Color Modeling and the Law of
Demeter
By David J. Anderson
Introduction
One of the most widely misunderstood elements of color
modeling and the Domain Neutral Component is its loosely coupled nature and the
use of what is often referred to as the Law of Demeter. In a DNC compliant
model, it is intended that classes only hold dependencies on their immediate
neighbors in the DNC. This reduces the coupling of architecture and allows
decisions on the packaging and coarse-grained componentization to be postponed
until much later. The DNC treats every class as a component and both its
interface and degree of coupling are carefully considered.
Dependency in the DNC
We are all used to seeing the Domain Neutral Component
drawn as it is in Figure 1. This diagram shows the static persistent
associations between the classes. However, consider the diagram in Figure 2
which shows the dynamic dependencies in the form of <<use>>
relationships between the classes in the DNC. Note how each class only holds dependencies
to its immediate neighbors. This is at the heart of the agile nature of color
modeling and the flexibility of the Domain Neutral Component. Building a
loosely coupled functional architecture in this fashion minimizes the future
impact of change and minimizes the regression effect when code needs to be
refactored. It also facilitates lean software development by allowing decisions
on coarse-grained componentization and the interfaces of coarse-grained
components to be postponed until the last responsible moment. The result is a
significant reduction in refactoring effort.
Law of Demeter
The Law of Demeter was first proposed by Ian Holland of Northeastern
University, Boston, MA, in 1987. It was named after the project on which the
style guide was first adopted. The law states,
Each
unit should have only limited knowledge about other units: only units
"closely" related to the current unit.
Or: Each unit should only talk to its friends; Don't talk to strangers.
Figure 1. The Domain Neutral Component (1999)
Peter Coad widely adopted the lessons of the Law of
Demeter (LoD) in his work, though he rarely referred to it by name. The trade
off with LoD is that you end up with large method signatures on your classes
because they spend a lot of time providing contextual message forwarding. The
advantage is loose coupling - a class only knows about its neighbors. The
opposite of LoD code is simply to call accessors on classes and then ask the
final one in the chain, the question that you needed answering. Figure 3 shows a
sequence diagram for a correctly posed question which is LoD compliant. Figure
4 shows the same query done is a typical accessor fashion which is rightly
coupled and not LoD compliant.
We can see clearly that the sequence diagram in Figure 4
shows the HotelDescription class is holding dynamic dependencies to
ConferenceVenue and Conference rather than its immediate neighbor Hotel. This
is a tightly coupled design which may require refactoring later on, if some
form of packaging or componentization is required. The design in Figure 4 is
less likely to encourage reuse of the Hotel and Hotel Description classes in
other applications.
Hence, reusability of an object-oriented design is
affected by low-level design choices at the individual feature level. Knowing
and applying the Law of Demeter as part of a coding style guide and enforcing
it with design and code reviews is a major contributor to code reuse and
sustained high project velocity.
Figure 2. DNC showing Dynamic Dependencies
Figure 3. List hotel availability for a conference (a) in an LoD compliant manner
Figure 4. List
hotel availability for a conference (b) in a tightly coupled manner
Summary
Color modeling with the Domain Neutral Component is a
very powerful technique which quickly allows you to create a highly flexible,
loosely coupled functional architecture for an object-oriented system in a
language like Java. It is not well understood that the DNC makes use of the Law
of Demeter and classes hold only dynamic dependencies to their immediate
neighbors. This is at the heart of the flexibility and loose coupling of the
DNC. Flexibility and loose coupling reduce the likelihood of refactoring
becoming necessary and reduce the regression effect of major refactoring or
restructuring when it does happen.
A loosely coupled DNC model treats each class as a
component. Decisions about coarse-grained, large, distributed components or
class packaging can be delayed until the last responsible moment. In the next
Coad Letter, I will discuss how to componentized and package color object
models.
About the author
David J. Anderson is the author of the recent
book, "Agile Management for Software Engineering - Applying the Theory of
Constraints for Business Results" published in Peter Coad's series by Prentice
Hall PTR in September 2003. He is Principal Consultant with VA Systems
Professional Services. David was one of the team which created the popular
agile development method, Feature Driven Development. He has introduced FDD at
two Fortune 100 companies Sprint (a telecommunications operator in the United
States) and Motorola. He writes the regular Agile Management
column at the Borland Developer Network website and publishes the his weblog at
agilemanagement.net. He holds a
degree in Computer Science and Electronics from the University of Strathclyde.
Email: fddmanager@yahoo.com
References
Coad Peter, Eric Le Febvre & Jeff De Luca, Java Modeling in Color with UML - Enterprise
Components and Process, Prentice Hall, Upper Saddle River, NJ, 1999
Karl Lieberherr, Law of Demeter, Northeastern University, http://www.ccs.neu.edu/research/demeter/demeter-method/LawOfDemeter/general-formulation.html
Palmer, Stephen R., A New Beginning, The Coad
Letter, 2002 http://www.thecoadletter.com/article/0,1410,29697,00.html
