agile software development lab spring 2008 r o o t s 1 sina golesorkhi([email protected])...

Agile Software Development Lab Spring 2008 R O O T S

1

Sina Golesorkhi([email protected])Thomas Schmickler([email protected])

XP: Extreme ProgrammingIntroduction into Refactoring, Refactoring to Patterns and Code Quality

Refactoring


2

Overview

Definition An Example in Eclipse Short Description Precondition of Refactoring Checklist Advantages of Refactoring Indicators for Refactoring are called „Bad Smells“


3

DefintionWhat is Refactoring ?

Refactor (verb): to restructure software by applying a series of refactorings.

Goals:

• Better readability, comprehensibility• Better design• Better maintainability and reusability

Refactoring (noun): a change made to the internal structure of software to make it easier to understand and cheaper to modify without changing ist observable behavior.


4

Example in Eclipse

You can download the code from http://www.schmickler.de/code.zip


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Short Description Better readability, comprehensibility

Sourcecode is for humans ! Bytecode for the machines.


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Some developers are proud of their cryptic code.


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Some developers are proud of their cryptic code.Some developers think they write the code only for themself.


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To maintain,reuse or extend a programm the code must be understood before from the programmer.


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-




To maintain,reuse or extend a programm the code must be understood before from the programmer.

We want to make good software not good documentationpapers.To understand what the code does it must be good comprehensible.Comprehesible code doesn't need to be documented comprehensive.


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Short Description Better design

Beginning with minimal or comprehensive Design


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Minimal (like in XP):Design grows with the Code. To get a good design we need to refactor.


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Given Design (Traditional SE):Good Design is given before coding begins. Nice. But with evolution the design changes. Maybe then it is not more good.


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What to do to keep the good design ?


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What to do to keep the good design ?

Refactor after or before all changes.Yippiyeah


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Short Description Better maintainability and reusability

Maintainability and Reusability:Changing any behaviour or adding new functionalities become much easier tasks if you understood the codefast and when the design is good.


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Short Description Better maintainability and reusability

Maintainability and Reusability:Changing any behaviour or adding new functionalities become much easier tasks if you understood the codefast and when the design is good.

When a feature has to be added to a program, if the code is not structured in a convenient way to add the feature, first refactor the program to make it easy to add the feature, then add the feature.

When a feature has to be added to a program, if the code is not structured in a convenient way to add the feature, first refactor the program to make it easy to add the feature, then add the feature.

Important:


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Precondition

Is there anything we have to do before and after a refactoring step ?

Question:


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Precondition

YES !


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Precondition

Regressiontests:

To be sure that you don't change the behavior of the code make tests after every refactoring.

Before starting refactoring, it is important to have a solid test suite, with self-checking test cases. In fact, after refactoring the program, it is convenient to perform regression testing automatically, relying on its output to gain some confidence that bugs have not been introduced.

Regressiontests:

To be sure that you don't change the behavior of the code make tests after every refactoring.

Before starting refactoring, it is important to have a solid test suite, with self-checking test cases. In fact, after refactoring the program, it is convenient to perform regression testing automatically, relying on its output to gain some confidence that bugs have not been introduced.


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Precondition

Should we make big or many refactorings in a single step ?

Question:


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Precondition

NO !


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Precondition

We should make little iterations.

1. Test2. little refactoring step3. Test4. Add a feature5. Test6. little refactoring step7. Test

and so on...


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Precondition



and so on...

Why is this important ?


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Precondition



and so on...

Thats important because if the test fails you don't have to search a long timethe bug.

Why is this important ?


