Upload
marisol-castro
View
31
Download
5
Embed Size (px)
DESCRIPTION
Aries: Refactoring Support Environment Based on Code Clone Analysis. Yoshiki Higo, Toshihiro Kamiya, Shinji Kusumoto, Katsuro Inoue Graduate School of Information Science and Technology, Osaka University Presto, Japan Science and Technology Agency - PowerPoint PPT Presentation
Citation preview
1Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Aries: Refactoring Support Environment Based on Code Clone Analysis
Yoshiki Higo, Toshihiro Kamiya, Shinji Kusumoto, Katsuro Inoue
Graduate School of Information Science and Technology, Osaka University
Presto, Japan Science and Technology Agency
{y-higo,kamiya,kusumoto,inoue}@ist.osaka-u.ac.jp
2Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Background
What is code clone?
a code fragment that has identical or similar fragments in the same or different files in a system
introduced in the source program because of various reasons such as reusing code by `copy-and-paste’
makes software maintenance more difficult.
copy-and-pastecopy-and-paste
3Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Requirements for Code Clone Detection
Appropriate code clones should be detected in compliance with demands.
To understand the amount and distribution of code clones, it is desirable to detect all code clones
To remove code clones (Restructuring or Refactoring), it is useful to detect code clones that can be removed, and also removing them improves software maintainability
4Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Research Objective and Approach
We aim to extract code clones which can be easily refactored
Approach
To detect code clones efficiently, we use a code clone detection tool, CCFinder.
Then, we extract the specific code clones easily refactored and provide applicable refactoring patterns for the code clones.
Finally, we develop a refactoring support tool and apply it to an open source program.
5Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Refactoring Process Support
Commonly used refactoring process
Step 1: Determine where refactoring should be applied
Step 2: Determine which refactoring patterns can/should be applied
Step 3: Investigate the effectiveness of the refactoring patterns
Step 4: Modify source code
Step 5: Conduct regression tests
Proposed method supports Steps1 and 2
High scalability: it take less of high time complexity.
Detect fine-grained clone: it detect more fine-graded code clone than method unit.
6Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Outline of CCFinder
CCFinder directly compares source code on token unit, and detects code clones
Normalization of name space
Replacement of names defined by user
Removal of table initialization
Consideration of module delimiter
CCFinder can analyze the system of millions line scale in practical use time
7Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Source files
Lexical analysis
Transformation
Token sequence
Match detection
Transformed token sequence
Clones on transformed sequence
Formatting
Clone pairs
1. static void foo() throws RESyntaxException { 2. String a[] = new String [] { "123,400", "abc", "orange 100" }; 3. org.apache.regexp.RE pat = new org.apache.regexp.RE("[0-9,]+"); 4. int sum = 0; 5. for (int i = 0; i < a.length; ++i) 6. if (pat.match(a[i])) 7. sum += Sample.parseNumber(pat.getParen(0)); 8. System.out.println("sum = " + sum); 9. }10. static void goo(String [] a) throws RESyntaxException {11. RE exp = new RE("[0-9,]+");12. int sum = 0;13. for (int i = 0; i < a.length; ++i)14. if (exp.match(a[i]))15. sum += parseNumber(exp.getParen(0));16. System.out.println("sum = " + sum);17. }
static void foo ( ) { String a
[ ] = new String [ ] { "123,400" ,
"abc" , "orange 100" } ;
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
sum
+= pat . getParen 0
; System . out . println ( "sum = "
+ sum ) ; }
throws RESyntaxException
Sample . parseNumber (
) )
if pat
. match a [ i ]( ) )
org . apache . regexp
. RE pat = new org . apache . regexp
. RE ( "[0-9,]+" ) ;
static void goo (
) {
String
a [ ]
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
System . out . println ( "sum = " + sum
) ; }
throws RESyntaxException
if exp
. match a [ i ]( ) )
exp =
new RE ( "[0-9,]+" ) ;
(
RE
sum
+= exp . getParen 0
;
parseNumber ( ) )(
(
(
[ ] = new String [ ] {
} ;
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
sum
+= pat . getParen 0
; System . out . println ( "sum = "
+ sum ) ; }
Sample . parseNumber (
) )
if pat
. match a [ i ]( ) )
pat = new
RE ( "[0-9,]+" ) ;
static void goo (
) {
String
a [ ]
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
System . out . println ( "sum = " + sum
) ; }
throws RESyntaxException
if exp
. match a [ i ]( ) )
exp =
new RE ( "[0-9,]+" ) ;
(
RE
sum
+= exp . getParen 0
;
parseNumber ( (
(
(
static void foo ( ) { String athrows RESyntaxException
$
RE
$ . ) )
Lexical analysis
Transformation
Token sequence
Match detection
Transformed token sequence
Clones on transformed sequence
Formatting
[ ] = new String [ ] {
} ;
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
sum
+= pat . getParen 0
; System . out . println ( "sum = "
+ sum ) ; }
Sample . parseNumber (
) )
if pat
. match a [ i ]( ) )
pat = new
RE ( "[0-9,]+" ) ;
static void goo (
) {
String
a [ ]
int sum = 0
; for ( int i = 0 ; i <
a . length ; ++ i )
System . out . println ( "sum = " + sum
) ; }
throws RESyntaxException
if exp
. match a [ i ]( ) )
exp =
new RE ( "[0-9,]+" ) ;
(
RE
sum
+= exp . getParen 0
;
parseNumber ( ) )(
(
(
static void foo ( ) { String athrows RESyntaxException
$
RE
$ .
