Refactoring

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Refactoring

• Dr. Dan Powell

SQI, Griffith University
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What is Refactoring

• When you recognize that the design, tests, or
  code proper have structural flaws, you need to
  refactor.
• Refactoring is the process of changing a software
  system such that the external behavior of the
  system does not change
• functional requirements are maintained but the
  internal structure of the system is improved
• that is, you do not modify the feature set at all

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Why?

• To improve the quality of the codebase
• Makes software easier to understand, therefore
  maintain, extend, verify,...
• It is difficult to get a design right the first
  time (iterative and incremental)
• Systems evolve maintenance is highest cost in
  SDLC.

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What refactoring isnt

• Refactoring is not an opportunity to add features
• Refactoring is not performance optimisation which
  often leads to code that is harder to understand.

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When?

• Rule of three
• If you do the same thing the same three times in
  an implementation its time to refactor to remove
  duplication.
• Refactor when you add new functionality
• before to make it easier to incorporate new
  function
• after to clean up
• If the code is not structured conveniently to add
  a feature, first refactor the program to make it
  easy to add the feature, then add the feature
• Refactor when you fix a defect
• Refactor as part of code review process (edit)

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When not to?

• When it is easier to start again
• Refactoring often requires changes to the
  interface, so refactoring is not a good idea
  when
• you do not own all of the calling code
• you have published the interface for other
  clients use
• Many refactorings have an adverse effect on
  efficiency (time/resource) if efficiency is a
  higher architectural priority than
  maintainability, dont apply refactorings if they
  will effect efficiency.
• This is rarely the case these days (exceptions
  include things like embedded and/or real-time
  systems).

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Safe Refactoring

• Use refactoring patterns
• Fowler et. al., Refactoring Improving the
  Design of Existing Code
• Test constantly
• Ensure you have an adequate set of tests before
  refactoring
• Make sure you are 100 compliant with tests
  before refactoring
• Refactor
• Make sure you are 100 compliant with tests after
  refactoring
• If program fails a test, you know about it
  straight away

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An example Refactoring

• Encapsulate Field
• To make clients of an object use methods to
  access a field rather than the field directly
• Encapsulate Field (cookbook)
• Create accessor and mutator methods for the field
• Locate all references to the field and replace
  all calls to field with calls to accessor and all
  changes to the field with calls to the mutator.
• Compile and test after changing each reference
• Declare the field as private
• Compile and test
• Refactoring works in tiny steps

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An example Refactoring

• Original Code
• public class Person public String name
• Original Client
• ...
• Person person
• person.name Joe Bloggs
• assertEquals(Joe Bloggs, person.name)
• ...

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An example Refactoring

• Step 1 Create Accessors and Mutators
• public class Person
• public String name
• public String getName() return name
• public void setName(String n) name n
• Client
• ...
• Person person
• person.name Joe Bloggs
• assertEquals(Joe Bloggs, person.name)
• ...

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An example Refactoring

• Step 2 Replace direct references with accessors
  and mutators
• public class Person
• public String name
• public String getName() return name
• public void setName(String n) name n
• New Client
• ...
• Person person
• person.setName(Joe Bloggs)
• assertEquals(Joe Bloggs, person.name)
• ...

Compile and Test
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An example Refactoring

• Step 2 Replace direct references with accessors
  and mutators
• public class Person
• public String name
• public String getName() return name
• public void setName(String n) name n
• New Client
• ...
• Person person
• person.setName(Joe Bloggs)
• assertEquals(Joe Bloggs, person.getName())
• ...

Compile and Test
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An example Refactoring

• Step 3 Make Field private
• public class Person
• private String name
• public String getName() return name
• public void setName(String n) name n
• New Client
• ...
• Person person
• person.setName(Joe Bloggs)
• assertEquals(Joe Bloggs, person.getName())
• ...

