Introduction to Refactoring

About Me

Vorleak ChyEmail (vorleak.chy@gmail.com)Yoolk Inc. (http://www.yoolk.com)Blog (http://vorleakchy.com)Rails Developer.NET Developer

What do we talk about Refactoring?

What? Why?

When? How?

Get Agile – Refactoring

Practices

Tools

What is Refactoring?

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.

Fowler, et al., Refactoring, 1999.

Before

Unreadable codeDuplicated codeComplex codeHard to modify… More

After

Easier to understandClean codeBetter codeCheaper to modify… More

Why do we Refactor?

The realityExtremely difficult to get the design “right” the first timeHard to fully understand the problem domainHard to understand user requirements, even if the user does!Hard to know how the system will evolve in five yearsOriginal design is often inadequateSystem becomes brittle over time, and more difficult to change

Why do we Refactor? (Cont.)

Helps us to deliver more business values fasterImprove code structure and design

Easier to maintain and understandEasier to modifyMore flexibilityEasier to add new featuresIncreased re-usability

Why do we Refactor? (Cont.)

Keep development at speedTo make the software easier to understand

Write for people, not the compilerUnderstand unfamiliar code

To help us find bugsRefactor while debugging to clarify the code

Readability

Which code segment is easier to read?

if (date.before(Summer_Start) || date.after(Summer_End))charge = quantity * winterRate + winterServiceCharge;

elsecharge = quantity * summerRate;

Sample 1

Sample 2if (isSummer(date))

charge = summerCharge(quantity);else

charge = winterCharge(quantity);

When should we refactor?

Add new featuresFix bugsDuring code reviewOnly refactor when refactoring. Do not add features during refactoring

When Not to Refactor

Sometimes you should throw things out and start againSometimes you are too close to a deadline

Costs of Not Refactoring

Bad code usually takes more code to do the same thing often because of duplication

How do we Refactor?

We looks for Code-SmellsThings that we suspect are not quite right or will cause us severe pain if we do not fix

Common Code Smells

Duplicated codeFeature EnvyCommentsLong MethodLong parameter listSwitch Statements

Duplicated code

There is obvious duplicationSuch as copy and paste

There are unobvious duplicationsSuch as parallel inheritance hierarchiesSimilar algorithms

RemedyExtract MethodPull Up Field

Feature Envy

A method that seems more interested in some other classes than the one it is inRemedy

Move MethodExtract Method

Extract Methodvoid printOwning(double amount){

printBanner();

// print detailsSystem.Console.WriteLine(string.Format(“name: “, name);System.Console.WriteLine(string.Format(“amount: “, amount);

}

void printOwning(double amount){printBanner();printDetails(amount);

}

void printDetails(double amount){System.Console.WriteLine(string.Format(“name: “, name);System.Console.WriteLine(string.Format(“amount: “, amount);

}

Comments

Comments – be suspicious!Comments represent a failure to express an idea in the codeRemedy

Extract MethodRename Method

Long Method

Good OO code is easiest to understand and maintain with shorter methods with good namesLong methods tend to be harder to read and understandRemedy

Extract MethodReplace Temp with QueryReplace Method with Method ObjectDecompose Conditional

Replace Temp with Query

double basePrice = _quanity *_itemPrice;

if(basePrice > 1000) {return basePrice * 0.95;

}else {

return basePrice * 0.98;}

if(getBasePrice() > 1000) {return getBasePrice() * 0.95;

}else {

return getBasePrice() * 0.98;}

double getBasePrice() {return _quanitiy * _itemPrice;

}

Long parameter list

Functions should have as few parameters as possibleRemedy

Replace Parameter with MethodPreserve Whole ObjectIntroduce Parameter Object

Introduce Parameter Object

amountInvoicedIn(Date start, Date end)amountRecivedIn(Date start, Date end)amountOverdueIn(Date start, Date end)

Customer

amountInvoicedIn(DateRange range)amountRecivedIn(DateRange range)amountOverdueIn(DateRange range)

Customer

Switch Statements

Type cases are evil because they tend to be duplicated many timesRemedy

Replace Type Code with SubclassesReplace Type Code with State / StrategyReplace Conditional with PolymorphismReplace Parameter with Explicit MethodsIntroduce Null Object

Replace Parameter with Explicit Methods

void setValue(String name, int value){

if(name.Equals(“height”)){

_height = value;return;

}if(name.Equals(“width”)){ _width = value;

return; }

}

void setHeight(int value){

_height = value;}

void setWidth(int value){

_width = value;}

Refactoring Cycle

Choose on appropriate Refactoring

to apply

Apply Refactoring

Run ALL Tests (Get GREEN

Bar)

No

Yes

Reached Desired

Structure?

Demo

Q & A

Essential Reading

Thank you!