Cs428 9 software engineering ii
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CS428/9: Software Engineering II. Darko Marinov (slides from Ralph Johnson). Administrative info. ACP third revision due on Thursday, March 15 HW3 posted on Wiki: metrics, due March 27 Will be graded by March 29 and covered in Demo 2 Demo 2: Functionality (code), testing, metrics

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CS428/9: Software Engineering II

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Cs428 9 software engineering ii

CS428/9: Software Engineering II

Darko Marinov

(slides from Ralph Johnson)


Administrative info

Administrative info

  • ACP third revision due on Thursday, March 15

  • HW3 posted on Wiki: metrics, due March 27

    • Will be graded by March 29 and covered in Demo 2

  • Demo 2: Functionality (code), testing, metrics

    • Will look more than before into your code and tests

    • After Spring Break, around April 3

    • Advisable to meet your TA before Spring Break

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Metrics

Metrics

  • Non-technical metrics about process

    • Number of people on project, time taken, money spent

    • Stories implemented

    • Bugs found (by testers/developers, by users)

    • Bugs fixed, features added

  • Technical metrics about product

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Technical metrics

Technical metrics

  • Size of code

    • Lines of code (LOC, SLOC, NCNB LOC)

    • Number of files, classes, processes

    • Function points

  • Complexity of code

    • Dependencies / coupling / cohesion

    • OO metrics

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Some ways to use metrics

Some ways to use metrics

  • Measure performance of programmers

    • Measure the amount of code produced each month by each programmer

    • Give high producers big raise

  • Prioritize programmers’ effort

    • Measure complexity of modules

    • Pick the most complex and rewrite it

  • Determine whether code is ready to ship

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Complexity

Complexity

  • Complex systems are

    • Hard to understand

    • Hard to change

    • Hard to reuse

      Cyclomatic complexity (covered last lecture)

      Cohesion and coupling (this lecture)

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Cyclomatic complexity

Cyclomatic complexity

CC = E – N + 2, where

E is the number of edges and

N is the number of nodes

def addInterest

for

if

acc.balance = ...

done

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Technical metrics for oo systems

Technical metrics for OO systems

  • Material from these readings

    • Bob Martin: OO Design Quality Metrics

    • Shyam R. Chidamber and Chris F. Kemerer:A Metrics Suite for Object Oriented Design

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Coupling and cohesion

Coupling and cohesion

  • Number and complexity of shared variables

    • Functions in a module should share variables

    • Functions in different modules should not

  • Number and complexity of parameters

  • Number of functions/modules that are called

  • Number of functions/modules that call me

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Dhama s coupling metric

Dhama’s coupling metric

Module coupling =

1 / (number of input parameters + number of output parameters + number of global variables used + number of modules called + number of modules calling)

.5 is low coupling, .001 is high coupling.

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Martin s coupling metric

Martin’s coupling metric

  • Ca: Afferent coupling: the number of classes outside this module that depend on classes inside this module

  • Ce: Efferent coupling: the number of classes inside this module that depend on classes outside this module

  • Instability = Ce / (Ca + Ce)

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Main sequence

Main sequence

  • A: Abstractness (# abstract classes in module / # classes in module)

(0,1)

Abstractness

(1,0)

Instability

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Technical metrics1

Technical metrics

  • A Metrics Suite for Object Oriented Design, Shyam R. Chidamber and Chris F. Kemerer IEEE Transactions on Software Engineering, June 1994, pp 476-493

  • Chapter 4 of Hamlet and Maybee, especially 4.3

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List of metrics

List of metrics

  • Weighted Methods per Class

  • Depth of Inheritance Tree

  • Number Of Children

  • Coupling Between Object classes

  • Response for a Class

  • Lack of Cohesion in Methods

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Weighted methods per class

Weighted Methods per Class

WMC: for each class, take the sum of the complexities of the methods in the class

Possible method complexities

  • 1 (number of methods)

  • Lines of code

  • Number of method calls

  • Cyclomatic complexity

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Weighted methods per class1

Weighted Methods per Class

  • The number of methods and the complexity of methods predicts the time and effort required to develop and maintain a class

  • The larger the number of methods in a class, the greater the potential impact on children

  • Classes with large numbers of methods are more likely to be application specific and less reusable

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Weighted methods per class2

Weighted Methods per Class

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Depth of inheritance tree

Depth of Inheritance Tree

  • DIT: Maximum length from a class to the root of the tree

  • The deeper a class is in the hierarchy, the more methods it inherits and so it is harder to predict its behavior

  • The deeper a class is in the hierarchy, the more methods it reuses

  • Deeper trees are more complex

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Depth of inheritance tree1

Depth of Inheritance Tree

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Number of children

Number Of Children

  • NOC: Number of immediate subclasses

  • More children is more reuse

  • A class might have a lot of children because of misuse of subclassing

  • A class with a large number of children is probably very important and needs a lot of testing

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Number of children1

Number of Children

  • Almost all classes have 0 children

  • Only a handful of classes will have more than five children

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Coupling between object classes

Coupling Between Object classes

  • CBO: Number of other classes to which a class is coupled

  • Class A is coupled to class B if there is a method in A that invokes a method of B

  • Want to be coupled only with abstract classes high in the inheritance hierarchy

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Coupling between object classes1

Coupling Between Object classes

  • Coupling makes designs hard to change

  • Coupling makes classes hard to reuse

  • Coupling is a measure of how hard a class is to test

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Coupling between object classes2

Coupling Between Object classes

  • C++ project: median 0, max 84

  • Smalltalk project: median 9, max 234

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Response for a class

Response For a Class

  • RFC: Number of methods in a class or called by a class

  • The response set of a class is a set of methods that can potentially be executed in response to a message received by an object of that class

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Response for a class1

Response For a Class

  • If a large number of methods can be invoked in response to a message, testing becomes more complicated

  • The more methods that can be invoked from a class, the greater the complexity of the class

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Response for a class2

Response For a Class

  • C++: median 6, max 120

  • Smalltalk: median 29, max 422

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Lack of cohesion in methods

Lack of COhesion in Methods

  • LCOM: Number of pairs of methods that don’t share instance variables - number of pairs of methods that share instance variables

  • Cohesiveness of methods is a sign of encapsulation.

  • Lack of cohesion implies classes should be split

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Lack of cohesion in methods1

Lack of COhesion in Methods

  • C++: median 0, max 200

  • Smalltalk: median 2, max 17

  • Smalltalk system had only a few zero

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What is best

What is best?

  • WMC or list of long methods?

  • DIT or list of classes with depth over 6?

  • NOC or list of classes with more than 6 children?

  • CBO or list of classes with high coupling?

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Questions

Questions?

  • Who looks at these figures?

  • What is done with the results?

  • How do you get the figures?

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The goal of measurement

The goal of measurement

  • Software measurement is a means to an end, not an end in itself

  • What is causing poor quality?

  • Where are we spending all our time on software development?

  • How accurate are our estimates?

  • What is the most cost-effective way to improve our quality?

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Next time

Next time

  • Software Measurement Guidebook

  • http://swg.jpl.nasa.gov/resources/NASA-GB-001-94.pdf

  • Pages 1-20 and 30-49

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