[ ] = [ ] {
} ;
=
; for ( = ; <
. ; ++ )
+= .
; . . (
+ ) ; }
. (
) )
if
. [ ]( ) )
=
( ) ;
static (
) {[ ]
=
; ( = ; <
. ; ++ )
. . ( +
) ; }
throws
if
. [ ]( ) )
=
new ( ) ;
(
+= .
;
( ) )(
(
(
static $ ( ) {throws
$
$ .
$ $ $ $
$ $
$ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
new
forfor
new
[ ] = [ ] {
} ;
=
; for ( = ; <
. ; ++ )
+= .
; . . (
+ ) ; }
. (
) )
if
. [ ]( ) )
=
( ) ;
static (
) {[ ]
=
; ( = ; <
. ; ++ )
. . ( +
) ; }
throws
if
. [ ]( ) )
=
new ( ) ;
(
+= .
;
( ) )(
(
(
static $ ( ) {throws
$
$ .
$ $ $ $
$ $
$ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $
$ $ $ $ $
Lexical analysis
Transformation
Token sequence
Match detection
Transformed token sequence
Clones on transformed sequence
Formatting
1. static void foo() throws RESyntaxException { 2. String a[] = new String [] { "123,400", "abc", "orange 100" }; 3. org.apache.regexp.RE pat = new org.apache.regexp.RE("[0-9,]+"); 4. int sum = 0; 5. for (int i = 0; i < a.length; ++i) 6. if (pat.match(a[i])) 7. sum += Sample.parseNumber(pat.getParen(0)); 8. System.out.println("sum = " + sum); 9. }10. static void goo(String [] a) throws RESyntaxException {11. RE exp = new RE("[0-9,]+");12. int sum = 0;13. for (int i = 0; i < a.length; ++i)14. if (exp.match(a[i]))15. sum += parseNumber(exp.getParen(0));16. System.out.println("sum = " + sum);17. }
Lexical analysis
Transformation
Token sequence
Match detection
Transformed token sequence
Clones on transformed sequence
Formatting
CCFinder:Clone Detection Process
8Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Definitions:Clone Pair and Clone Set
Clone Pair: a pair of identical or similar fragmentsClone Set: a set of identical or similar fragments
CCFinder detects code clones as a clone pairAfter detection process, clone pairs are transformed into clone sets
C1
C5
C4
C3
C2
Clone Pair Clone Set
(C1, C4) {C1, C4, C5}
(C1, C5) {C2, C3}
(C2, C3)
(C4, C5)
9Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Extraction of code clones easily refactored
Structural code clones are regarded as the target of refactoring
Detect clone pairs by CCFinder
Transform the detected clone pairs into clone sets
Extract structural parts as structural code clones from the detected clone sets
What is structural code clone ?