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Simple Design! In priority order, the code must:

• Run all the tests • Contain no duplicate

code • Express all the ideas

the author wants to express

• Minimize classes and methods

(Kent Beck)

• Run all the tests • Follow the once and

only once rule• Has high cohesion

(clarity) • Has loose coupling

(Alan Shalloway)

Checklist


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Steps for improvement

Eliminating temporary variables

Splitting the invoice()-method

Moving subroutines to appropriate classes

Replacing the pricecode dependentswitch-statements with message Moving the method Apply the Strategy pattern

MoviegetPriceCode()setPriceCode()getTitle()

1

1*

*Customerinvoice()

RentalgetDaysRented()getMovie()getCharge()getFrequentRenterPoints()


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Moving the charge()-method from Rental to Movie: Prior

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)


}}

class Rental ... public double getcharge() {

double result = 0;switch (getMovie().getPriceCode()) {

case Movie.REGULAR: result += 2;if (getDaysRented() > 2)


case Movie.NEW_RELEASE:result +=getDaysRented()*3;break;

case Movie.CHILDRENS:result += 1.5;if (getDaysRented() > 3)


}}

Movie

getPriceCode()setPriceCode()getTitle()getcharge()

1

1*

*Customer

invoice()

Rental

getDaysRented()getMovie()getCharge()getFrequentRenterPoints(


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Moving the charge()-method from Rental to Movie: Afterwards

class Movie ... public double getCharge(int daysRented) {

double result = 0;switch ( getPriceCode()) {

case Movie.REGULAR: result += 2;if ( daysRented > 2)

result += ( daysRented -2)*1.5;break;

case Movie.NEW_RELEASE:result += daysRented *3;break;

case Movie.CHILDRENS:result += 1.5;if ( daysRented > 3)


}

}

class Movie ... public double getCharge(int daysRented) {

double result = 0;switch ( getPriceCode()) {

case Movie.REGULAR: result += 2;if ( daysRented > 2)


case Movie.NEW_RELEASE:result += daysRented *3;break;

case Movie.CHILDRENS:result += 1.5;if ( daysRented > 3)


}

}

class Rental ...public double getCharge() {

return _movie.charge(_daysRented);}

class Rental ...public double getCharge() {

return _movie.charge(_daysRented);}

MoviegetPriceCode()setPriceCode()getTitle()getCharge()

1

1*

*Customerinvoice()

RentalgetDaysRented()getMovie()getCharge()getFrequentRenterPoints(


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Moving bonusPoints()-method from Rental to Movie

class Movie ...public int getFrequentRenterPoints(int daysRented) {

if ( (this.getPriceCode()==NEW_RELEASE) && daysRented>1)return 2;

elsereturn 1;

}

class Movie ...public int getFrequentRenterPoints(int daysRented) {

if ( (this.getPriceCode()==NEW_RELEASE) && daysRented>1)return 2;

elsereturn 1;

}

class Rental ...public int getFrequentRenterPoints() {

return _movie.getFrequentRenterPoints(_daysRented); }


return _movie.getFrequentRenterPoints(_daysRented); }


if ((getMovie().getPriceCode()==Movie.NEW_RELEASE) && getDaysRented()>1)

return 2;else

return 1;}


if ((getMovie().getPriceCode()==Movie.NEW_RELEASE) && getDaysRented()>1)

return 2;else

return 1;}

MoviegetPriceCode()setPriceCode()getTitle()getCharge()getFrequentRenterPoints()

1

1*

*Customerinvoice()

RentalgetDaysRented()getMovie()getCharge()getFrequentRenterPoints()


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Polymorphism

Polymorphism(from the Greek, meaning “many forms”) is a feature that allows one interface to be used for a general class of actions


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ChildrensPrice NewReleasePrice RegularPrice

Polymorphism via Inheritance

Movie

priceCode:int

getPriceCode()setPriceCode()getCharge(days:int)getFrequentRenterPoints(days:int)

1

1Customer

totalCharge()totalBonusPoints()invoice()invoiceAsHtml()

*

*

Rental

daysRented:int

charge()bonusPoints ()

getCharge(days:int) getCharge(days:int) getFrequentRenterPoints(days:int)

getCharge(days:int)

Not applicable here: A movie would always have a fixed pricecategory


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Polymorphism via State Pattern

Movie

– priceCode:int

getPriceCode()setPriceCode()charge(days:int)bonusPoints(days:int)