Compile and Test
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How? Look for Bad Smells - Fowler

• Duplicated Code
• Long methods
• Large Classes
• Long parameter lists
• Divergent Change
• Shotgun Surgery
• Feature Envy
• Data Clumps
• Primitive Obsession
• Switch statements
• Parallel Inheritance Hierarchies

• Lazy Class
• Speculative Generality
• Temporary Field
• Message Chain
• Middle Man
• Inappropriate Intimacy
• Alternate classes different names
• Incomplete Library Class
• Data Class
• Refused Bequest
• Comments

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Some Common Refactorings

• Extract Method
• Replace Temp with Query
• Move Method

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Extract Method

• When?
• Duplicate Code
• extract out common code into separate method
• Long Method
• extract a number of smaller, more cohesive
  methods
• Comments
• extract individually commented blocks to a
  separate method

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Extract Method - Example

• public double correlation(double x, double
  y, int len)
• // calculate sum of xy products
• int i 0 double sumProd 0
• while (i lt len)
• sumProd sumProd xiyi
• ...
• // calculate sum of squares x
• i 0 double sumSquaresX 0
• while (i lt len)
• sumSquaresX sumSquaresX xixi
• // calculate sum of squares y
• i 0 double sumSquaresY 0
• while (i lt len)
• sumSquaresY sumSquaresY yiyi
• ...

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Extract Methods

• Smells, Symtoms and Refactorings
• Comments in code commenting blocks
• extract method extract separate methods for
  each block
• Duplicate Code
• extract method extract common code to a single
  method

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Original - Comments

• // calculate sum of xy products
• int i 0 double sumProd 0
• while (i lt len)
• sumProd sumProd xiyi

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Refactoring - Comments

• public double sumProd( double x,double y,
  int len )
• int i 0 double sumProd 0
• while (i lt len)
• sumProd sumProdxiyi
• return sumProd
• double sumProd sumProd(x,y,len)

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Original Duplicate Code

• // calculate sum of squares x
• i 0 double sumSquaresX 0
• while (i lt len)
• sumSquaresXsumSquaresXxixi
• // calculate sum of squares y
• i 0 double sumSquaresY 0
• while (i lt len)
• sumSquaresYsumSquaresYyiyi

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Refactoring Duplicate Code

• public double sumSquares(double a, int len )
• int i 0 double sumSquares 0
• while (i lt len)
• sumSquaressumSquaresaiai
• return sumSquares
• double sumSquaresXsumSquares(x,len)
• double sumSquaresYsumSquares(y,len)

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Refactoring

• public double correlation(double x, double
  y, int len)
• ...
• double sumProd sumProd(x,y,len)
• double sumSquaresX sumSquares(x,len)
• double sumSquaresY sumSquares(y,len)
• ...

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Long Method

• Symptoms
• Large number of lines in a method (some people
  say over 5 to 10 is too many)
• Causes
• Doing more than one cohesive thing in a method
• Refactoring
• Extract Method to break up method to smaller
  methods

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Long Method Example

• public static void report(Writer out, List
  machines, Robot robot)
• throws IOException
• out.write("FACTORY REPORT\n")
• Iterator line machines.iterator()
• while (line.hasNext())
• Machine machine (Machine) line.next()
• out.write("Machine " machine.name())
• if (machine.bin() ! null)
• out.write(" bin" machine.bin())
• out.write("\n")
• out.write("\n")
• out.write("Robot")
• if (robot.location() ! null)
• out.write(" location"
  robot.location().name())
• if (robot.bin() ! null)
• out.write(" bin" robot.bin())
• out.write("\n")
• out.write("\n")

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Long Method Example

• public static void report(Writer out, List
  machines, Robot robot)
• throws IOException
• out.write("FACTORY REPORT\n")
• Iterator line machines.iterator()
• while (line.hasNext())
• Machine machine (Machine) line.next()
• out.write("Machine " machine.name())
• if (machine.bin() ! null)
• out.write(" bin" machine.bin())
• out.write("\n")
• out.write("\n")
• out.write("Robot")
• if (robot.location() ! null)
• out.write(" location"
  robot.location().name())
• if (robot.bin() ! null)
• out.write(" bin" robot.bin())
• out.write("\n")
• out.write("\n")

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Long Method Example

• public static void report(Writer out, List
  machines, Robot robot) throws IOException
• reportHeader(out)
• reportMachines(out, machines)
• reportRobot(out, robot)
• reportFooter(out)

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Long Parameter List

• Symptoms
• A method has many parameters (some say more than
  one or two)
• Causes
• Attempt to minimise coupling
• Genralised algorithm
• Refactoring
• If parameter can be obtained from another object
  already known to this one, Replace Parameter with
  Method.
• If the parameters come from a single object, try
  Preserve Whole Object
• If the data is not from one logical object, maybe
  group them with Introduced Parameter

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Long Parameter List - Example

• public void paintComponent(Graphics gr, Component
  renderer, Container parent, int x, int y, int
  width, int height, Boolean shouldValidate)
• Introduce Parameter Object
• public void paintComponent(Graphics gr, Component
  renderer, Container parent, Rectangle rect,
  Boolean shouldValidate)

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Replace Temp with Query

• Symptoms
• Using temporary variable to hold result of
  calculation (expression)
• Refactoring
• Extract expression to method and replace all
  references to temp with call to method.