example: Java language
Declaration: class declaration, interface declaration
Method: method body, constructor, static initializer
statement: do, for, if, switch, synchronized, try, while
10Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
609: reset();610: grammar = g;611: // Lookup make-switch threshold in the grammar generic options612: if (grammar.hasOption("codeGenMakeSwitchThreshold")) {613: try {614: makeSwitchThreshold = grammar.getIntegerOption("codeGenMakeSwitchThreshold");615: //System.out.println("setting codeGenMakeSwitchThreshold to " + makeSwitchThreshold);616: } catch (NumberFormatException e) {617: tool.error(618: "option 'codeGenMakeSwitchThreshold' must be an integer",619: grammar.getClassName(),620: grammar.getOption("codeGenMakeSwitchThreshold").getLine()621: );622: }623: }624:625: // Lookup bitset-test threshold in the grammar generic options626: if (grammar.hasOption("codeGenBitsetTestThreshold")) {627: try {628: bitsetTestThreshold = grammar.getIntegerOption("codeGenBitsetTestThreshold");
623: }624:625: // Lookup bitset-test threshold in the grammar generic options626: if (grammar.hasOption("codeGenBitsetTestThreshold")) {627: try {628: bitsetTestThreshold = grammar.getIntegerOption("codeGenBitsetTestThreshold");629: //System.out.println("setting codeGenBitsetTestThreshold to " + bitsetTestThreshold);630: } catch (NumberFormatException e) {631: tool.error(632: "option 'codeGenBitsetTestThreshold' must be an integer",633: grammar.getClassName(),634: grammar.getOption("codeGenBitsetTestThreshold").getLine()635: );636: }637: }638:639: // Lookup debug code-gen in the grammar generic options640: if (grammar.hasOption("codeGenDebug")) {641: Token t = grammar.getOption("codeGenDebug");642: if (t.getText().equals("true")) {
fragment 1
fragment 2
Code clones which CCFinder detects
Code clones which proposed method detects
11Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
1527: if ( inputState.guessing==0 ) {1528: t=a.getText();1529: }1530: {1531: _loop84:1532: do {1533: if ((LA(1)==COMMA)) {1534: match(COMMA);1535: id();1536: if ( inputState.guessing==0 ) {1537: t+=","+b.getText();1538: }1539: }
1007: if ( inputState.guessing==0 ) {1008: buf.append(a.getText());1009: } 1010: {1011: _loop144:1012: do {1013: if ((LA(1)==WILDCARD)) {1014: match(WILDCARD);1015: a=id();1016: if ( inputState.guessing==0 ) {1017: buf.append('.'); buf.append(a.getText());1018: }1019: }
fragment 3
fragment 4
Code clones which CCFinder detects
12Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Provision of applicable refactoring patterns
Following refactoring patterns[1][2] can be used to remove clone sets including structural code clones
Extract Class,Extract Method,Extract Super Class,Form Template Method,Move Method,Parameterize Method,Pull Up Constructor,Pull Up Method,
For each clone set, the proposed method determines which refactoring pattern is applicable by using several metrics.
[1]: M. Fowler: Refactoring: Improving the Design of Existing Code, Addison-Wesley, 1999.
[2]: http://www.refactoring.com/, 2004.
13Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Metrics(1):Volume Metrics for Clone SetLEN, POP, DFL
LEN(S): is the average length of token sequence for a clone set S
POP(S): is the number of elements (code fragments) of a clone set S
DFL(S): indicates an estimation of how many tokens would be removed from source files when all code fragments in a clone set S are reconstructed
new sub routinecaller statements
14Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Metrics(2): Coupling Metrics for Clone SetNRV, NSV
NRV(S): represents the average number of externally defined variables referred in the fragments of a clone set S
NSV(S): represents the average number of externally defined variables assigned to in the fragments of a clone set S
Definition
Clone set S includes fragment f1, f2, ・・・ , fn
si is the number of externally defined variable which fragment fi refersti is the number of externally defined variable which fragment fi assigns
int a , b; … if( … ){ …;
… = b; a = …; …;
} …
assignment
reference
Fragment f1
example :・ Clone set S includes fragments f1 and f2. ・ In fragment f1 , externally defined variable b is referred and a is assigned to.・ Fragment f2 is same as f1.
then , NRV(S) = ( 1 + 1 ) / 2 = 1 NSV(S) = ( 1 + 1 ) / 2 = 1
15Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Metrics ( 3 ): Inheritance Metric for Clone SetDCH
DCH(S): represents the position and distance between each fragment of a clone set S
Definition
Clone set S includes fragment f1, f2, ・・・, fn
Fragment fi exists in class Ci
Class Cp is a class which locates lowest position in C1, C2, ・・・,Cn on class hierarchy
If no common parent class of C1 , C 2,・・・, Cn exists, the value of DCH(S) is -1This metric is measured for only the class hierarchy where target software exists.
