Department of Computer Science, Graduate School of Information Science & Technology, Osaka University A Metric-based Approach for Reconstructing Methods in Object-Oriented Systems Tatsuya Miyake, Yoshiki Higo, Katsuro Inoue

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Research Overview Propose a method that automates some of refactoring steps – Identify where the software should be refactored – Determine which refactoring pattern should be applied – Estimate the effect of the refactoring ⇒ Support to perform appropriate refactorings with low cost. 2016/6/27WoSQ2008 2

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Refactoring What’s Refactoring – A set of operations to improve internal attributes of a software system without changing the external behavior of it – One of trenchant countermeasures to handle large-scale and complex software systems Refactoring Procedure – Step 1: Identify where the software should be refactored – Step 2: Determine which refactoring pattern should be applied to the identified place – Step 3: Estimate the effect of the refactoring – Step 4: Apply the refactoring – Step 5: Maintain the consistency between the refactored program code and other software artifacts 2016/6/27WoSQ2008 3

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Issue with Refactoring Refactoring often requires the high costs  A good deal of knowledge and experiences There are few skillful developers Inappropriate refactorings may be performed instead of appropriate ones  A lot of time and energy It is difficult to precisely estimate the effect of refactorings on the early stage of the refactoring  The refactoring may not be worth the cost 2016/6/27WoSQ2008 4

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Research Purpose Automatization of a part of refactoring process  Reduction of the refactoring cost  Appropriate refactoring Improve the maintainability  Efficient and effective refactoring Estimate the effect of refactoring 2016/6/27WoSQ2008 5

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ2008 6

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ2008 7

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ2008 8

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Identify where the software should be refactored Measure metrics on each method and each block statements in it  Cyclomatic complexity The complexity of control logics The number of linearly independent paths from the start node to the end one of a graph  Represented by the number of conditional expressions plus 1  LOC (Lines Of Code) Used for measurement of the identified fragment’s occupancy rate (OR) in the method  ALV ( Available Local Variables ) Identify the block statements having undesirable metrics value with their surrounding code 2016/6/27WoSQ2008 9

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Local Variable Encapsulation 2016/6/27WoSQ To hide unused local variables may improve maintainability of a mehtod  Extract the code fragment as a new method  Access by arguments and a return value as needed Local variable encapsulation ⇒ Reduce the number of available variables Attributes of classes should be hidden for improving maintainability  Declare a field as a private attribute  Use accessor methods as needed Field encapsulation

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of Local Variable Encapsulation 2016/6/27WoSQ public void sample ( ) { int i = 0; String str; int j = 0; boolean b; String str2; ・ ・ while( hoge() ) { int k = bar(); jar( str ); foo(i, k); } ・ } There are 6 available variables i, str, j, b, str2, k j, b, str2 are not used public void sample ( ) { int i = 0; String str; int j = 0; boolean b; String str2; ・ newMethod(i, str); ・ } void newMethod(int i, String str) { while( hoge() ) { int k = bar(); jar( str ); foo(i, k); } } Available variables are only i, str, k Only the used variables are referenced as arguments Refactoring ALV : 6 ALV : 5 ALV : 3 The unused variables, j, b, str2, are hidden

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Relation between block statements 2016/6/27WoSQ public void sample ( ) { int i = 0; String str; while( hoge() ) { str = “string”; if( jar( I ) ) { make(str); } foo(i); for( int j = 0; j < I ; j++) { sam( i ); if( log ) { System.out.println(i); } ・ while( ) { bar(i); make(str); } sample ( ) while iffor if Vertical relation – Between a block statement and its outer or inner block statements. Horizontal relation – Between two block statements in the same scope

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Vertical Coupling between Block Statements A coupling between block statements that have the vertical relation each other Measure based on the number of outer variables that are the used local variables defined outside the bock statement – NRV ( Number of Referenced Variables ) Pass the values to the new method as arguments, If extract the new method – NAV ( Number of Assigned Variables ) The new method may have to return the assigned variables as return value 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of Vertical Coupling ~ Low Coupling~ 2016/6/27WoSQ public void sample ( ) { ・ newMethod(); ・ } private void newMethod ( ) { while( hoge() ) { int i = 0; String str = “string”; foo(i); } public void sample ( ) { ・ while( hoge() ) { int i = 0; String str = “string”; foo(i); } ・ } Just extract as a new method NRV : 0 NAV : 0