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Replace Temp with Query e.g.

• public double correlation(double x, double
  y, int len)
• ...
• double sumProd sumProd(x,y,len)
• double sumSquaresX sumSquares(x,len)
• double sumSquaresY sumSquares(y,len)
• ...
• return (lensumProd sumXsumY) /
• (squareRoot( (lensumSquareX square(sumX))
  
• (lensumSquareY square(sumY) ))

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Replace Temp with Query - Refactoring
public double correlation(double x, double
y, int len) return (lensumProd(x,y,len)
sum(x,len)sum(y,len)) / (squareRoot(
(lensumSquare(x,len)- square(sum(x,len)))
(lensumSquare(y,len)- square(sum(y,len)))
))
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Move Method

• Symptoms
• A method is using more features (attributes and
  operations) of another class than the class on
  which it is defined
• Causes
• Often caused as a result of other refactoring
• Refactoring
• Create a new method with a similar body in the
  class it uses most.
• Either turn the old method into a simple
  delegation, or remove it altogether

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Move Method Example

• public class Report
• public static void report(Writer out, List
  machines, Robot robot)
• throws IOException
• reportHeader(out)
• reportMachines(out, machines)
• reportRobot(out, robot)
• reportFooter(out)
• ...
• private void reportRobot(Writer out, Robot
  robot)
• out.write("Robot")
• if (robot.location() ! null)
• out.write(" location"
  robot.location().name())
• if (robot.bin() ! null)
• out.write(" bin" robot.bin())
• out.write("\n")

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Move Method Example

• Move method to Robot Class
• public class Robot
• ...
• public void reportRobot(Writer out)
• out.write("Robot")
• if (location() ! null)
• out.write(" location"
• location().name())
• if (bin() ! null)
• out.write(" bin" bin())
• out.write("\n")
• ...

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Move Method Example

• Change call in client
• public class Report
• public static void report(Writer out, List
  machines, Robot robot)throws IOException
• reportHeader(out)
• reportMachines(out, machines)
• robot.reportRobot(out)
• reportFooter(out)
• ...
• private void reportRobot(Writer out, Robot
  robot)
• ...

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Move Method Example

• Remove method from client
• public class Report
• public static void report(Writer out, List
  machines, Robot robot) throws IOException
• reportHeader(out)
• reportMachines(out, machines)
• robot.reportRobot(out)
• reportFooter(out)

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Replace Conditional withPolymorphism

• Symptoms
• You have a conditional that chooses different
  behavior depending on the type of an object
• Refactoring
• Move each leg of the conditional to an
  overriding method in a subclass.
• Make the original method abstract

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Refactoring

• Get rid of the switch statement
• class Movie double getCharge( int
  daysRented ) double result 0
  switch (getPriceCode()) case
  Movie.REGULAR result 2
  if (daysRented gt 2)
  result (daysRented - 2) 1.5
  break case Movie.NEW_RELEASE
  result daysRented 3
  break case Movie.CHILDRENS
  result 1.5 if
  (daysRented gt 3) result
  (daysRented - 3) 1.5 break
  return result

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Refactoring

• ?

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Refactoring

• Flaw
• A movie may change its classification during its
  lifetime.
• An object cannot change its class during its
  lifetime.

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Refactoring

• Solution
• Use State design pattern.

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Refactoring

• Replace price (type) code
• Apply Replace Type Code with State.
• Compile and test after each step.