example 1:・ Clone set S includes fragments f1 and f2.・ If all fragments of clone set S are included in a same class,
then , DCH(S) = 0
class A
fragment f1
fragment f2
class A
class B class C
fragment f1 fragment f2
example 2 :・ Clone set S includes fragments f1 and f2.・ If all fragments of clone set S are included in a class and its direct child classes,
then , DCH(S) = 1
16Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Aries: Refactoring Support ToolOverview
Target: Java programs
Runtime environment: JDK1.4 or above
Implementation
Analysis component: Java 32,000 Lines
CCFinder is used as code clone detection component
JavaCC is used to construct syntax and semantic analysis component
GUI component: Java14,000 Lines
User can specify target clone sets through GUI operations.
17Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Case Study: AntOverview
Ant is one of build tools like ‘make’
Input for Aries
Source files of Ant: 627
LOC: about 180,000
It took 30 seconds to extract structural code clones
We got 151 clone sets.
Environment
OS: FreeBSD 4.9
CPU: Xeon 2.8G x 2
Memory: 4GB
18Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Case Study: AntExtract Method (conditions)
To apply ‘Extract Method’ pattern, we filtered clone sets by using following conditions
The unit of clone is statement (do, for, if, …)
Set the value of DCH(S) = 0 All fragments of a clone set are included in a class
Set the value of NSV(S) < 2 Each fragment of a clone set assigns any value to 1 or no externally defined variable.
32 clone sets satisfied these conditions
19Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Case Study: AntExtract Method(result)
32 clone set can be categorized as followings
category number
No parameter, no return value 3Addition of some parameters, no return value 18
Addition of some parameters and return the value 7Others 4
if (!isChecked()) { // make sure we don't have a circular reference here Stack stk = new Stack(); stk.push(this); dieOnCircularReference(stk, getProject()); }
if (iSaveMenuItem == null) { try { iSaveMenuItem = new MenuItem(); iSaveMenuItem.setLabel("Save BuildInfo To Repository"); } catch (Throwable iExc) { handleException(iExc); } }
assignment
if (name == null) { if (other.name != null) { return false; }} else if (!name.equals(other.name)) { return false;}
// javacoptsif (javacopts != null && !javacopts.equals("")) { genicTask.createArg().setValue("-javacopts"); genicTask.createArg().setLine(javacopts); }
local variable
20Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Conclusion
We have
proposed refactoring support method
implemented a refactoring support tool, Aries
conducted a case study to Ant, which is an open source program, and most of filtered clone sets could be removed.
21Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Future Works
As future works, we are going toevaluate whether or not each refactoring should be done as the viewpoint of software quality (support Step 3)find a group of clone sets that can be refactored at once to conduct refactoring more effectively
Commonly used refactoring processStep 1: Determine where refactoring should be appliedStep 2: Determine which refactoring patterns can/should be ap
pliedStep 3: Investigate the effectiveness of the refactoring patternsStep 4: Modify source codeStep 5: Conduct regression tests
22Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
23Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Code clone detection for refactoring:Related Works
Detect similar sub-graphs as clone on program dependency graph [1].
High accuracy: This approach finds out data-dependence and control dependence in source codes.
High time complexity: It takes O(n2) time to construct program dependency graph.
Detect similar methods and functions as clone using metrics [2].
Low accuracy: if the size of target method or function is small, the values of metric make no difference.
detection unit restriction: only method and function unit clone can be detected.
[1] R. Komondoor and S. Horwitz, “Using slicing to identify duplication in source code”, In Proc. of the 8th International Symposium on Static Analysis, Paris, France, July 16-18, 2001.[2] Magdalena Balazinska, Ettore Merlo, Michel Dagenais, Bruno Lague, and Lostas Kontogiannis, “Advanced Clone-Analysis to Support Object-Oriented System Refactoring”, WCRE 2000, pp. 98-107
24Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
The difference between ‘diff’ and clone detection tools
Diff finds the longest common sub-string.
Given a code portion, diff does not report two or more same code portions (clones).
Clone detection tool finds all the same or similar code portions.
25Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Suffix-tree
Suffix tree is a tree that satisfies the following conditions.1. A leaf node represents the starting
position of sub-string.2. A path from root node to a leaf node
represents a sub-string.3. First characters of labels
of all the edges from one node are different from each other.
