The Law of Demeter (LoD) is a design guideline for developing software, particularly object-oriented programs. The guideline was invented at Northeastern University in the fall of 1987, and can be succinctly summarized as "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).
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 to request its services. Doing so would mean that object A implicitly requires greater knowledge of object B's internal structure. Instead, B's class should be modified if necessary so that object A can simply make the request directly of object B, and then let object B propogate the request to any relevant subcomponents. If the law is followed, only object B knows its internal structure.
More formally, the Law of Demeter for functions requires that any method M of an object O may only invoke the methods of the following kinds of objects:
- itself
- its parameters
- any objects it creates/instantiates
- its direct component objects
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.
A disadvantage of the Law of Demeter is that it requires writing a large number of small "wrapper" methods to propogate method calls to the components - these can increase initial development time, increase space overhead, and noticeably decrease performance. Automated tools exist to at least partially counteract these problems.
Basili et al published experimental results in 1996 suggesting that the Law of Demeter was a valid way to reduce the probability of software faults.
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