Code Review Defects. Authors: Mika V. Mäntylä and Casper Lassenius Original version: 4 Sep, 2007 Made available online: 24 April, 2013

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1 Code Review s Authors: Mika V. Mätylä ad Casper Lasseius Origial versio: 4 Sep, 2007 Made available olie: 24 April, 2013 This documet cotais further details of the code review defects preseted i [1]. of each defect is give as well as otes describig the most typical cases. [1] Mätylä, M. V. ad Lasseius, C. "What Types of s Are Really Discovered i Code Reviews?" IEEE Trasactios o Software Egieerig, vol. 35, o 3, May/Jue 2009, pp , Available olie: Documetatio - Textual s Idustrial Studet Idustrial Studet Namig Problems relatig to software elemet (=methods, classes, variables, etc) ames Most of the Namig defects were due either to ot coformig to the compay s amig policy or havig uiformative ames. I some cases, amig defects led to loger discussios o how to ame certai elemets as the compay did ot have policy for amig every code elemet. Some amig problems were iherited from legacy code that did ot coform to the curret amig policy. Discussios with the idustrial reviewers made it clear that the compay strogly believed selfdescriptive amig over code commetig explaig the differece i Namig issue shares. Most of the Namig defects foud by studets poited out uiformative ames. Ofte the ames were too short, or too geeral, e.g. arg1 ad arg2. I a few cases, the ames were icorrect whe compared with the behavior. Occasioally, the amig did ot follow Java codig stadard by SUN. Commets Problems i code commets Commets were eeded, e.g., to explai complicated programmig logic. Some commets were icorrect, which was ofte the result of old legacy commets or copy-paste-programmig. Some commets lacked iformatio required by the compay s commetig policy. May Commets defects were related to the omissio of JavaDoc elemets, e.g., missig method descriptios. Some commets were requested to further explai the rules implemeted. Fially, some defects poited out icorrect commets or just simple spellig mistakes. Debug Ifo Problems with debuggig messages. Debug Ifo is placed i this sub-group because it improves programs static ad rutime documetatio. All the Debug Ifo defects requested more iformatio o the error coditio Other Textual s Other Textual s that could ot be placed to other defect classes. Should commets or method prototypes be preseted i the header files or i the source files, geeral commets of laguage usage affectig all commets ad ames. 2. Documetatio - Supported By Laguage s Elemet Type

2 2 Idustrial Studet Idustrial Studet The software elemet is with wrog type (oly cases ot causig rutime failure) All Elemet Type defects were i Retur value types, e.g., chagig the retur value from it to boolea, or returig a value istead of a void. Iterestigly, the differet teams of the compay had somewhat differet policies regardig retur values. I oe team, void was allowed as a retur value type, while i may others it was ot permitted. I the studet reviews, the Elemet Type defects were more diverse. I additio to defects related to retur value types, there were excessively geeral descriptio of exceptios (e.g. throws Exceptio versus throws IOExceptio ), missig a implemeted iterface i the class descriptio, havig the wrog type of variables, claimig that a method throws a exceptio whe it does ot, ad uecessarily extedig the java.lag.object class as it is exteded by default. Immutable Not declarig variable to be immutable whe it should have bee or declarig it immutable whe it should have ot bee This was preset i both reviews as both laguages Java ad C support such mechaism. Visibility Software elemet (e.g. method, variable, class) has too much or too restricted visibility I the idustrial reviews, all the Visibility defects addressed method visibility, ad i the studets reviews there were cases of both the methods ad variables visibility Void Parameter Usig empty brackets istead of keyword void as parameter This was oly preset i the idustrial reviews as Java does ot have such optio. Elemet Referece Referrig to software elemet with icomplete ame This was oly preset i studet reviews ad all defects foud idicated that keyword "this" should have bee used whe referrig to istace variables or methods 3. Visual Represetatio Both Reviews Bracket Usage Icorrect usage of brackets or braces I both reviews these issues mostly referred to cases i which the developer had omitted brackets whe havig oly a sigle statemet after a coditioal brach. Occasioally, brackets were requested to clarify the precedece operatios i complex comparisos or mathematical statemets. Idetatio Icorrect idetatio Ofte the developers thought the idetatio might be correct i the code, ad it was wrogly ideted oly i the pritig. However, sice they from the poit of view of the review sessios were defects, we caot remove them from our aalysis simply because the developer believes this is ot a problem i the code. The Blak Lie Usage Blak Lie Usage is icorrect This occurred mostly due to havig a excess of blak lies or too few blak lies. However, it also icluded cases i which lies were split at icorrect positios. Log Lie Log Lie meas defects where a log code statemet was cotaied i a excessively log sigle lie, ofte more tha 80 characters. Space Usage Space Usage defects referred to the usage of the blak space character i the code. Groupig Groupig refers to the groupig of code elemets, e.g., i header files, oe should group fuctios so that closely related fuctios are close to each other i the file, itroducig class variables at the

