The Law of Demeter

Back in the 1980s, a group of programmers working on a project called the Demeter system realized that certain qualities in their object-oriented code led to the code being easier to maintain and change. Qualities such as low coupling; information hiding; localization of information, and narrow interfaces between objects. They asked themselves: “Is there a simple heuristic that humans or machines can apply to code to determine whether it has these positive qualities?”.

The answer they came up with came to be known as the “Law of Demeter”

I’ve heard about the Law of Demeter, but I didn’t know what it was. Avdi Grimm explains it nicely, with an example or two. Worth the read.

Oh, the law? Here ’tis:

For all classes C, and for all methods M attached to C, all objects to which M sends a message must be instances of classes associated with the following classes:

1. The argument classes of M (including C).
2. The instance variable classes of C.

(Objects created by M, or by functions or methods which M calls, and objects in global variables are considered as arguments of M.)

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