Designing and Maintaining Software (DAMS) Louis Rose
Recap: Cohesion
“A measure of the degree to which the elements of a module belong together.” - Yourdon & Constantine Structured Design, 1979
Cohesion Metrics
Indicate the extent to which elements are used together (and hence belong together?)
Corollary There are lots of cohesion metrics!
LCOM Lack of Cohesion of Methods
An inverse metric: a lower score is better We are using the 4th revision: LCOM4 Partitions an element (e.g., class) into sets of related elements
- Briand, Daly and Wust A Unified Framework for Cohesion Measurement in Object-Oriented Systems Empirical Software Engineering, 3, 1998
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
bake
oven
LCOM4 Example class Pizza attr_reader :toppings, :oven
toppings
def title toppings.title end
title
def cost toppings.cost + 4 end
cost
def bake oven.bake(self) end end
oven
bake
LCOM4 Example @toppings
@oven
toppings
oven
class Pizza attr_reader :toppings, :oven def title toppings.title end def cost toppings.cost + 4 end def bake oven.bake(self) end end
title
cost
bake
LCOM4 Example @toppings
@oven
toppings
oven
class Pizza attr_reader :toppings, :oven def title toppings.title end def cost toppings.cost + 4 end def bake @oven.bake(self) end end
title
cost
bake
LCOM4 Example @toppings
@oven
toppings
oven
class Pizza attr_reader :toppings, :oven def title toppings.title end def cost toppings.cost + 4 end def bake @oven.bake(self) end end
title
cost
bake
LCOM4 Example @toppings
@oven
toppings
oven
class Pizza attr_reader :toppings, :oven def title toppings.title end def cost toppings.cost + 4 end def bake @oven.bake(self) end end
title
cost
bake
Divergent Change What is it? A class can change for several reasons Why is it problematic? Changes are harder to make with confidence When does it arise? 👎 Violation of Single Responsibility Principle 👍 You know that the triggering changes won’t occur
Data Clumps What is it? Several pieces of data are often used together Why is it problematic? Behaviour that operates on the clump has no home (and consequently is often duplicated) When does it arise? 👎 High cohesion of the clump has not been detected 👍 Domain is not thoroughly understood (yet)
Primitive Obsession What is it? Using built-in types to represent domain concepts. Why is it problematic? Behaviour that operates on the domain object has no home (and consequently is often duplicated) When does it arise? 👎 Fear of OO (often due to performance concerns) 👍 If any extracted object would be very small
Feature Envy What is it? An object is more concerned with another object’s attributes or methods than its own. Why is it problematic? Behaviour probably belongs on the envied object (and consequently is often duplicated) When does it arise? 👎 Violation of Tell, Don’t Ask principle 👍 Behaviour is much more likely to change when the envying class changes
Summary Classes should have a single responsibility: a single reason to change Classes with low cohesion normally have more than one responsibility Extract classes to redistribute responsibilities
Designing and Maintaining Software (DAMS) - GitHub
What is it? Several pieces of data are often used together. Why is it problematic? Behaviour that operates on the clump has no home. (and consequently is often duplicated). When does it arise? High cohesion of the clump has not been detected. Domain is not thoroughly understood (yet) ...
ASTs are tree data structures that can be analysed for meaning (following JLJ in SYAC 2014/15) ... More Cohesive. Avoids Duplication. Clearer. More Extensible.
Open-source. Influenced by Perl, Smalltalk, Eiffel, Ada and Lisp. Dynamic. Purely object-oriented. Some elements of functional programming. Duck-typed class Numeric def plus(x) self.+(x) end end y = 5.plus(6) https://www.ruby-lang.org/en/about · http
Page 1. Getting Lean. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Lean software⦠Has no extra parts. Solves the problem at hand and no more. Is often easier to change (i.e., is more habitable). Page 3. The Advice I Want to Give.
