Law of Demeter
The Law of Demeter says that an object cannot request services from an object via a different object.
"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:
- O itself
- M's parameters
- any objects created/instantiated within M
- O's direct component objects" --[Wikipedia]
A common (but certainly not the only) indicator of a violation of the law in some languages is more than one dot in a line of code, for example man.getTrousers().checkPocketContents().
Consequences
A violation of the Law of Demeter means that the caller object is dependant on the internal structure of the object it accesses. In the above example, if the Trousers class were to change (to say, Shorts, or perhaps a more abstract piece of clothing) the original caller would need to be updated.
Solution
The best solution is to write wrapper methods in the class to allow callers to ask that class for information about objects it contains. Unfortunately this may mean the class presents a very large public interface.
Example
To come soon...
