Extracting Code Clones for Refactoring Using Combinations of Clone Metrics

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Extracting Code Clones for Refactoring Using
Combinations of Clone Metrics
Eunjong Choi, Norihiro Yoshida, Takashi
Ishio,Katsuro Inoue, and Tateki Sano
Osaka University, Japan Nara Institute of
Science and Technology , Japan NEC Corporation,
Japan
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Background Clone Set

• A set of code clones that is similar or identical
  to each other

similar
identical
Clone Set
S1Code Clone 1, Code Clone 3
S2Code Clone 2, Code Clone 4, Code Clone 5
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Background Refactoring Code Clone

• Merge code clones into a single program unit

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Background Language-dependent Code Clone

• It is unavoidable to exist in source code
• because of features of the used program
  language.

/ Code Clone in a clone set whose RNR(S) is the
second highest in Ant 1.7.0 / else // is
the zip file in the cache file) if null)
(file)
Example of the language-dependent code
clone (Consecutive setter invocations)
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Background Clone Metrics Higo2007

• Quantitative information on clone sets
• E.g., LEN(S), RNR(S), POP(S)
• Purposes
• To check features of code clones in software
• To extract code clones for several purposes
• E.g., refactoring, defect-prone code clones

Higo2007 Yoshiki Higo, Toshihiro Kamiya,
Shinji Kusumoto, Katsuro Inoue, "Method and
Implementation for Investigating Code Clones in a
Software System", Information and Software
Technology, pp. 985-998 (2007-9)
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Clone Metrics LEN(S)

• The average length of token sequences of code
  clones in a clone set S

A token sequence c c is detected as a code
clone from a token sequence ltc c c a bgt
LEN(S) 2
Superscript indicated that the token is in a
repeated token sequence
Clone set S
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Clone Metrics RNR(S)

• The ratio of non-repeated token sequences of code
  clones in a clone set S

A token sequence c c is detected as a code
clone from a token sequence ltc c c a bgt
Clone set S
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Clone Metrics POP(S)

• The number of code clones in a clone set S

POP(S) 6
Clone set S
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Single Clone Metric (1/2)

• Clone sets whose RNR(S) is higher
• They do not organize a single semantic unit
• semantic unit many instructions forming a
  single functionality

/ Code Clone in a clone set whose RNR(S) is the
second highest in Ant 1.7.0 / else // is
the zip file in the cache ZipFile zipFile
(ZipFile) zipFiles.get(file) if (zipFile
null) zipFile new ZipFile(file)
zipFiles.put(file, zipFile)
ZipEntry entry zipFile.getEntry(resourceName)
if (entry ! null) x
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Single Clone Metric (2/2)

• Clone sets whose POP(S) is higher
• They Include many language-dependent code clones

/ Code Clone in a clone set whose POP(S) is the
first highest in Ant 1.7.0 /
out.println("\"gt")
out.println("") out.print("lt!ELEMENT
project (target ")
out.print(TASKS) out.print(" ")
out.print(TYPES)
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Key Idea

• It is not appropriate to extract refactorable
  code clones using just a single clone metric
• According to our experiences
• We propose a method based on combined clone
  metrics
• To improve the weakness of single-metric-based
  extraction

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Combined Clone Metrics

• Clone sets whose RNR(S), POPS(S) are higher
• Each code clone organizes a single semantic units

/ Code Clone in a clone set whose RNR(S),
POP(S) are higher than others/ if (ifProperty !
null p.getProperty(ifProperty) null)
return false else if
(unlessProperty ! null
p.getProperty(unlessProperty) ! null)
return false return true

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Case Study (1/2)

• Goal validating our key idea
• Using combined clone metrics is a feasible method
  to extract code clone for refactoring
• Target System
• Industrial Java software developed by NEC
• 110KLOC, 736 clone sets

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Case Study (2/2)

• Experimental Step
• Selected 62 clone sets from CCFinder's output
  using clone metrics.
• Conducted a survey about these clone sets and got
  feedback from a developer.

Survey
Feed back
CCFinder
Source files
Clone sets using clone metrics
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Subject Code Clones (1/2)

• Clone sets whose either clone metric value is
  high
• Clone sets whose LEN(S) value is top 10 high
• Clone sets whose RNR(S) value is top 10 high
• Clone sets whose POP(S) value is top 10 high

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Subject Code Clones (2/2)

• Clone sets whose combined clone metrics values
  are high
• 15 clone sets whose LEN(S) and RNR(S) values are
  high rank in the top 15
• 7 clone sets whose LEN(S) and POP(S) values are
  high rank in the top 15
• 18 clone sets whose RNR(S) and POP(S) values are
  high rank in the top 15
• 1 clone set whose LEN(S), RNR(S) and POP(S)
  values are high rank in the top 15

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Results of Case Study (1/2)
Filtering Selected Clone Sets Refactoring Precision
Each Single Clone metric 30 14 0.47
Combined Clone metrics 41 34 0.87

• Selected Clone Sets The number of selected
  clones
• Refactoring The number of clone sets marked as
  Perform refactoring in survey

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Results of Case Study (2/2)
Filtering Selected Clone Sets Refactoring Precision
Each Single Clone metric 30 14 0.47
Combined Clone metrics 41 34 0.87

• Precision How many refactoring candidates were
  accepted by a developer?

