Software Refactoring Part I: Introduction Bartosz Walter Advanced Object-Oriented Design Lecture 5

Motto „Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” Martin Fowler

Agenda 1.Refactoring: ideas, motivation, examples 2.Verification of refactorings 3.Bad smells in code

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Motivation for refactoring  High cost of maintenance  Yourdon: 80% of TCD  Software decays during development  Low understanding of code  Design does not fit the requirements/ functionality  YAGNI = Y ou A ren't G oing to N eed I t

Refactoring  a change made to the internal structure of software void doSth()  to make it easier to understand and cheaper to modify  without changing its observable behaviour Refactoring is: Source: W. Opdyke

Simplest example Extract Method void compAndPrint() { // compute X // compute Y... // print X // print Y... // store X // store Y } void compute () { // compute X // compute Y print(X, Y); // store X // store Y } void print(X,Y) { // print X // print Y }  One method – one function  Allows localizing the potential bugs

Simplest example (reverted) Inline Method void compAndPrint() {... X++; Y++;... } void compute () { X = next(X); Y = next(Y); } int next(N) { return N++; }  Removal of very short methods

Cost of refactoring The cost depends on:  the language used  support from CASE tools  the type of refactorings  number and quality of test cases Refactoring is costly because it does not add new functions to the system. Important factors:  cost of documentation update  cost of test cases update

When to do and not do refactoring?  Three strikes and refactor  When adding new functions  When fixing a bug  While inspecting code  At close deadlines  Prematurely published interfaces  Unstable, rubbish code Unfinished refactoring is like going into debt. You can live with it, but it is costly. Ward Cunningham

Verification of refactorings  Automated verification  Implemented in many IDEs  Verification requires testing  Tests need to be manually created SIMPLEHARD

Simple refactorings... void doSth() If before....  Verification of pre-conditions  Static analysis of code Then after....

... and hard ones  Static code analysis, preconditions  Role of unit tests void doSth() If before.... Then after....

Bad smells If it stinks, change it Kent Beck grandma discussing child-rearing philosophy

Duplicated Code Same or similar code appears all over the system  in same class: extract out the common bits into their own method ( extract method )  in sibling classes: extract method with a shared functionality and then pull up the method to a common superclass  in unrelated classes: extract class

Long Method Same or similar code appears all over the system  Too many options, causing the method to do too many things  Not enough support from other methods, causing the method to do tasks at a lower level than it should  Overly complicated exception handling  the rule of one screen/twenty lines  extract code

Large class Same or similar code appears all over the system  No clear definition of what the class should do, resulting in it doing rather a lot of different things  Out-of-control inner classes  Numerous static and instance methods  Excessive numbers of convenience methods  Cut-and-pasted code  extract new class/subclass/interface  pull up methods/fields

Long Parameter List Method needs to much external information  replace parameter with method (receiver explicitly asks sender for data via sender getter method)  replace parameters with a member fields in a dedicated object

Divergent Change Same or related code appears all over the system Many different changes are necessary  Separate out the varying code into varying classes (extract class) that either subclass or are contained by the non-varying class  Use Visitor or Self Delegation patterns

Feature Envy Method in one class uses lots of pieces from another class  move method to the other class  use Visitor or Self Delegation patterns

Data Clumps Data that's always hanging with each other (e.g. name street zip)  Extract out a class ( extract class ) for the data  Related to long parameter list.  Introduce a Parameter Object  Preserve whole object

Summary  Refactoring decreases cost of maintenance  Refactoring preserves software behaviour  Testing and analysis as methods of verification  Code smells indicate a need for refactoring

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