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The Law of Demeter (LoD) or principle of least knowledge is a design guideline for developing software, particularly object-oriented programs. In its general form, the LoD is a specific case of loose coupling. The guideline was proposed at Northeastern University towards the end of 1987, and can be succinctly summarized in each of the following ways: * 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. The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents), in accordance with the principle of "information hiding". It is so named for its origin in the Demeter Project, an adaptive programming and aspect-oriented programming effort. The project was named in honor of Demeter, “distribution-mother” and the Greek goddess of agriculture, to signify a bottom-up philosophy of programming which is also embodied in the law itself. ==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 should not "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:# O itself# m's parameters# Any objects created/instantiated within m# O's direct component objects# A global variable, accessible by O, in the scope of mIn 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 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.抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Law of Demeter」の詳細全文を読む スポンサード リンク
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