Why not duplicate? Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Bad Practice. Page 4. Don't Repeat Yo
âWe have tried to demonstrate that it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which
Tools: Vagrant. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bugs that appear in production and that can't be reproduced by a developer on their machine are really hard to fix. Problem: âIt works on my machineâ. Page 3. Why does
Clear Documentation. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Bad documentation. Misleading or contradictory find_customer(id). CustomerGateway. Used to look up a customer by their customer number. Page 3. Bad documentation. Red
%w.rack tilt date INT TERM..map{|l|trap(l){$r.stop}rescue require l};. $u=Date;$z=($u.new.year + 145).abs;puts "== Almost Sinatra/No Version has taken the stage on #$z for development with backup from Webrick". $n=Module.new{extend. Rack;a,D,S,q=Rack
R&D: sketch habitable solutions on paper, using UML. 4. Evaluate solutions and implement the best, using TDD. Probably start again at 3. 5. Give to the product owner to validate. Probably start again at 1. 6. Put into production for customers to eval
Habitable Software. Leaner. Less Complex. Loosely Coupled. More Cohesive. Avoids Duplication. Clearer. More Extensible ??? Page 3. Lean. âPerfection is finally achieved not when there is no longer anything to add, but when there is no longer anythi
Fixes issue #42. Users were being redirected to the home page after login, which is less useful than redirecting to the page they had originally requested before being redirected to the login form. * Store requested path in a session variable. * Redi
Observers. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Page 3. Delivery people need to know when pizzas are ready class Pizza def initialize(delivery_person). @delivery_person = delivery_person end def bake cook # blocking call. @d
âWe want the reading of code to be easy, even it makes the writing harder. (Of course, there's no way to write code without also reading it, soâ¦)â - Bob Martin. Clean Code. Prentice Hall, 2009. Page 5. Page 6. User Experience. âA person of av
Getting loose coupling. Designing and Maintaining Software (DAMS). Louis Rose ... should not depend on low-level modules. Both should depend on abstractions.â âAbstractions should not depend on details. Details should depend on abstractions.â -
Automatically detect similar fragments of code. class StuffedCrust def title. "Stuffed Crust " +. @toppings.title +. " Pizza" end def cost. @toppings.cost + 6 end end class DeepPan def title. "Deep Pan " +. @ingredients.title +. " Pizza" end def cost
Ruby Testing Frameworks. 3 popular options are: RSpec, Minitest and Test::Unit. We'll use RSpec, as it has the most comprehensive docs. Introductory videos are at: http://rspec.info ...
Clear Names. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Naming is hard. âThere are only two hard things in Computer. Science: cache invalidation and naming things.â - Phil Karlton http://martinfowler.com/bliki/TwoHardThings.ht
Coupling Between Objects. Counts the number of other classes to which a class is coupled (other than via inheritance). CBO(c) = |d â C - (1cl U Ancestors(C))| uses(c, d) V uses(d, c). - Chidamber and Kemerer. A metrics suite for object-oriented des
Reducing duplication. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Tactics. Accentuate similarities to find differences. Favour composition over inheritance. Know when to reach for advanced tools. (metaprogramming, code generation).
Plug-ins. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Problem. Page 3. Current Architecture. Shareable. Likeable. Food. Pizza. Liking and sharing foods are primary business concerns, so shouldn't be implemented as delegators. Page
When we are testing the way that a unit behaves when a condition is met, use a stub to setup the condition. Solution: use stubs for queries class Subscription ... def bill(amount) unless payments.exists(subscription_id: id) payments.charge(subscripti
Getting Cohesion. Designing and Maintaining Software (DAMS). Louis Rose. Page 2. Single Responsibility. Principle. A class should have only one reason to change. - Martin and Martin. Chapter 8, Agile Principles, Patterns and Practices in C#, Prentice
Size != Complexity. âImagine a small (50 line) program comprising. 25 consecutive "IF THEN" constructs. Such a program could have as many as 33.5 million distinct control paths.â - Thomas J. McCabe. IEEE Transactions on Software Engineering, 2:4,