Law of Demeter

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 proposed at Northeastern University towards the end of 1987, and can be succinctly summarized in one of the following ways

• Each unit should have only limited knowledge about other units: only units "closely" related to the current unit.
• Each unit should only talk to its friends; don't talk to strangers.
• Only talk to your immediate friends.

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), in accordance with the principle of "information hiding".

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.

Advantages

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.[3] published experimental results in 1996 suggesting that a lower Response For a Class (RFC, 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 (WMC, 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.

Disadvantages

Although the LoD increases the adaptiveness of a software system, it may also result in having to write many wrapper methods to propagate calls to components; in some cases, this can add noticeable time and space overhead.[3][4][5]

At the method level, the LoD leads to narrow interfaces, giving access to only as much information as it needs to do its job, as each method needs to know about a small set of methods of closely related objects.[6] On the other hand, at the class level, the LoD leads to wide (i.e. enlarged) interfaces, because the LoD requires introducing many auxiliary methods instead of digging directly into the object structures. One solution to the problem of enlarged class interfaces is the aspect-oriented approach,[7] where the behavior of the method is specified as an aspect at a high level of abstraction. This is done by having an adaptive method that encapsulates the behaviour of an operation into a place, with which the scattering problem is solved. It also abstracts over the class structure that results in avoiding the tangling problem. The wide interfaces are managed through a language that specifies implementations. Both the traversal strategy and the adaptive visitor use only a minimal set of classes that participate in the operation, and the information about the connections between these classes is abstracted out.[4][7]

Since 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.