ChildrensPrice

getPriceCode()getcharge(days:int)

NewReleasePrice

getPriceCode() getcharge(days:int)getFrequentRenterPoints(days:int)

RegularPrice

getPriceCode()getcharge(days:int)

Price

– priceCode:int

getPriceCode()

charge(days:int)getFr...Points(days:int)

1

*

1

1Customer

invoice()– totalCharge()– totalBonusPoints()invoiceAsHtml()

*

*

Rental

– daysRented:int

getDaysRented():intgetMovie():Moviecharge()bonusPoints ()

charge(days:int) { return price.charge(days)}


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Polymorphismus via State Pattern

Movie

– priceCode:int

getPriceCode()setPriceCode()getCharge(days:int)getFr..Points(days:int)

NewReleasePrice

getPriceCode()getCharge(days:int)getFr...Points(days:int)

RegularPrice

getPriceCode()getCharge(days:int)

Price

– priceCode:int

getPriceCode()

getCharge(days:int)getFr..(days:int)

1

*

Steps

1. Create classes Price, …, RegularPrice

2. Implement getPriceCode()-methods therein

3. Replace the pricecode with a Price objekt (in Movie) setPriceCode(int) getPriceCode Construktor

4. Move charge() und bonusPoints() from Movie to Price

5. Replace switch-statements with polymorphism Move each occurance of the charge()

method from Price to the charge()-method of a subclass

Analogous for bonusPoints()


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Steps 1-3: Replacement of pricecode with Price object

class Movie { ... private int _priceCode;

public Movie(String name, int priceCode) {_name = name;_priceCode = priceCode;

}public int getPriceCode() {

return _priceCode;}public void setPriceCode(int arg) {

_priceCode = arg; }}

class Movie { ... private int _priceCode;

public Movie(String name, int priceCode) {_name = name;_priceCode = priceCode;


return _priceCode;}public void setPriceCode(int arg) {

_priceCode = arg; }}

3

abstract class Price {public abstract int getPriceCode();

}

abstract class Price {public abstract int getPriceCode();

}class RegularPrice extends Price {

public int getPriceCode() { return Movie.REGULAR;}

}

class RegularPrice extends Price {public int getPriceCode() { return Movie.REGULAR;}

}class ChildrensPrice extends Price {

public int getPriceCode() { return Movie.CHILDRENS;}

}

class ChildrensPrice extends Price {public int getPriceCode() { return Movie.CHILDRENS;}

}class NewReleasePrice extends Price {

public int getPriceCode() { return Movie.NEW_RELEASE;}

}

class NewReleasePrice extends Price {public int getPriceCode() { return Movie.NEW_RELEASE;}

}

1+2

class Movie ...private Price _price;

public void setPriceCode(int arg) {switch (arg) {

case REGULAR:_price = new RegularPrice();break;

case CHILDRENS:_price = new ChildrensPrice();break;

case NEW_RELEASE:_price = new NewReleasePrice();break;

default:throw new

IllegalArgumentException( “Incorrect price code“);

}}

}

public Movie(String name, int priceCode) {_name = name;setPriceCode(priceCode);


return _price.getPriceCode();}


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class Price { ... public double getCharge(int daysRented) {

double result = 0; switch (getPriceCode()) {

case Movie.REGULAR:result += 2;if (daysRented() > 2)

result += (daysRented()-2)*1.5;break;

case Movie.CHILDRENS:result += 1.5;if (daysRented() > 3)

result += ( daysRented()-3)*1.5;break;

case Movie.NEW_RELEASE:result +=daysRented()*3;break;

} }

class Price { ... public double getCharge(int daysRented) {

double result = 0; switch (getPriceCode()) {

case Movie.REGULAR:result += 2;if (daysRented() > 2)

result += (daysRented()-2)*1.5;break;

case Movie.CHILDRENS:result += 1.5;if (daysRented() > 3)

result += ( daysRented()-3)*1.5;break;

case Movie.NEW_RELEASE:result +=daysRented()*3;break;

