Smells in Code Parallel Inheritance Hierarchies (stench 6) If when ever you make a subclass in one corner of the hierarchy, you must create another subclass in another corner Move Method/Field - get one hierarchy to refer to the other 94

95 Bad Smells in Code Lazy Class (stench 4) If a class (e.g. after refactoring) does not do much, eliminate it. Collapse Hierarchy- for subclasses Inline Class - remove a single class 95

96 Bad Smells in Code Temporary Field (stench 3) If a class has fields that are only set in special cases, extract them Extract Class- for the special fields 96

97 Bad Smells in Code Message Chains (stench 3) BazObject b =foo.getbar().getbaz() Long chains of messages to get to a value are brittle as any change in the intermittent structure will break the code Hide Delegate - remove one link in a chain Extract Method - change the behavior to avoid chains As an example of Hide Delegate, we can introduce the following method into foo s class: BazObject getbaz() { return bar.getbaz(); And then we can just call BazObject b = foo.getbaz(); 97

98 Bad Smells in Code Middle Man (stench 3) An intermediary object is used too often to get at encapsulated values Remove Middle Man - to talk directly to the target Replace Delegation with Inheritance - turns the middle man into a subclass of the real object 98

99 Bad Smells in Code Inappropriate Intimacy (stench 5) Classes may make too much use of each other s fields and methods Move Method/Field - to separate pieces in order to reduce intimacy Extract Class - make a common class of shared behavior/data Replace Inheritance with Delegation -when a subclass is getting too cozy 99

100 Bad Code Smells: Inappropriate Intimacy // class Movielist public void writeto(writer destination) throws IOException{ Iterator movieiterator = movies.iterator(); while movieiterator.hasnext()) { Movie movietowrite = (Movie)movieIterator.next(); destination.write(movietowrite.getname()); destination.write( ); destination.write(movietowrite.getcategory().tostring()); destination.write( ); try{ destination.write(integer.tostring(movietowrite.getrating())); catch (UnratedException ex) { destination.write( -1 ); destination.write( \n ); 100

101 Bad Code Smells Inappropriate Intimacy A class knows too much about another s internal details WriteTo in MovieList has and uses full knowledge of the structure of Movie If the structure of Movie changes (e.g. multiple ratings), WriteTo in MovieList must change to match Move the loop body into a new writeto method in Movie // class Movielist public void writeto(writer destination) throws IOException{ Iterator movieiterator = movies.iterator(); while movieiterator.hasnext()) { Movie movietowrite = (Movie)movieIterator.next(); movietowrite.writeto(destination); 101

102 The rule of three (XP) The first time you code a task, just do it. The second time you code the same idea, wince and code it up again. The third time you code the same idea, it s time to refactor! Any programming construct can be made more abstract but that s not necessarily a good thing. Generality (flexibility) costs too Don t spin you wheels designing and coding the most abstract system you can imagine. Practise Just-in-Time abstraction. Expect that you will be re-arranging your code constantly. Don t worry about it. Embrace it. 102