Software Development Using Proper Separation of Concerns: Talk only to your friends who share your concerns (Law of Demeter for Concerns)

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Software Development UsingProper Separation of
ConcernsTalk only to your friends who share
your concerns(Law of Demeter for Concerns)

• Theme/Introduction

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General

• Many interpretations for talk and friends.
• LOD Only talk to your friends
• LoDC Only talk to your friends who share your
  concerns
• LoDC implies LoD
• LoD a module should not know about too many
  other modules.

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LoD OO

• Send messages only to preferred supplier objects.
• Preferred suppliers ??

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LODC

• Crossing boundaries.
• Specify separately what is foreign
• ClassGraph, Strategy, Visitor
• LoD organize inside a set of concerns
• LoDC separate outside concerns concern-based
  growth phases modularize each growth phase

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Summing LoDC

• Find all Salary-objects Traversal concern
  WhereToGo concern only deals with traversal
• Visitor separately expresses WhenAndWhatToDo.
  Describes where traversal is extended
• Start c 0
• Salary cvalue

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Weaving

• cg.traverse(o, whereToGo, whatAndWhereToDo)
• Weaves four concerns
• Structure concern (cg)
• Instantiation concern (o)
• Traversal concern (whereToGo)
• Collaboration concern (whatAndWhereToDo)

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How do we find all Salary objects

• Follow LoD, not good enough
• Follow DRY
• Abstract from class graph
• From Company to Salary
• And not company.divisions.departments.
• workGroups.employees.getSalary()

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Relational Formulation
from x1 to x2 (from Company to Salary)
From object o of class x1, to get to x2, follow
edges in the set POSS(x1,x2)e x1
lt.e.gt.(lt.C.gt).lt x2
Can easily compute these sets for every x1, x2
via transitive-closure algorithms. POSS
abbreviation for following these edges it is
still possible to reach a x2-object for some
x1-object rooted at o.
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Adaptation Dilemma

• When a parameterized abstraction P(Q) is given
  with a broad definition of the domain of the
  allowed actual parameters, we need to retest and
  possibly change the abstraction P when we modify
  the actual parameter, i.e., we move from P(Q1)
  to P(Q2).

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Consequence of Adaptation Dilemma

• When class graph changes, need to test all
  strategies

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Producing Traversal code

• For some class graph and strategy combinations we
  might produce a large number of traversal methods
  if we are not careful (exponential in worst
  case).
• DJ (DemeterJ, DJ, DAJ) avoid this problem. Have
  built-in efficient code generation.

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PaperBoy Example

• // customer.wallet.totalMoney
• // customer.apartment.kitchen.
• // kitchenCabinet.totalMoney
• // customer.apartment.bedroom.
• // mattress.totalMoney 
• we widen the interface of the Customer class and
  add a method
• int customer.getPayment(..)

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Implications of LoD

• A general problem of following the LoD is that it
  leads to wide class interfaces because there are
  so many ways of traversing through a complex
  object structure for different purposes.
• Use abstraction to organize the wide interfaces
  using the strategy language

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Other LoDC Applications

• Growth plans for class graphs
• Constructing data structures

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Other Applications of LoD

• Object creation
• Constructor call call on a class object
• Minimize number of such calls follows the LoD
  (Principle of Minimal Knowledge)
• A.parse(object description)