} }

Steps 4-5: Replace Switch with Polymorphism

abstract class Price ...abstract public double getCharge(int days);

abstract class Price ...abstract public double getCharge(int days);

class RegularPrice extends Price { ...public double getCharge(int daysRented)

{ double result =2;if (daysRented > 2)

result += (daysRented -2)*1.5;return result;

}

class RegularPrice extends Price { ...public double getCharge(int daysRented)

{ double result =2;if (daysRented > 2)

result += (daysRented -2)*1.5;return result;

}

class ChildrensPrice extends Price { ...public double getCharge (int

daysRented){double result = 1.5;if (daysRented > 3)

result += (daysRented -3) * 1.5;return result;

}

class ChildrensPrice extends Price { ...public double getCharge (int

daysRented){double result = 1.5;if (daysRented > 3)

result += (daysRented -3) * 1.5;return result;

}

class NewReleasePrice extends Price { ...public double getCharge (int daysRented)

{return daysRented * 3;

}

class NewReleasePrice extends Price { ...public double getCharge (int daysRented)

{return daysRented * 3;

}

class Movie { ... public double charge(int daysRented) { return _price.charge(daysRented); }

class Movie { ... public double charge(int daysRented) { return _price.charge(daysRented); }

4

5


36

The same for getFrequentRenterPoints

class Rental...int getFrequentRentedPoints(int daysRented) {

if ((getPriceCode() == Movie.NEW_RELEASE) && daysRented > 1)return 2;

elsereturn 1;

}

class Rental...int getFrequentRentedPoints(int daysRented) {

if ((getPriceCode() == Movie.NEW_RELEASE) && daysRented > 1)return 2;

elsereturn 1;

}

class NewReleasePrice...int getFrequentRenteroints (int daysRented) {

return (daysRented > 1) ? 2: 1; }

class NewReleasePrice...int getFrequentRenteroints (int daysRented) {

return (daysRented > 1) ? 2: 1; }

class Rental...int getFrequentRentedPoints (int daysRented) {

return _movie.bonusPoints(daysRented); }

class Rental...int getFrequentRentedPoints (int daysRented) {

return _movie.bonusPoints(daysRented); }

class Movie...int getFrequentRenteroints(int daysRented) {

return _price.getFrequentPoints(daysRented); }

class Movie...int getFrequentRenteroints(int daysRented) {

return _price.getFrequentPoints(daysRented); }

class Price...int getFrequentRenteroints (int daysRented) {

return 1; }

class Price...int getFrequentRenteroints (int daysRented) {

return 1; }


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Extreme Code Quality


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Benefits of refactoring

1. Improved Design

2. Better understanding of the code

3. Better bug detection

4. Faster development!


39

Simplicity is Code Quality

Everything we write must

Run all the tests Express every idea that we need to express Say everything once and only once Have the minimum number of classes and

methods consistent with the above

(Ron Jeffries et al.: Extreme Programming Installed)


40

Bad Smells

In the community of computer programming, code smell is any symptom that indicates something may be wrong. It generally indicates that the code should be refactored or the overall design should be reexamined. The term appears to have been coined by Kent Beck on WardsWiki.

Different Kinds of Bad Smells Duplicated code Long Method Large Class Switch Statements Temporary Field ….

Bad Smells in Code by Kent Beck and Martin Fowler


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Refactoring-Catalog

Composition of Methodes Extract Method Inline Method Replace Temp with Query Inline Temp Split Temporary Variable Remove Assignments to Parameters Replace Method with Method Object ...


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Extract Method

void printOwing(double amount) {printBanner();

// print detailsSystem.out.println(“name“+_name);System.out.println(“amount“+ amount);

}

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

}

void printDetails (double amount) {System.out.println (“name“+_name);System.out.println (“amount“+ amount);

}

How to indicate? Code-Blocks that are logically related to each other

How to handle? Replace with a well-named methode .


