Law of Demeter

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Law of Demeter
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Law of Demeter

• What is it Style Rule for building systems.
• Proposed by my research group The Demeter
  Research Group in 1987, published in 1988.
• Covered in many major books on OO design and
  programming.

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Law of Demeter Principle

• Each unit should only use a limited set of other
  units only units closely related to the
  current unit.
• Each unit should only talk to its friends.
  Dont talk to strangers.
• Main Motivation Control information overload. We
  can only keep a limited set of items in
  short-term memory.

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Law of Demeter
FRIENDS
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closely related
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Application to OO

• Unit method
• closely related
• methods of class of this/self and other argument
  classes
• methods of immediate part classes (classes that
  are return types of methods of class of
  this/self)
• In the following we talk about this application
  of the Law of Demeter Principle to OO example
  follows in a few slides.

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Citibank Quote Law of Demeter

• The Law of Demeter forms one of the cornerstones
  of the design approach of the Global Finance
  Application Architecture (quote from Global
  Finance Application Architecture Business
  Elements Analysis, Oct. 1991, Citibank
  confidential document)
• Widely used in big projects, for example, at JPL
  for the Mars exploration software.

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Jet Propulsion Laboratory(JPL) Quote Law of
Demeter

• The Law of Demeter has taken a firm hold in
  many areas of JPL. Major systems which have used
  LoD extensively include Mars Pathfinder
  Software (begun in 1993). We are going to use LoD
  as a foundational software engineering principle
  for the X2000 Europa orbiter mission.

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What others say about the Law of Demeter

• Two examples
• Booch
• Rumbaugh

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Booch and the Law of Demeter

• Context
• Chapter Classes and Objects, Section On
  Building Quality Classes and Objects, Subsection
  Choosing Relationships

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Booch and the Law of Demeter

• Quote The basic effect of applying this Law is
  the creation of loosely coupled classes, whose
  implementation secrets are encapsulated. Such
  classes are fairly unencumbered, meaning that to
  understand the meaning of one class, you need not
  understand the details of many other classes.

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Rumbaugh and the Law of Demeter

• Context
• Chapter Programming Style, Section Extensibility

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Rumbaugh and the Law of Demeter

• Quote Avoid traversing multiple links or
  methods. A method should have limited knowledge
  of an object model. A method must be able to
  traverse links to obtain its neighbors and must
  be able to call operations on them, but it should
  not traverse a second link from the neighbor to a
  third class.

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Law of Demeter(alternative formulation)

• A method should have limited knowledge of an
  object model.
• Leads to another Demeter favorite Use grammars
  to define both class structure and an
  application-specific language. See the
  Structure-Shy Object Pattern.

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Agreement that LoD Good Idea

• How to follow LoD good solutions exist but not
  widely known. Two approaches to following LoD
• OO approach
• Adaptive approaches
• Traversal support
• APPC
• DemeterJ

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The Law of Demeter (cont.)Violation of the Law

• class A public void m() P p() B b
• class B public C c
• class C public void foo()
• class P public Q q()
• class Q public void bar()
• void Am()
• this.b.c.foo() this.p().q().bar()

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Violations Dataflow Diagram
m
foo()
2c
1b
B
C
A
4q()
bar()
3p()
P
Q
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OO Following of LoD
m
foo2
foo()
c
1b
B
C
A
2foo2()
4bar2()
bar2
bar()
3p()
q()
P
Q
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Adaptive Following of LoD

• void Am()
• (C)
• Traversal.long_get(this,A-gtC).foo()
• (Q)
• Traversal.long_get(this,A-gtQ).bar()
• void Am()
• this.b.c.foo() this.p().q().bar() //
  violation

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Law of Demeter
FRIENDS
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What if your friends are far away?

• You pay them to travel to you or you send an
  agent to them to collect the information you
  need.
• Approximate Directions You give them or your
  agent directions about what kind of information
  to collect but you dont care about accidental
  details of the travel.
• Detailed Directions You give them or your agent
  detailed travel directions.

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Adaptive Following LoD
C
A
FRIENDS
a
S
X
c
b
aFrom S to A bFrom S to B cFrom S via X
to C
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Traversal strategies create friends

• Class Traversal is an intermediate class between
  classes that need to communicate

Traversal.long_get(Object o, Strategy s)
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Are not friends for accidental reasons

• Other classes exist for other reasons
• Ideal class graph all are friends, even far
  away classes.