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Refactoring

• public class Movie private int
  _priceCode public Movie( String title, int
  priceCode ) _title title
  _priceCode priceCode public int
  getPriceCode() return _priceCode
  public void setPriceCode( int arg )
  _priceCode arg

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Refactoring

• public class Movie private int
  _priceCode public Movie( String title, int
  priceCode ) _title title
  setPriceCode(priceCode) public int
  getPriceCode() return _priceCode
  public void setPriceCode( int arg )
  _priceCode arg

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Refactoring

• Add new state classesabstract class Price
  public abstract int getPriceCode()class
  RegularPrice extends Price public int
  getPriceCode() return Movie.REGULAR
  class NewReleasePrice extends Price
  public int getPriceCode() return
  Movie.NEW_RELEASE class ChildrensPrice
  extends Price public int getPriceCode()
  return Movie.CHILDRENS

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Refactoring

• Replace price type codes with instances of price
  state classes

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Refactoring

• public class Movie private int
  _priceCode public int getPriceCode()
  return _priceCode public
  void setPriceCode( int arg ) _priceCode
  arg

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• public class Movie private Price
  _price public int getPriceCode()
  return _price.getPriceCode()
  public void setPriceCode( int arg )
  switch (arg) case REGULAR
  _price new RegularPrice()
  break case NEW_RELEASE
  _price new NewReleasePrice()
  break case CHILDRENS
  _price new ChildrensPrice()
  break default
  throw new IllegalArgumentException( Incorrect
  price code )

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• class Movie double getCharge( int
  daysRented ) double result 0
  switch (getPriceCode()) case
  Movie.REGULAR result 2
  if (daysRented gt 2)
  result (daysRented - 2) 1.5
  break case Movie.NEW_RELEASE
  result daysRented 3
  break case Movie.CHILDRENS
  result 1.5 if
  (daysRented gt 3) result
  (daysRented - 3) 1.5 break
  return result

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Refactoring

• Move getCharge() to Price class
• Apply Move Method.

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• class Price double getCharge( int
  daysRented ) double result 0
  switch (getPriceCode()) case
  Movie.REGULAR result 2
  if (daysRented gt 2)
  result (daysRented - 2) 1.5
  break case Movie.NEW_RELEASE
  result daysRented 3
  break case Movie.CHILDRENS
  result 1.5 if
  (daysRented gt 3) result
  (daysRented - 3) 1.5 break
  return result

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• class Movie double getCharge( int
  daysRented ) return _price.getCharge(
  daysRented )

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• class Price double getCharge( int
  daysRented ) double result 0
  switch (getPriceCode()) case
  Movie.REGULAR result 2
  if (daysRented gt 2)
  result (daysRented - 2) 1.5
  break case Movie.NEW_RELEASE
  result daysRented 3
  break case Movie.CHILDRENS
  result 1.5 if
  (daysRented gt 3) result
  (daysRented - 3) 1.5 break
  return result

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Refactoring

• Replace switch statement in getCharge()
• For each case, add overriding method.
• Define abstract method.
• Apply Replace Conditional with Polymorphism.

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• class RegularPrice double getCharge( int
  daysRented ) double result 2
  if (daysRented gt 2) result
  (daysRented - 2) 1.5 return result
  class NewReleasePrice double
  getCharge( int daysRented ) return
  daysRented 3 class ChildrensPrice
  double getCharge( int daysRented )
  double result 1.5 if (daysRented gt 3)
  result (daysRented - 3) 1.5
  

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• class Price abstract double
  getCharge( int daysRented )

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Refactoring

• Replace if statement in getFrequentRenterPoints()
  
• Apply Replace Conditional with Polymorphism.

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Refactoring

• class Price int getFrequentRenterPoin
  ts( int daysRented ) if
  ((getPriceCode() Movie.NEW_RELEASE)
  daysRented gt 1) return 2
  else return 1

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Refactoring

• class Price int getFrequentRenterPoin
  ts( int daysRented ) return 1
  class NewReleasePrice int
  getFrequentRenterPoints( int daysRented )
  return (daysRented gt 1) ? 2 1

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Refactoring

• Result of state design pattern
• easy to change price behavior
• can add new price codes
• rest of application does not know about this use
  of the state pattern

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Refactoring

• Benefits of second refactoring
• easy to change movie classifications
• easy to change rules for charging and frequent
  renter points

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Refactoring
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References

• Refactoring
• M. Fowler et al.
• Addison-Wesley, 1999
• Refactoring Workbook
• W. Wake
• Addison-Wesley, 2003
• http//xp123.com/rwb/ - Sample chapter, exercises
  and sample code that needs refactoring