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of Vertical Coupling ~ Strong Coupling~ 2016/6/27WoSQ public void sample ( ) { int i = 0; boolean bool ; String str; str = newMethod(i, bool); ・ return str; } private String newMethod (int i, boolean bool) { String str; while( hoge() ) { str = “string”; foo(i); bar( bool ); } return str; } public void sample ( ) { int i = 0; boolean bool ; String str; ・ while( hoge() ) { str = “string”; foo(i); bar( bool ); } ・ return str; } NRV : 2 NAV : 1 Two outer variables, i and bool, are referenced These value have to be passed to the new method as arguments Extract as a new method An outer variable, str, is assigned str is subsequently referenced Have to return a value of the assigned variable If the number of assigned outer variables is more than one Strong vertical coupling ⇒ a lot of operations to complete the refactoring

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Horizontal Coupling between Block Statements A coupling between two block statements that have horizontal relation each other Measured based on the number and the kind of data flows between the two block statements 2016/6/27WoSQ Strong couplingLow coupling Usage of the same variables in both block statements no data flow Extract as a new single method Extract as different new methods The kind of data flows Refactoring threshold indirect data flows

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University public void newMethod2 (int i, String str){ while( ) { bar(i); make(str); } public String newMethod1 (int i) { String str; while( hoge() ) { str = “string”; foo(i); } return str; } Example of Horizontal Coupling ~Strong coupling~ 2016/6/27WoSQ public void sample ( ) { int i = 0; String str; ・ while( hoge() ) { str = “string”; foo(i); } ・ while( ) { bar(i); make(str); } ・ } Extract as different new methods public void sample ( ) { int i = 0; String str; ・ str = newMethod1 ( i ); ・ newMethod2( i, str ); ・ } Two variables, str and i, are referenced or assigned Two same variables, str and i, are referenced or assigned The two variables, str and i, have to be shared

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of Horizontal Coupling ~Strong coupling~ 2016/6/27WoSQ public void sample ( ) { int i = 0; String str; ・ while( hoge() ) { str = “string”; foo(i); } ・ while( ) { bar(i); make(str); } ・ } Public void sample () { int i = 0; ・ newMethod ( i ); ・ } public String newMethod1 (int i) { String str; while( hoge() ) { str = “string”; foo(i); } ・ while( ) { bar(i); make(str); } Extract as a new single method Easier than extracting as different methods All you have to do is to pass variable i The signature is simple

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of Horizontal Coupling ～ Low coupling ～ Extract as a new single method  The signature of the new method is simple Few arguments Need no return value Extract as different new methods  Division based on functionality Each variable has its own role 2016/6/27WoSQ public void sample ( ) { int i = 0; String str; ・ while( hoge() ) { str = “string”; foo(i); } ・ String str2 = str; ・ while( ) { bar(); make(str2); } ・ } There is no outer variable used in the both while statement A data flow between two while-statement by the assignment of str to str2

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Determine which refactoring should be applied Determine based on what kinds of members are used in the target fragment  Members of its own class are mainly used ⇒ Apply “Extract Method”  Only members of the super class are used ⇒ Apply “Pull Up Method”  Members of other classes are mainly used Local feature envy  Only the identified spot may be interested in other classes Extract the identified spot as a method of the class whose members are most used ⇒ Apply both “Extract Method” and “Move Method” 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Estimate the effect of refactoring Definition of the effect of refactoring  Change of complexity metrics values Cyclomatic complexity  The number of conditional expressions in the extracted code LOC( Lines of Code)  The number of lines of the code extracted as a new method ALV( Available Local Variables )  The number of local variable declaration in the extracted code Coupling between classes  May change in the case of applying “Move Method” 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Proposal Technique -Overview- Identify where the software should be refactored  Target: A code fragment within a method Several software metrics Coupling of block statements with their surrounding code  Vertical coupling  Horizontal coupling Determine which refactoring pattern should be applied to the identified place  By reference to the used variables and the invoked methods  Turn the fragment into a new method Extract Method Pull Up Method Move Method Estimate the effect of the determined refactoring  Based on the change of software metrics 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Experiment Target language: Java Target software  High-quality software JHotDraw  Low-quality software Programs written by undergraduate students  Several versions of the same software Evaluation standard  Adequacy of automatic identification of the spot should be refactored  Adequacy of automatic determination of the applied refactoring  Change of complexity metrics value 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Case Study 2016/6/27WoSQ The number of available outer variable is 13 The number of really used outer variable is 3 Low vertical coupling ↓ It is easy to extract from outer block statements Low horizontal coupling ↓ The upper for- statement is not included in the extracted part There is no outer variable referenced in both “for-statement” Identification is adequate from functional standpoint because of the code comment

