law of demeter

During the course of this course (Software Construction, Design, and Architecture), there have been design concepts that are very easy to grasp at first glance, and those that take much more time to digest. I was surprised to see that the Law of Demeter, or Principle of Least Knowledge, is a fairly intuitive rule, but feels almost too restrictive in nature.

Essentially, the Law of Demeter is the concept that methods in an object should not communicate with any element that isn’t ‘adjacent’ to it. According to a blog post by JavaDevGuy (and thinking of a Java application to the rule), the elements that are allowed by the law are the object itself (this), objects in the argument of the method, instance variables of the object, objects created by the method, global variables, and methods that the method calls.

This is most easily explained by a negative example. For example, if a class Car has a method with a Dashboard object as an argument, it can technically call something like dashboard.getVersion(). But if a class Garage method has a Car argument, the method should not call something like car.getDashboard().getVersion(). Maybe this is a silly example, but this applies to more practical code as well.

JavaDevGuy goes further to say that most Getter methods violate this law. This interpretation seems restrictive, as it makes it much more difficult to just get work done (of course, I’m not the most experienced in the design aspect of software engineering so I could be wrong). It seems more practical to use the law to get rid of chaining in your code, as it causes needless complexity. Chaining methods together, regardless of how necessary it is, always ends up looking kind of ugly. I do feel like it is a necessary evil sometimes though.

As it stands, I can understand that this sort of practice can minimize that amount of complexity and reduce code repetition, but it does feel like sometimes you sort of need to put things together in this way to get the desired output. The aforementioned blog post seems to explain when code is violating the law, but unless my eyes and brain are too tired to read properly, the author doesn’t really give any good replacement options for the code. The few alternatives given don’t seem very desirable. This is typically the problem with negative rules, it imposes a restriction without a solution, and so you have to scramble to figure out how to work it.

Perhaps I’ll understand better when we cover this material in class.