1. Evaluating the impact of refactorings on internal and external quality indicators

2. 2 Problem statement  How can we optimally apply refactorings in order to maximize the improvement regarding specific quality attributes? –Which quality attributes? –What is the impact of a refactoring on these quality attributes? –How can we evaluate this impact?

3. 3 Selecting quality attributes  Pervading methods of organization [Coad&Yourdon90] : –Distinguishing Objects and their attributes Trees and their size –Distinguishing Wholes and their parts Trees and their branches –Distinguishing Classes Trees and Stones  Leads to Coupling & Cohesion

4. 4 Coupling and cohesion  Too coarse-grained  Categorization of Briand et. al in 5 coupling and 2 cohesion dimensions –{Import, Export, General, Aggregation, Inheritance} coupling –{Normalized, Non-normalized} cohesion

5. 5 Requirements f/a categorization  Orthogonal categories  Semantically sane (ito design practice)  Assumed/known relation to external properties

6. 6 Analyzing impacts  Express refactoring postconditions and metrics in a uniform formalism  Derive the impact of the refactoring on the metric in this formalism  Impact is conditionally expressed  Use this qualitive feedback to optimize the refactoring process.

7. 7 Example of conditional impact Quality attribute spectrum DeteriorationImprovement Tight Class Cohesion as cohesion indicator  Applying Move Method can both improve and deteriorate normalized cohesion. Move method with local references Move method without local references

8. 8 Using qualitative refactoring feedback  From the conditions specified in the refactoring impact, specific guidelines for improvement can be composed.  These guidelines should be validated empirically for external quality attributes  This allows us to focus on those refactoring opportunities that improve specific quality attributes. Quality attribute spectrum DeteriorationImprovement

9. 9 Evaluating impacts  For specific maintenance tasks… –Analyze software system to narrow down improvement target –Apply analytically derived guidelines in a controlled environment –Measure known maintenance performance indicators

10. 10 Maintenance performance  Correctness –Objective evaluation Change points …  Execution time

11. 11 Aftermath  Current assumption: external quality as monotonic function of internal –Where and how to apply refactorings  Alternative: polynomial relation –When to apply refactorings. Internal quality attribute value External quality attribute value