43

Steps Defining new and well-named methodes

always „private“

Copy the Code

Searching for the local variables in extracted code Variables that will be used just in new methodes

local variables of new methodes

Variables which are changed in new methodes and will be used in old methodes

If only one: give it back as the result of new method more than one: Parts that can not be extracted!

(„Replace Temp with Query“ or try to „Split Temp Variable“)

Variables that will be read in new methodes Parameters of new methodes


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Steps(2)

Compiling

in original methode Replacing the extracted code by calling the new methode Deleting of the delclaration of local variables which have use anymore

Compiling Testing


45

Example: no local variables

void printOwing(double amount) {

Enumeration e = :orders.elements();double outstanding = 0.0;

// print bannerSystem.out.println("**********************");System.out.println("*** Customer owes ****");System.out.println("**********************");

// calculate outstandingwhile (e.hasMoreElements()) {

Order each = (Order) e.nextElement();outstanding += each.getAmount();

// print detailsSystem.out.println(“name“+ _name);System.out.println(“amount“+ outstanding);

}

Extraction of code for print banner


46

Example: no local variables



printBanner();



// print detailsSystem.out.println(“name“+ _name);System.out.println(“amount“+ outstanding);

}

Extraction of code for print banner Local variable which is not altered („outstanding“)

private void printBanner() {System.out.println("**********************");System.out.println("*** Customer owes ****");System.out.println("**********************");

}


47

Local variable which is not altered



printBanner();



printDetails(outstanding);

}

Extraction of codes for calculation Local variable , which will be altered and finally used(„outstanding“) Local variable , which will be altered and will not be used anymore „e“

private void printDetails(double outstanding) {System.out.println(“name“+ _name);System.out.println(“amount“+ outstanding);

}


48

Example:Local variable wchich will be altered



printBanner();

outstanding = getOutstanding();


}

Extracting code for calculation If local variable is assigned before in original method

private double getOutstanding() {Enumeration e = orders.elements();double outstanding = 0.0;while (e.hasMoreElements()) {


}return outstanding;

}


49

Exapmle : local variable that was altered even before


double outstanding = amount *1.2;

printBanner();

outstanding = getOutstanding(outstanding);


}

private double getOutstanding(double startValue) {Enumeration e = orders.elements();double result = startValue;while (e.hasMoreElements()) {

Order each = (Order) e.nextElement();result += each.getAmount();

}return result;

}

Now are more Rafactorings possible twice „inline temp“ for assigning to local variable „outstanding“ In this way we can eliminate „outstanding“ from „printOwing“-Methode


50

Example: Elimination of „outstanding“


double outstanding = amount *1.2;

printBanner();

outstanding = getOutstanding(outstanding);


}


51

Example : End state of printOwing()


printBanner();

printDetails(getOutstanding(amount *1.2));

}


52

Overview Refactoring Tools

Commercial Together since Version 5.5

Only a few refactorings www.togethersoft.com

jFactor Plug-in for JBuilder and VisualAge

for Java Extensive functionality www.instantiations.com/jfactor/

IDEA www.intellij.com/idea/ IDE with builtin refactorings

JBuilder since version 6 Not tested yet

Retool New www.chive.com

Free Smalltalk Refactoring Browser

First tool at all Very powerfull http://chip.cs.uiuc.edu/users/brant/Refactory/

Xrefactory Plug-in for Emacs http://www.xref-tech.com/speller

JavaRefactor Plug-in for jEdit http://plugins.jedit.org/plugins/JavaRefactor

Eclipse IDE Preview, Veto and Undo of changes Extract Method, Rename Method, ... www.eclipse.org


53

Refrences

Slides of SWT Lecture (Günter Kniesel) Slides of Exterme Programming for Nanjing Univesity(Günter Kniesel) Osborne - Java 2--Complete Reference (5th Ed 2002) Exapmles of Kent Beck and Martin Fowler From Berkley University

http://www.berkeley.edu/ http://sis36.berkeley.edu/projects/streek/agile/bad-smells-in-code.html

agile software development lab spring 2008 r o o t s 1 sina golesorkhi([email protected])...

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