Editing Law of Demeter (section)

==In object-oriented programming==
An object <code>a</code> can request a service (call a method) of an object instance <code>b</code>, but object <code>a</code> should not "reach through" object <code>b</code> to access yet another object, <code>c</code>, to request its services. Doing so would mean that object <code>a</code> implicitly requires greater knowledge of object <code>b</code>'s internal structure.

Instead, <code>b</code>'s interface should be modified if necessary so it can directly serve object <code>a</code>'s request, propagating it to any relevant subcomponents. Alternatively, <code>a</code> might have a direct reference to object <code>c</code> and make the request directly to that. If the law is followed, only object <code>b</code> knows its own internal structure.

More formally, the Law of Demeter for functions requires that a method <code>m</code> of an object <code>a</code> may only invoke the methods of the following kinds of objects:<ref>{{cite web
 | title      = The Paperboy, The Wallet, and The Law Of Demeter
 | url        = http://www.ccs.neu.edu/research/demeter/demeter-method/LawOfDemeter/paper-boy/demeter.pdf
 | last     = Bock
 | first      = David
 | page       = 5
 | publisher  = College of Computer and Information Science, Northeastern University
 | access-date = 2012-07-05
}}</ref>
* <code>a</code> itself;
* <code>m</code>'s parameters;
* any objects instantiated within <code>m</code>;
* <code>a</code>'s attributes;
* global variables accessible by <code>a</code> in the scope of <code>m</code>.

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".<ref>{{cite book |page=81 |title=Practical Object-Ortiented Design: An Agile Primer Using Ruby |first=Sandi |last=Metz |edition=Second |publisher=Addison-Wesley |year=2019 |isbn=978-0134456478 |lccn=2018939833 }}</ref> That is, the code <code>a.m().n()</code> breaks the law where <code>a.m()</code> 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.