Law of Demeter - in Object-oriented Programming

In Object-oriented Programming

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, C, to request its services. Doing so would mean that object A implicitly requires greater knowledge of object B's internal structure. Instead, B's interface should be modified if necessary so it can directly serve object A's request, propagating it to any relevant subcomponents. Alternatively, A might have a direct reference to object C and make the request directly to that. If the law is followed, only object B knows its own internal structure.

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

In particular, an object should avoid invoking methods of a member 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.b.Method breaks the law where a.Method does not. As a simple example, when one wants a dog to walk, one would not command the dog's legs to walk directly; instead one commands the dog which then commands its own legs.