3 3 begiig of the class. 4. Structure - Orgaizatio s Idustrial Studet Idustrial Studet Both reviews Idustrial Studet Idustrial Studet Move Fuctioality Move Fuctioality, refers to the eed to move fuctios, part of fuctios, or other fuctioal elemets to a differet class, file, or module. Log Sub-routie Fuctio, procedure or method is of excessive legth ad fuctioality. Some of these recommedatios suggested particular elemets that should be extracted to a ew method while others merely poited out that method is too log ad difficult to uderstad Dead Code Code that is ot executed or used i the software. Idustrial Dead Code defects were evely distributed ad they cosisted of uecessary variables, uecessary headers, ucalled fuctios, ad braches of code that are ever executed. The most promiet Dead Code defect i the studet code reviews was a uecessary header with 6 metios. Other Dead Code defects i the studet reviews were a uecessary type cast, a uecessary variable, a uecessary method, ad a uecessary retur statemet. Duplicatio Code that is duplicated The Duplicatio issues icluded fuctios that were partially duplicated, completely duplicated fuctios, duplicated compariso operatios, ad duplicated variables. They types were similar to the issues of the idustrial review. I the studet code review, may of the duplicated code segmets were quite small three to five lies of code, but there were duplicates up to 20 lies of code. Complex Code a piece of code that is difficult to comprehed Some Complex Code defects were quite geeral, e.g. a method is messy ad eeds to be rewritte, poiter usage is wild, or the implemetatio is uecessarily complex. Other defects were more specific, e.g., a if-statemet has too may comparisos or uecessary use of egative compariso whe usig a positive result would be more readily uderstood. Statemet Issues These require splittig, combiig or otherwise reorgaizig a statemet iside a fuctio. idetified oce i idustrial reviews, whe a reviewer suggested combiig a declaratio ad a iitializatio of a variable to oe code lie Six Statemet issues were recogized i the studet code reviews. Three of them suggested splittig a log boolea retur statemet to several if-else statemets. Three Statemets issues suggested combiig statemet, two of them suggested combiig variable itroductio with iitializatio ad oe them suggested combiig statemets rather tha usig temporary values to store the results. Cosistecy Meas the eed to keep code cosistet i a sese that similar code elemets operate i a similar fashio ad are more or less symmetrical. For example, similar tasks i similar classes should have similar implemetatios Examples the code should ot alter betwee usig pre-defied values ad umeric values, fuctios performig similar task i similar classes should have similar amig ad amig ad implemetatio, ad compariso statemets should be cosistet with each other or eve extracted to their ow fuctios to remove repetitio. I studet reviews oly oe Cosistecy issues was foud. This issue poited out icosistet variable iitializatio, as some variables were iitialized at the variable declaratio, others at the method s costructor ad some of the variables where ot iitialized at all. Other Other Orgaizatio defects