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Summary In this study, we  proposed ALV of new metrics for representing the maintainability of software  proposed the method that automates some of refactoring steps based on ALV and other metrics  Showed there are cases where our method is effective 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Future work We are going to  experiment more to confirm the adequacy of our refactoring method Quantitative data derived from the result of our planned experiment Evaluation of the result of our method by developers  investigate the relation between ALV and the quality of software Whether there is correlation between ALV and bugs or not  propose and implement program slicing method to identify more adequately the surrounding code of the block statement 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Thank you for your attention Please speak slowly, if you have some questions. 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Target software High-quality software  JHotDraw5.3b About loc About 288 classes Low-quality software  Programs written by under undergraduate students Support code clone analysis About 3000 loc About 107 classes Several versions of the same software 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Refactoring What’s Refactoring – A set of operations to improve internal attributes of a software system without changing the external behavior of it – One of trenchant countermeasures to handle large-scale and complex software systems Effect of Refactoring – Enhance readability of a target software – Improve software design Maintainability Scalability Reusability – Find bugs of a target software 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Procedure of Refactoring Step 1: Identify where the software should be refactored  Long Method  Complicated Control Structure Step 2: Determine which refactoring should be applied to the identified place  Various refactoring patterns have been proposed Step 3: Estimate the effect of the refactoring  Prevent from performing inefficient or ineffective refactorings Step 4: Apply the refactoring Step 5: Maintain the consistency between the refactored program code and other software artifacts 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of applying “Extract Method” 2016/6/27WoSQ class A { public boolean field1; public void a2(); public void a3(); public void sample ( ) { ・ while( field1() ) { a2(); b.b1(); a3(); } ・ } Class B { public b1(); public b2(); } Members of the same class are mainly used class A { public boolean field1; public void a2(); public void a3(); public void sample(); public void newMethod ( ) { while( field1() ) { a2(); b.b1(); a3(); } Class B { public b1(); public b2(); } Extract as a new method of the same class

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Example of applying “Pull Up Method” 2016/6/27WoSQ class A extends B{ public void a1(); public void a2(); public void a3(); public void sample ( ) { a1(); a2(); while( fieldB ) { b1(); b2(); } a3(); } class B { protected bool fieldB; protected void b1(); protected void b2(); } class A extends B{ ・・・・・・・・・・ public void sample ( ) { a1(); a2(); newSuperMethod(); a3(); } class B { protected bool fieldB; protected void b1(); protected void b2(); protected void newSuperMethod() { while( fieldB ) { b1(); b2(); } Extract as a new method of the super class Using only members of the super class in the extraction target

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Case Study (2/2) 2016/6/27WoSQ Low vertical coupling with owner method Low horizontal coupling Strong horizontal coupling

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Determine which refactoring should be applied Determine based on what kinds of members are used in the target fragment  Members of its own class are mainly used ⇒ Apply “Extract Method”  Only members of the super class are used ⇒ Apply “Pull Up Method”  Members of other classes are mainly used Local feature envy  Only the identified spot may be interested in other classes Extract the identified spot as a method of the class whose members are most used ⇒ Apply both “Extract Method” and “Move Method” 2016/6/27WoSQ

Department of Computer Science, Graduate School of Information Science & Technology, Osaka University Estimate the effect of refactoring Definition of the effect of refactoring  Change of complexity metrics values Cyclomatic complexity  The number of conditional expressions in the extracted code LOC( Lines of Code)  The number of lines of the code extracted as a new method ALV( Available Local Variables )  The number of local variable declaration in the extracted code Coupling between classes  May change in the case of applying “Move Method” 2016/6/27WoSQ