© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.1 7. Problem Detection Metrics  Software quality  Analyzing trends Duplicated Code  Detection techniques  Visualizing duplicated code

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.2 Why Metrics in OO Reengineering (ii)? Assessing Software Quality  Which components have poor quality? (Hence could be reengineered)  Which components have good quality? (Hence should be reverse engineered)  Metrics as a reengineering tool! Controlling the Reengineering Process  Trend analysis: which components did change?  Which refactorings have been applied?  Metrics as a reverse engineering tool!

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.3 ISO 9126 Quantitative Quality Model Leaves are simple metrics, measuring basic attributes

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.4 Product & Process Attributes

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.5 External & Internal Attributes

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.6 External vs. Internal Product Attributes ExternalInternal Advantage: close relationship with quality factors Disadvantage: relationship with quality factors is not empirically validated Disadvantages: measure only after the product is used or process took place data collection is difficult; often involves human intervention/interpretation relating external effect to internal cause is difficult Advantages: can be measured at any time data collection is quite easy and can be automated direct relationship between measured attribute and cause

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.7 Metrics and Measurements [Wey88] defined nine properties that a software metric should hold. Read [Fenton] for critiques. For OO only 6 properties are really interesting [Chid 94, Fenton] 1. Noncoarseness: Given a class P and a metric m, another class Q can always be found such that m (P)  m(Q) not every class has the same value for a metric 2. Nonuniqueness. There can exist distinct classes P and Q such that m(P) = m(Q) two classes can have the same metric 3. Monotonicity m(P)  m (P+Q) and m(Q)  m (P+Q), P+Q is the “combination” of the classes P and Q.

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.8 Metrics and Measurements (ii) 4. Design Details are Important The specifics of a class must influence the metric value. Even if a class performs the same actions details should have an impact on the metric value. 5. Nonequivalence of Interaction m(P) = m(Q)  m(P+R) = m(Q+R) where R is an interaction with the class. 6. Interaction Increases Complexity m(P) + (Q) < m (P+Q). when two classes are combined, the interaction between the too can increase the metric value Conclusion: Not every measurement is a metric.

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.9 Selecting Metrics Fast  Scalable: you can’t afford log(n2) when n  1 million LOC Precise  (e.g. #methods — do you count all methods, only public ones, also inherited ones?)  Reliable: you want to compare apples with apples Code-based  Scalable: you want to collect metrics several times  Reliable: you want to avoid human interpretation Simple  Complex metrics are hard to interpret

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.10 Assessing Maintainability Size of the system, system entities  Class size, method size, inheritance  The intuition: large entities impede maintainability Cohesion of the entities  Class internals  The intuition: changes should be local Coupling between entities  Within inheritance: coupling between class- subclass  Outside of inheritance  The intuition: strong coupling impedes locality of changes

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.11 Sample Size and Inheritance Metrics Class Attribute Method Access Invoke BelongTo Inherit Inheritance Metrics hierarchy nesting level (HNL) # immediate children (NOC) # inherited methods, unmodified (NMI) # overridden methods (NMO) Class Size Metrics # methods (NOM) # instance attributes (NIA, NCA) # Sum of method size (WMC) Method Size Metrics # invocations (NOI) # statements (NOS) # lines of code (LOC)

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.12 Sample class Size (NIV)  [Lore94] Number of Instance Variables (NCV)  [Lore94] Number of Class Variables (static) (NOM)  [Lore94] Number of Methods (public, private, protected) (E++, S++) (LOC) Lines of Code (NSC) Number of semicolons [Li93]  number of Statements (WMC) [Chid94] Weighted Method Count  WMC = ∑ c i  where c is the complexity of a method (number of exit or McCabe Cyclomatic Complexity Metric)

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.13 Hierarchy Layout (HNL) [Chid94] Hierarchy Nesting Level, (DIT) [Li93] Deep of Inheritance Tree, HNL, DIT = max hierarchy level (NOC) [Chid94] Number of Children (WNOC) Total number of Children (NMO, NMA, NMI, NME) [Lore94] Number of Method Overridden, Added, Inherited, Extended (super call) (SIX) [Lore94]  SIX (C) = NMO * HNL / NOM  Weighted percentage of Overridden Methods

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.14 Method Size (MSG) Number of Message Sends (LOC) Lines of Code (MCX) Method complexity  Total Number of Complexity / Total number of methods  API calls= 5, Assignment = 0.5, arithmetics op = 2, messages with params = 3....

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.15 Sample Metrics: Class Cohesion (LCOM) Lack of Cohesion in Methods [Chid94] for definition [Hitz95a] for critique I i = set of instance variables used by method M i let P = { (I i, I j ) | I i  I j =  } Q = { (I i, I j ) | I i  I j   } if all the sets are empty, P is empty LCOM =|P| - |Q|if |P|>|Q| 0otherwise Tight Class Cohesion (TCC) Loose Class Cohesion (LCC) [Biem95a] for definition Measure method cohesion across invocations

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.16 Sample Metrics: Class Coupling (i) Coupling Between Objects (CBO) [Chid94a] for definition, [Hitz95a] for a discussion  Number of other classes to which it is coupled Data Abstraction Coupling (DAC) [Li93a] for definition  Number of ADT’s defined in a class Change Dependency Between Classes (CDBC) [Hitz96a] for definition  Impact of changes from a server class (SC) to a client class (CC).

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.17 Sample Metrics: Class Coupling (ii) Locality of Data (LD) [Hitz96a] for definition LD = ∑ |L i | / ∑ |T i | L i = non public instance variables + inherited protected of superclass + static variables of the class T i = all variables used in M i, except non-static local variables M i = methods without accessors

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.18 The Trouble with Coupling and Cohesion Coupling and Cohesion are intuitive notions  Cf. “computability”  E.g., is a library of mathematical functions “cohesive”  E.g., is a package of classes that subclass framework classes cohesive? Is it strongly coupled to the framework package?

© S. Demeyer, S. Ducasse, O. Nierstrasz Duplication.19 Conclusion: Metrics for Quality Assessment Can internal product metrics reveal which components have good/poor quality? Yes, but...  Not reliable false positives: “bad” measurements, yet good quality false negatives: “good” measurements, yet poor quality  Heavy Weight Approach Requires team to develop (customize?) a quantitative quality model Requires definition of thresholds (trial and error)  Difficult to interpret Requires complex combinations of simple metrics However...  Cheap once you have the quality model and the thresholds  Good focus (± 20% of components are selected for further inspection) Note: focus on the most complex components first!