4 4 Idustrial Studet I the idustrial reviews, these defects icluded splittig up a large file cotaiig several classes ad 2000 lies of code ito several files; defects were a logical piece of code was uecessarily split ito several places; the eed to remove wrog coupligs betwee software elemets; the eed to split up fuctioality ito several implemetatios; ad reducig the umber of differet error hadlig mechaisms. I the studet reviews, they icluded havig several retur statemets i a method; usig loop variables outside of the loop structure; i-liig a method as part of a other method; startig idexig from zero istead of oe; usig egative retur values istead of positive; havig too may temporary variables; havig variables i class scope istead of method scope; havig a method with too may parameters, ad commeted code that should be deleted. 5. Structure - Solutio Approach s Idustrial Studet Idustrial Studet Idustrial Idustrial Sematic Duplicatio Meas sytactically differet code blocks with equal itet, e.g., differet sortig algorithms such as quicksort ad heapsort have equal itet but they are ot idetical at the code level. I the idustrial case, we had o defects where the code uder review had Sematic Duplicatio with itself. Istead, all the Sematic Duplicatio defects were idetified based o the reviewers kowledge of existig fuctioality located elsewhere i the software system. E.g. fuctioality is already implemeted i some other place ad there is o eed to duplicate this fuctioality i the code uder review. All Sematic Duplicatio defects i the studet reviews suggested replacig code with pre-built fuctioality icluded i the Java class library. Sematic Dead Code Code fragmets that are executed, but they do ot serve ay meaigful purpose ad/or have o effect o the result I the idustrial reviews, two code review sessios idetified early half of the Dead Code ad Sematic Dead Code defects. This was caused by code templates that were re-used ad modified. All the Sematic Dead Code defects of the studet reviews were uecessary checks performed i coditioal statemets while i the idustrial review the reasos were more varyig Chage Fuctio Need to chage a certai fuctio call to aother whe the program used old or deprecated fuctios Ofte the reaso for chagig a fuctio was that a better alterative was available, for example, the compay had provided wrapper fuctios o top of may basic C-fuctios that added extra features or made usig the fuctios easier. Use Stadard Method Use Stadard Method cotais defects where a stadardized way of workig should be used. I the idustry reviews these ofte meat usig predefied costats rather tha magic umbers. I the studet the use exceptios for error messagig istead of retur values was metioed ofte. Other issue i this category were use of eumeratio istead of itegers; ad use accessor-methods to access a class. New Fuctioality Need of ew fuctioality to esure evolvability I the idustrial reviews, we saw defects where the creatio of ew fuctioality or classes was required to make the software more evolvable. We categorized these uder the headig of New Fuctioality. No such defects were idetified i the studet reviews. Other Other cotais a wide rage of defects that truly represet the Alterative Approach i its most fruitful form I the idustrial reviews, this type cotaied implemetatio chages such as usig arrays istead of other more complex memory maagemet structures, chagig the code to eable a easier removal of several data items from the database, usig dedicated arrays for each data elemet istead of a

5 5 Studet Idustrial shared array, ad usig a simpler ad more efficiet way of keepig records i a database. I the studet reviews Other defects icluded suggestig a simpler way of performig computig ad compariso operatios, usig Java s Geerics data structures, ad cachig umeric values rather tha recomputig them. Mior Mior gathers implemetatio chages, but the defects are easier to fix ad seemed less importat tha those categorized uder Other These defects were oly idetified i the idustrial review. Examples of such defects are, havig a default brach i a switch block, chagig a if-else block to a switch-block, chagig compariso elemet from a class ame to a class id. 6. Resource s Variable Iitializatio Meas defects where variables are left uiitialized prior to use. Uiitialized variables may cotai ay value ad usig such variable for compariso or calculatio produces arbitrary results. Oly preset i idustrial reviews because studets used Java laguage that forces primitive variable iitializatio Memory Maagemet Meas a defect where a mistake is made i hadlig the system memory. I idustrial reviews these iclude allocatig too little memory, ot freeig memory after its use, ad freeig the same memory more tha oce. I the Java laguage used by the studets, memory maagemet is automatic, thus, the studets idetified o memory allocatio defects. Data & Resource Maipulatio s related to maipulatig or releasig data or other resources, For example ot releasig database cursor, ad mistakes i data modificatio. There was oly sigle Data & Resource maipulatio defect i the studet code reviews, ad it is likely due to the applicatio, which did ot require may data structures. 7. Check s Check Fuctio Meas that whe a fuctio is called there is also a eed to check that the value retured is valid ad that o error occurred I idustrial reviews 4 istaces were eeded to check the result of memory allocatio operatio Check Variable Meas that there is a eed to check variable Ofte such variables were fuctio parameters, or loop cotrol variables. I idustrial reviews poiter checkig was foud 6 istaces Check User Iput Check User Iput meas the eed to validate user iput No additioal descriptio

6 6 8. Iterface s Fuctio Call Meas that a fuctio call to aother part of the system or class library is icorrect or missig. Parameter Meas that a fuctio call or other iteractio mechaism has a icorrect or missig parameter 9. Logic s Compare meas a mistake i a compariso statemet Examples: the wrog elemet is compared, a required compariso is missig, or the wrog type of compariso is made Compute Such defects are made whe computatios produce icorrect results. Wrog Locatio meas that a correct operatio is performed, but it is doe too soo or too late. Algorithm/Performace meas that a iefficiet algorithm is used. Examples: performig uecessary searches, passig large data arrays by value, ad re-calculatig values rather tha storig them i variables Other All other defect i this catogory Examples: the eed to use a loop istead of a sigle fuctio call, missig a etire compariso statemet ad the required logic to hadle the particular case, ad usig if-else blocks with two logically urelated if blocks. 10. Larger s Completeess A feature is partially implemeted GUI s i the user iterface code relatig to the cosistecy of the user-iterface, ad to the optios made possible to the user i each situatio. Check outside code s that required that part of the applicatio code that was ot uder review to be checked, as it was likely to cotai icorrect code based o the curret review.