Law of Demeter

Source: 
http://en.wikipedia.org/wiki/Law_of_Demeter

The Law of Demeter (LoD), or Principle of Least Knowledge, is a simple design style for developing software, particularly object-oriented programs. In this general form, the LoD is a more specific case of the Low Coupling Principle.

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.

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:

1. O itself
2. M's parameters
3. any objects created/instantiated within M
4. O's direct component objects
5. a global variable, accessible by O, in the scope of M

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 sometimes requires writing a large number of small “wrapper” methods (sometimes referred to as Demeter Transmogrifiers) to propagate method calls to the components