Law of Demeter: Principle of Least Knowledge

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.

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