Refactoring in Extreme Programming

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Refactoring in Extreme Programming

• Burak Celebi
• burak.celebi_at_gmail.com

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Outline

• Basics of Extreme Programming
• Why Refactoring is important for XP?
• Refactoring
• Conclusion

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Warming up...

• There are two ways of constructing a software
  design.
• One way is to make it so simple that there are
  obviously no eficiencies.
• And the other way is to make it so complicated
  that there are no obvious deficiencies.
• C.A.R. Hoare

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What is Extreme Programming?

• XP is a set of principles and practices that
  guide software development. It is an agile
  process in that it makes every effort to
  eliminate unnecessary work, instead focusing on
  tasks that deliver value to the customer.1
• 1 Extreme Programming Cookbook

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Four Principles in XP

• Simplicity
• Communication
• Feedback
• Courage

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Simplicity

• Perfection (in design) is achieved not when
  there is nothing more to add, but rather when
  there is nothing more to take away. Antoine de
  Saint-Exupéry
• Do The Simplest Thing That Could Possibly Work
• You Aint Gonna Need It (YAGNI)
• Refactoring

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Communication

• Customers
• No Long Design Phase. No heavy documents! Just do
  it..
• CRC Cards
• Pair Programming - pairprogramming.com
• Team Room
• http//www.scissor.com/resources/teamroom/Sdadsad

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Feedback

• Test-First approach.
• Unit Testing
• Integration Test
• Acceptance Tests

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Courge

• Always design and code for today and not for
  tomorrow.
• For managers, Pair Programming can be hard to
  accept.
• Just the code remains from a project.

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Refactoring

• Any fool can write code that
• a computer can understand.
• Good programmers write code that
• humans can understand.
• Martin Fowler

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Outline of Refactoring Section

• What is Refactoring?
• Why You Should Refactor?
• When Sould You Refactor?
• When Shouldnt You Refactor?
• Refactoring Examples
• Refactoring Tools

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What is Refactoring?

• Refacoring is the process of changing a software
  system in such a way that it does not alter the
  external behavior of the code yet improves its
  internal structure.
• It is a disciplined way to clean up code that
  minimizes the chances of introducing bugs.

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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.
• 1 Refactoring Improving The Design of Existing
  Code, Martin Fowler

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Why Sould You Refactor?

• Refactoring Improves the Design of Software
• Refactoring Makes Software Easier to Undersand,
  therefore maintain, extend, verify.
• Refactoring Helps You Find Bugs
• Refactoring Helps You Program Faster

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When Sould You Refactor?

• Refactor When You Add Function
• Refactor When You Need to Fix a Bug
• Refactor As You Do a Code Review

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When Shouldnt You Refactor?

• If the current code does not work.
• If you are close to a deadline.

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What about Managers?

• Who cares?

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Refactoring and Performance

• To make the software easier to understand, you
  often make changes that will cause the program to
  run more slowly.
• However, it makes the software easier to tune
  during optimization
• Result Rapid and clean development

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Bed Smells in the Code

• Duplicated Code
• Long Methods
• Large Classes
• Long Parameter Lists
• Speculative Generality
• Too Many Comments

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Selected Topics from Fowlers Catalog

• Extract Method
• Introduce Explaining Variable
• Preserve Whole Object
• Remove Double Negative
• Replace Conditional with Polymorphism
• Replace Error Code with Exception
• Replace Iteration with Recursion
• Replace Error Code with Exception
• Replace Magic Number with Symbolic Constant
• Replace Method with Method Object
• Replace Error Code with Exception
• Replace Parameter with Method
• Introduce Parameter Object

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Extract Method

• You have a code fragment that can be grouped
  together.
• Turn the fragment into a method whose name
  explains the purpose of the method.
• void printOwing()
• printBanner()
• //print details
• System.out.println ("name " _name)
  System.out.println ("amount "
  getOutstanding())
• void printOwing()
• printBanner()
• printDetails(getOutstanding())
• void printDetails (double outstanding)
• System.out.println ("name " _name)
  System.out.println ("amount " outstanding)

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Introduce Explaining Variable

• You have a complicated expression.
• Put the result of the expression, or parts of the
  expression, in a temporary variable with a name
  that explains the purpose.
• if ( (platform.toUpperCase().indexOf("MAC") gt -1)
  (browser.toUpperCase().indexOf("IE") gt -1)
  wasInitialized()
• resize gt 0 )
• // do something
• final boolean isMacOs platform.toUpperCase().ind
  exOf("MAC") gt -1
• final boolean isIEBrowser browser.toUpperCase().
  indexOf("IE") gt -1
• final boolean wasResized resize gt 0
• if (isMacOs isIEBrowser wasInitialized()
  wasResized)
• // do something

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Preserve Whole Object

• You are getting several values from an object and
  passing these values as parameters in a method
  call.
• Send the whole object instead.
• int low daysTempRange().getLow()
• int high daysTempRange().getHigh()
• withinPlan plan.withinRange(low, high)
• withinPlan lan.withinRange(daysTempRange())