→ A common path means a clone
x
y
z%
%
xyxyz%
y
xyz%
z%
xyz%
z%
1
2
43
56
71 2 3 4 5 6 7x x y x y z %
1 2 3 4 5 6 7x x y x y z %
1 x *2 x * *3 y *4 x * * *5 y * *6 z *7 % *
26Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Example of transformation rules in Java
All identifiers defined by user are transformed to same tokens.
Unique identifier is inserted at each end of the top-level definitions and declarations.
Prevents detecting clones that begin at the middle of class definition and end at the middle of another one.
”java. lang. Math. PI” is transformed to ”Math. PI”.
By using import sentence, a class is referred to with either full package name or a shorter name
” new int[] {1, 2, 3} ” is transformed to ” new int[] {$} ”
Eliminates table initialization code.
27Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
The output ofCCFinder
Output of CCFinder
#version: ccfinder 3.1
#langspec: JAVA
#option: -b 30,1
#option: -k +
#option: -r abcdfikmnprsv
#option: -c wfg
#begin{file description}
0.0 52 C:\Gemini.java
0.1 94 C:\GeneralManager.java
:
:
#end{file description}
#begin{clone}
0.1 53,9 63,13 1.10 542,9 553,13 35
0.1 53,9 63,13 1.10 624,9 633,13 35
0.2 124,9 152,31 0.2 154,9 216,51 42
:
:
#end{clone}
Object file ID( file 0 in Group 0 )
Location of a clone pair( Lines 53 - 63 in file 0.1 and Lines 542 - 553 in file 1.10 are identical or similar to each other)
It is difficult to analyze source code by only this text-based information of the location of clone pairs.
28Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
The analysis of comparison among students (non-gapped clones only)
A
B
The corresponding code
A (2 students)
Similar code fragments were from source code of sample compiler described in textbook.
B (4 students)
Many code fragments were similar even with respect to name of variables or comments.
29Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Clone class metrics
LEN (C ): Length of token sequence of each element in clone class C LNR (C) : Length of non-repetitive token sequence of LEN(C) POP (C ): Number of elements in clone class C DFL (C ): Estimation of how many tokens would be removed from source
files when all code fragments of clone class C are replaced with caller statements of a new identical routine
RAD (C ): Distribution in the file system of elements in clone class C
new sub routinecaller
statements
30Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Comparison with AST approach
Features of AST approachExtract the same sub-trees of AST as a cloneThe result is precise because of strict syntax analysis.High space and time complexity
Features of Our approachHybrid approach of CCFinder’s quick but inaccurate clone detection and CCShaper’s filtering considering syntax structure.
31Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
The other approaches
AST(Abstract syntax tree) approachClone = the same sub-trees in an ASTDeep dependence on program language
PDG(Program dependency Graph) approachClone = the same sub-graph in a PDGGraph comparison is difficult
Code metricClone = the routines which have the same metric valuesSevere restriction in granularity
CCFinder&CCShaperClone = the code fragments which have the same syntax structureLimited precision
32Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Why I choose “a”
I selected the clones by the following criteria
All clone code fragments appear in the same class
The metric LEN is high
The code fragment includes a whole method body
33Software Engineering Laboratory, Department of Computer Science, Graduate School of Information Science and Technology, Osaka University
Metrics ( 3 ): Inheritance Metric for Clone SetDCH
DCH(S): represents the position and distance between each fragment of a clone set S
Definition
Clone set S includes fragment f1, f2, ・・・, fn
Fragment fi exists in class Ci
Class Cp is a class which locates lowest position in C1, C2, ・・・,Cn on class hierarchy
If no common parent class of C1 , C 2,・・・, Cn exists, the value of DCH(S) is -1This metric is measured for only the class hierarchy where target software exists.
example 1:・ Clone set S includes fragments f1 and f2.・ If all fragments of clone set S are included in a same class,
then , DCH(S) = 0
class A
fragment f1
fragment f2
class A
class B class C
fragment f1 fragment f2
example 2 :・ Clone set S includes fragments f1 and f2.・ If all fragments of clone set S are included in a class and its direct child classes,
then , DCH(S) = 1
fragment f1 fragment f2
class A class B
example 3 :・ Clone set S includes fragments f1 and f2.・ If all classes which include f1 and f2 don’t have common parent class,
then , DCH(S) = -1