Refactoring
Precision
Selected Clone Sets
Combined clone metrics is more accepted as
refactoring candidates by a developer
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Summary and Future Work

• Summary
• Our Industrial case study shows that our key idea
  is appropriate.
• Future Work
• Investigate about recall
• Conduct case studies of open source software
• Suggest a new metric

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Thank You
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Clone sets whose RNR(S) is higher than others

• Each code clone in a clone set S consists of more
  non-repeated token sequences

/ Code Clone in a clone set whose RNR(S) is the
second highest in Ant 1.7.0 / else // is
the zip file in the cache ZipFile zipFile
(ZipFile) zipFiles.get(file) if (zipFile
null) zipFile new ZipFile(file)
zipFiles.put(file, zipFile)
ZipEntry entry zipFile.getEntry(resourceName)
if (entry ! null) / /
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Clone sets whose RNR(S) is lower than others

• Consists of more repeated token sequences
• Involve in language-dependent code clone

/ Code Clone in a clone set whose RNR(S) is the
lowest in Ant 1.7.0 / String
sosCmdDir null skip
code. private String
filename null private boolean noCompress
false private boolean noCache false
private boolean recursive false private
boolean verbose false / /
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Survey Format About Clone set XXX

• (1) Do you think that this clone set need a
  practice?
• Yes No(?Jump to next clone set)
•  
• (2) If you marked Yes in your answer to (1),
  what practice is appropriate for this clone set?
• Refactoring
• Write comments about code clones, but dont
  perform refactoring.
• Change nothing.
• Others. (
• (3) Write the reason why did you mark in your
  answer to (2)
• Reason

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Results, and Precision of each clone set in the
survey
Filtering Selected Clone Sets Refactoring Precision
Clone sets whose LEN(S) value is top 10 high 10 7 0.70
Clone sets whose RNR(S) value is top 10 high 10 4 0.40
Clone sets whose POP(S) value is top 10 high 10 3 0.30
Clone sets whose LEN(S) and RNR(S) values are high rank in the top 15 15 13 0.87
Clone sets whose LEN(S) and POP(S) values are high rank in the top 7 6 0.86
RNR(S) and POP(S) values are high rank in the top 15 18 14 0.78
Clone sets whose 1 clone set whose LEN(S), RNR(S), and POP(S) values are high rank in the top 15 1 1 1.00
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Clone metric RNR(S) (1/2)

• File
• F1 a b c a b,
• F2 c c c a b,
• F3 d a b, e f
• F4 c c d e f
• Superscript indicated that the token is in a
  repeated token sequence
• RNR(S1) of Clone Set S1 is

Clone Set S1 , , ,
ab
ab
ab
ab
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Clone metric RNR(S) (2/2)

• File
• F1 a b c a b,
• F2 c c c a b,
• F3 d a b, e f
• F4 c c d e f
• Superscript indicated that the token is in a
  repeated token sequence
• RNR(S2) of Clone Set S2 is

Clone Set S2 , ,
c c
c c
c c
1 0 1 2 2 2
RNR(S2) 100 33.3
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Subject Code Clones

• 62 clone sets
• clone sets whose individual clone metric value is
  high
• SLEN Clone sets whose LEN(S) value is top 10
  high.
• SRNR Clone sets whose RNR(S) value is top 10
  high.
• SPOP Clone sets whose POP(S) value is top 10
  high.
• clone sets whose combined clone metrics values
  are high
• SLENRNR 15 clone sets whose LEN(S) and RNR(S)
  values are high rank in the top 15.
• SLENPOP 7 clone sets whose LEN(S) and POP(S)
  values are high rank in the top 15.
• SRNRPOP 18 clone sets whose RNR(S) and POP(S)
  values are high rank in the top 15.
• SLENRNRPOP 1 clone set whose LEN(S), RNR(S) and
  POP(S) values are high rank in the top 15.

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The Number of Duplicate Clone Set

• SRNR n SPOP n SRNR POP 1
• SRNR n SRNR POP 2
• S POP n SRNR POP 2
• SLEN RNR n SLEN POP n SRNR POP
  n SLEN RNR POP 1

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Example of clone set that are not selected

• It is too short to organize a semantic unit.
• RNR metric sometimes extract unintentional code
  clones
• E.g., Language-dependent code clones

boolean isEqual(final DeweyDecimal other)
final int max Math.max(other.components.length,
components.length) for (int i 0 i lt max
i) final int component1 (i lt
components.length) ? components i 0
final int component2 (i lt other.components.lengt
h) ? other.components i 0 if (