From Brad_Appleton-GBDA001@aieg.mot.com Thu Oct 24 13:22:36 1996
From: Brad Appleton <Brad_Appleton-GBDA001@aieg.mot.com>
Subject: Re: (OTUG) Law of Demeter
To: otug@rational.com (Object Technology user's group)
Cc: lieber@ccs.neu.edu (Karl Lieberherr)
Reply-To: Brad_Appleton-GBDA001@email.mot.com
Organization: Motorola Automotive and Industrial Electronics Group

Andreas_Helmer writes:

> has someone of you ever heard about the "Law of Demeter" ?

The name "Demeter" refers to the Demeter System and Tools being
developed by Karl Lieberherr and friends at Northeastern Univeristy.
The "Law of Demeter" refers to a paper that was published in
IEEE Software in 1989.

    Lieberherr, Karl. J. and Holland, I.
    Assuring good style for object-oriented programs
    IEEE Software, September 1989, pp 38-48

I think they also had a paper along the same lines at OOPSLA'89 (give
or take a year). There is also a "Law of Demeter" URL located at
http://www.ccs.neu.edu/home/lieber/LoD.html which is reachable from
the "Demeter Home Page".

Basically, they came up with the following "Law of Demeter":

    A method "M" of an object "O" should invoke *only* the
    the methods of the following kinds of objects:
          1. itself
          2. its parameters
          3. any objects it creates/instantiates
          4. its direct component objects

The basic idea is to avoid invoking methods of an object that is
returned by another method. When you do this, you are making some
structural assumptions about the container object that may be likely
to change.  The container may later need to be modified to contain a
different number of the contained objects, or it may end up being
changed to contain another object which contains the original
component object.

Thus using the Law of Demeter (abbreviated as "LoD") you instead ask the
container to invoke a method on its elements and return the result.
The details of how the container propgates the message to its elements
are encapsulated by the containing object.

A side-effect of this is that if you conform to LoD, while it may quite
likely increase the maintainability and "adaptiveness" of your software
system. you also end up having to write *lots* of little wrapper methods
to propagate methods calls to its components (which can add noticeable
time and space overhead).

The Demeter system and tools is about "Adaptive" programming, which is
an addition to OOP that attempts wait to bind algorithms to
data-structures until as late as possible. There is also a "Law of
Demeter for Adaptive Programs" (abbreviated LoD-AP) which is a bit
stronger than the LoD described above (which Lieberherr calls the Law
of Demeter for Functions (or Methods).

The Demeter/Adaptive Programming home page is at
    http://www.ccs.neu.edu/research/demeter/

Dr. Lieberherr also published a book on the Demeter method last
September:

      Adaptive Object-Oriented Software:
         The Demeter Method with Propagation Patterns
      by Dr. Karl J. Lieberherr
      PWS Publishing Company, 1996 ISBN: 0-534-94602-X

I was fortunate enough to take a graduate class from Dr. Lieberherr
on the Demeter method and Adaptive O-O Programming.
--
 Brad_Appleton@email.mot.com        Motorola AIEG, Northbrook, IL USA
 "And miles to go before I sleep."  DISCLAIMER: I said it, not my employer!