Principle of least knowledge or Law of Demeter (LoD)

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.

· 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.

In particular, an object should avoid invoking methods of an 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.m().n() breaks the law where a.m() does not. As an analogy, when one wants a dog to walk, one does not command the dog's legs to walk directly; instead one commands the dog which then commands its own legs.

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 implementation can be changed without reworking their callers.

Disadvantages

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