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Remove Double Negative

• You have a double negative conditional.
• Make it a single positive conditional
• if ( !item.isNotFound() )
• if ( item.isFound() )

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Replace Conditional with Polymorphism

• You have a conditional that chooses different
  behavior depending on the type of an object.
• Move each leg of the conditional to an overriding
  method in a subclass. Make the original method
  abstract.
• double getSpeed()
• switch (_type)
• case EUROPEAN
• return getBaseSpeed()
• case AFRICAN
• return getBaseSpeed() - getLoadFactor()
  _numberOfCoconuts
• case NORWEIGIAN_BLUE
• return (_isNailed) ? 0 getBaseSpeed(_voltage
  )
• throw new RuntimeException ("Should be
  unreachable")

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Replace Error Code with Exception

• A method returns a special code to indicate an
  error.
• Throw an exception instead.
• int withdraw(int amount)
• if (amount gt _balance)
• return -1
• else
• _balance - amount
• return 0
• void withdraw(int amount) throws
  BalanceException
• if (amount gt _balance) throw new
  BalanceException()
• _balance - amount

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Replace Iteration with Recursion
You have a loop, and it is not obvious what each
iteration is doing Replace Iteration with
Recursion

• unsigned greatest_common_divisor (unsigned a,
  unsigned b)
• while (a ! b)
• if (a gt b)
• a - b
• else if (b gt a)
• b - a

• unsigned greatest_common_divisor (unsigned a,
  unsigned b)
• if (a gt b)
• return greatest_common_divisor ( a-b, b )
• else if (b gt a)
• return greatest_common_divisor ( a, b-a )
• else // (a b)
• return a

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Replace Error Code with Exception

• A method returns a special code to indicate an
  error.
• Throw an exception instead.
• int withdraw(int amount)
• if (amount gt _balance)
• return -1
• else
• _balance - amount
• return 0
• void withdraw(int amount) throws
  BalanceException
• if (amount gt _balance) throw new
  BalanceException()
• _balance - amount

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Replace Magic Number with Symbolic Constant

• You have a literal number with a particular
  meaning.
• Create a constant, name it after the meaning, and
• replace the number with it.
• double potentialEnergy(double mass, double
  height)
• return mass height 9.81
• double potentialEnergy(double mass, double
  height)
• return mass GRAVITATIONAL_CONSTANT height
• static final double GRAVITATIONAL_CONSTANT
  9.81

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Replace Method withMethod Object
You have a long method that uses local variables
in such a way that you cannot apply Extract
Method Turn the method into its own object so
that all the local variables become fields on
that object. You can then decompose the method
into other methods on the same object.

• class Order...
• double aMethod(int a, Obj o) double
  primaryBasePrice
• double secondaryBasePrice double
  tertiaryBasePrice
• // long computations
• private int otherMethod(int a)
• ...

class Order.. price() return new
PriceCalculator(this).compute() Class
PriceCalculator... double primaryBasePrice double
secondaryBasePrice double tertiaryBasePrice
compute() ... private int otherMethod() ...
// it uses global variables
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Replace Error Code with Exception

• A method returns a special code to indicate an
  error.
• Throw an exception instead.
• int withdraw(int amount)
• if (amount gt _balance)
• return -1
• else
• _balance - amount
• return 0
• void withdraw(int amount) throws
  BalanceException
• if (amount gt _balance) throw new
  BalanceException()
• _balance - amount

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Replace Parameter with Method

• An object invokes a method, then passes the
  result as a parameter for a method. The receiver
  can also invoke this method.
• Remove the parameter and let the receiver invoke
  the method.
• int basePrice _quantity _itemPrice
• discountLevel getDiscountLevel()
• double finalPrice discountedPrice (basePrice,
  discountLevel)
• int basePrice _quantity _itemPrice
• double finalPrice discountedPrice (basePrice)

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Introduce Parameter Object

• You have a group of parameters that naturally go
  together.
• Replace them with an object.
• mountInvoicedIn(start Date, end Date)
• amountReceivedIn(start Date, end Date)
• amountOverdueIn(start Date, end Date)
• amountInvoicedIn(DateRange)
• amountReceivedIn(DateRange)
• amountOverdueIn(DateRange)

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Big Refactorings

• gt These have more influence on software design.
• Tease Apart Inheritance
• Convert Procedural Design to Objects
• Separate Domain from Presentation
• Extract Hierarchy

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Refactoring Tools

• Smalltalk
• Smalltalk Refactoring Browser
• Java
• IntelliJ Idea, Eclipse, RefactorIt, XRefactory,
  etc.
• .Net
• ReSharper, C Refactory, C Refactoring Tool
• C/Cr
• SlickEdit, Ref
• Python
• Bicycle Repair Man

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Thank you..

• I'm not a great programmer I'm just a good
  programmer with great habits.
• Kent Beck, one of the three creators of XP