Law of Demeter: Principle of Least Knowledge

The Law of Demeter (LoD) is a simple style rule for designing object-oriented systems. "Only talk to your friends" is the motto. The style rule was first proposed at Northeastern University in the fall of 1987 by Ian Holland and popularized in books by Booch, Budd, Coleman, Larman, Page-Jones, Rumbaugh and others. A 2000 book that describes it well is The Pragmatic Programmer by Andrew Hunt and David Thomas. The name "Law of Demeter" was chosen because the style rule was discovered while working on the The Demeter Project which ever since was strongly influenced by the Law of Demeter. The Demeter Project develops tools that make it easier to follow the Law of Demeter. (Demeter = Greek Goddess of Agriculture; grow software in small steps.) For example, DemeterF is an exciting Java tool to follow the Law of Demeter in Java.

In 2003/2004 the Law of Demeter was revisited for Karl Lieberherr's ICSE 2004 keynote paper and presentation. The Law of Demeter was refined from "Only talk to your friends" to "Only talk to your friends who share your concerns" and this refined form is called the Law of Demeter for Concerns (LoDC). The LoDC is best followed by using Aspect-Oriented Software Development (AOSD) techniques such as AspectJ or DJ. On the other hand, the LoDC leads to better AOSD. Properly following the LoDC has two key benefits: It leads to better information hiding (using the technique of structure-shyness or the more general technique of concern-shyness) and to less information overload for the software developer.

This paragraph is added in September 2011. I am pleased that there is continued attention in the software development domain to the Law of Demeter. The Clean Code Talks series in the Google Tech Talks refers to the Law of Demeter as a promoter of clean code. Here is an enjoyable talk by Miskov Hevery from Google. The Clean Code Talks - Don't Look For Things! I also recommend: OO Design for Testability also by Miskov Hevery. There is also continued activity in the blog sphere. This search finds about a dozen posts from Google that mention the Law of Demeter. From the Google "Testing on the Toilet" Loose Coupling with Demeter. Here is another recent example from July 2011: Demeter: It's not just a good idea. It's the Law by Avdi Grimm in Virtuous Code. A recent paper from the University of Zurich from Fall 2011: An Empirical Validation of the Benefits of Adhering to the Law of Demeter (local copy: An Empirical Validation of the Benefits of Adhering to the Law of Demeter ) Conclusion: quote: "we conclude that obeying the Law of Demeter is a straight-forward approach for developers to reduce the number of bugs in their software."

This paragraph is added June 2013: The talk by Ben Orenstein at RailsConf 2013 provides a very good visualization of the Law of Demeter. See his first lightening talk at the beginning: Visualizing Law of Demeter. His topic is interesting: How to talk to Developers. You learn to sing at the end of the talk!

A general formulation of the Law of Demeter
Karl Lieberherr and Ian Holland, Assuring Good Style for Object-Oriented Programs. IEEE Software, September 1989, pages 38-48.
Formulations and Benefits of the Law of Demeter (SIGPLAN paper, in .ps and .tex)
Brad Appleton Explains Law of Demeter and Adaptive Programming
David Bock Explains Law of Demeter
Links to interesting discussions of LoD
Other tools that help you follow the Law of Demeter.
- DJ: Dynamic Adaptive Programming in Java. This paper and tool shows you a clever way to follow the Law of Demeter when you program in Java.
- DAJ. This tool shows you a clever way to follow the Law of Demeter when you program in AspectJ.
- Support Mailing List for tools that help you follow the Law of Demeter (DJ, DemeterJ, DemeterF).
- Law of Demeter Checkers for Java written in AspectJ
Static versus dynamic analysis applied to LoD
How the Law of Demeter is used at the Jet Propulsion Laboratory
```
>From David.E.Smyth@jpl.nasa.gov Mon May 26 17:33:30 1997
>From: "David E. Smyth" 
>To: lieber@ccs.neu.edu
>Subject: Law of Demeter
>
>I have been using LoD pervasively since about 1990, and it has taken
>firm hold in many areas of the Jet Propulsion Laboratory. Major systems
>which have used LoD extensively include the Telemetry Delivery System (a
>real-time database begun in 1990), the Flight System Testbed, and Mars
>Pathfinder flight software (both begun in 1993). We are going to use LoD
>as a foundational software engineering principle for the X2000 Europa
>orbiter mission. I also used it within a couple of commercial systems
>for Siemens in 91-93, including a Lotus Notes like system, and a email
>system.
...
>   We have found, however, that breaking the LoD is very expensive! On
>   Mars Pathfinder, the integration costs of the law breaking parts of
>   the system were at least an order of magnitude higher, and we had
>   to pay that cost on every integration cycle -- not just once! We
>   should have paid the cost once by implementing a more sophisticated
>   protocol that did obey the LoD. 
```
Minsky's home page (contains reference to checking Law of Demeter)
Using a Law-Governed Approach to Enforcing the Law of Demeter
Bibtex references
Limitations of Data Encapsulation
Discussions of Law of Demeter and Adaptive Programming
Discussions of Law of Demeter for Socrates and AspectJ
Law of Demeter confirmed by experiment
Publications by Demeter Group, including LoD papers from the late 80's

Synonyms: Principle of Least Knowledge.

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Professor Karl Lieberherr

College of Computer and Information Science, Northeastern University

Boston, MA 02115

lieber at CCS dot NEU dot EDU

Fax: (617) 373 5121