The Law of Demeter (LoD) or Principle of Least Knowledge is a design guideline for developing software, particularly object-oriented programs. In its general form, the LoD is a specific case of loose coupling. The guideline was invented at Northeastern University towards the end of 1987, and can be succinctly summarized in one of the following ways:
The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents).
It is so named for its origin in the Demeter Project, an adaptive programming and aspect-oriented programming effort. The project was named in honor of Demeter, “distribution-mother” and the Greek goddess of agriculture, to signify a bottom-up philosophy of programming which is also embodied in the law itself.
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When applied to object-oriented programs, the Law of Demeter can be more precisely called the “Law of Demeter for Functions/Methods” (LoD-F). In this case, an object A can request a service (call a method) of an object instance B, but object A cannot "reach through" object B to access yet another object, C, to request its services. Doing so would mean that object A implicitly requires greater knowledge of object B's internal structure. Instead, B's interface should be modified if necessary so it can directly serve object A's request, propagating it to any relevant subcomponents. Alternatively, A might have a direct reference to object C and make the request directly to that. If the law is followed, only object B knows its own internal structure.
More formally, the Law of Demeter for functions requires that a method M of an object O may only invoke the methods of the following kinds of objects:
In particular, an object should avoid invoking methods of a member object returned by another method. For many modern object oriented languages that use a dot as field identifier, the law can be stated simply as "use only one dot". That is, the code a.b().Method() breaks the law where a.Method() does not. As a simple example, when one wants to walk a dog, it would be folly to command the dog's legs to walk directly; instead one commands the dog and lets it take care of its own legs.
The advantage of following the Law of Demeter is that the resulting software tends to be more maintainable and adaptable. Since objects are less dependent on the internal structure of other objects, object containers can be changed without reworking their callers.
Basili et al.[1] published experimental results in 1996 suggesting that a lower Response For a Class (the number of methods potentially invoked in response to calling a method of that class) can reduce the probability of software bugs. Following the Law of Demeter can result in a lower RFC. However, the results also suggest that an increase in Weighted Methods per Class (the number of methods defined in each class) can increase the probability of software bugs. Following the Law of Demeter can also result in a higher WMC; see Disadvantages.
A Multilayered architecture can be considered to be a systematic mechanism for implementing the Law of Demeter in a software system. In a layered architecture, code within each layer can only make calls to code within the layer and code within the next layer down. "Layer skipping" would violate the layered architecture.
Although LoD may increase the maintainability and "adaptiveness" of your software system, it results in having to write many wrapper methods to propagate methods calls to its components (which can add noticeable time and space overhead)[2].
At the method level, the LoD leads to narrow interfaces because each method needs to know about a small set of methods of closely related objects. On the other hand, at the class level, the LoD leads to wide interfaces because the LoD requires that we introduce many auxiliary methods instead of digging directly into the object structures. Wide interfaces for classes are a problem and the solution is an aspect-oriented approach where the behavior of the method is specified as an aspect at a high level of abstraction. We still have wide interfaces but we manage them through a language that specifies implementations[3].
Because the LoD exemplifies a specific type of coupling, and does not specify a method of addressing this type of coupling, it is more suited as a metric for Code smell as opposed to a methodology for building loosely coupled systems.