In OO software, do not assume knowledge of anything else.
The Law of Demeter (LoD) is the principle of least knowledge for developing software.
Roughly speaking other ways of expressing this are:
- software should be loosely coupled
- principle of encapsulation
- principle of “information hiding”
For modern OO languages, when using dot notation simply use 1 dot. i.e. something like this would be forbidden:
as you’re assuming knowledge.
Much longer explanation here: http://en.wikipedia.org/wiki/Law_of_Demeter