Law of demeter programming

Its the only one i know of that has a nearmathematical definition. Law of demeter the law of demeter was developed early during the demeter project by ian holland et al. The law of demeter is a heuristic for writing good object oriented code which was introduced by k. It is so named for its origin in the demeter project, an adaptive programming and aspectoriented programming effort. The genius of the law of demeter the new java developer.

As the original paper points out, it was developed during design and implementation of the demeter system hence the name and was. I find this topic an extremely important for having the code clean, welldesigned and maintainable. The call to dosomething propagates outwards till it gets to z. This is a very useful and simple law to learn and then apply to all our programming, whether were writing an individual line of code or designing an entire architecture. This law was proposed by ian holland in 1987 when he and his colleagues were programming a system called demeter using oriented. The law of demeter might be one of the most welldefined, useful, and concisely written rules of objectoriented software development ever. The law of demeter helps you apply the rules of object oriented programming for reusable, loosely coupled code a scalable and maintainable application. Apr 08, 2017 the are no real laws in programming, and thats sort of the only law. Loose coupling is nearly always a virtue but is just on. One of my favorite design principles is the law of demeter which enforces one of the strongest principles of objectoriented programming. The name law of demeter was chosen because the style rule was discovered while working on the the demeter project which ever since was strongly influenced by the law of demeter. It re stricts the number of types the programmer has to be aware of when writing a. Law of demeterlod is more of a guideline than a principle to help reduce. Though demeter is often described simply as the goddess of the harvest, she presided also over the sacred law, and the cycle of life and death.

Unfortunately, whether a program satisfies the object form is undecidable. May 14, 2017 the law of demeter is the law of the land in objectoriented development. Complying with the law technically and in spirit everywhere is the minimum required to produce proper, well designed objectoriented code. The law of demeter is the law of the land in objectoriented development. The law of demeter intends to promote loose coupling between components, so that each component affects as few other components as possible when it changes.

The law of demeter itself doesnt present a methodology for good demand. Loose coupling states that each given object should only be aware and interact with the components closest to it. The guideline was proposed by ian holland at northeastern university towards the end of 1987, and can be succinctly summarized in each. The law of demeter is an interesting programming principle. In its general form, the lod is a specific case of loose coupling. She and her daughter persephone were the central figures of the eleusinian mysteries, a religious tradition that predated the olympian pantheon, and which may have its roots in the mycenaean period c. Usually it is in the form of a question asking what is demeter. In my experience, seeing it broken is a huge smell for bad design. In the above example, if following the law of demeter, im only allowed to call methods on xand not on gety and getz. Named for its origin in the demeter project, an adaptive programming and aspectoriented programming effort, itself named after the greek goddess demeter. At best, its a useful heuristic for identifying potentially problematic areas of code. In object oriented programming classes are basically black boxes of code with well defined interfaces. The law of demeter lod or principle of least knowledge is a design guideline for developing software, particularly objectoriented programs wikipedia this law was proposed by ian holland in 1987 when he and his colleagues were programming a system called demeter using oriented object programming.

They all explain what it is and show an example of breaking the law but that is easy. That being said not all fluent interfaces violate the law of demeter, just as long as they return the same type as their caller. Now that you understand responsibilities, dependencies and interfaces, youre equipped to explore the law of demeter. Jul 14, 2009 despite its name, violating the law of demeter will not get you on an episode of cops nor is it some inviolable law of nature. The demeter programmers wrote up their experiences in a paper called objectoriented programming. Of course, theres a whole world of refactoring out there. The law of demeter was originally formulated as a style rule for designing. Lod tells us that it is a bad idea for single functions to know the entire navigation structure of the system. Law of demeter lod is more of a guideline than a principle to help reduce coupling between components.

Mar 23, 2012 demeters is one of the great ideals of good coding practice in my opinion. Demeters is one of the great ideals of good coding practice in my opinion. In the above example, both findall and sort return the same type of object as the original list. The law of demeter practical object oriented design in ruby. The law of demeter lod is an object oriented programming oop design guideline that fits well with this last principle. There is probably lots of ways to obey this law good design and planning being one but in very simple terms would this be a way to obey it. Mar 15, 2017 the law of demeter is a heuristic for writing good object oriented code which was introduced by k. The law of demeter lod or principle of least knowledge is a design guideline for developing software, particularly objectoriented programs. Before getting into the theory, i think the law of demeter is best explained through a source code example, in this case, a java example. Oot is a mini series on writing maintainable object oriented code without pulling your hair out. In his article, kola summarizes that according to the law of demeter, an object should only call methods that. Here then is a java class which attempts to demonstrate what method calls are considered okay according to the law of demeter. The law of demeter practical object oriented design in.

Any time i see articles about law of demeter the author never seems to give a solid example of how to obey this law. Deviations from the law can be considered a code smell. Let me illustrate the law of demeter by spotting the violations of it. The lodc is best followed by using aspectoriented software development aosd techniques such as aspectj or dj. At lower level, a chunk of code that has multiple dots on line. There are wellknown abstract concepts in objectoriented programming, like encapsulation. Youll find few guidelines that connect or touch upon so many other core principles of quality programming. Fcl can also be used to encode design rules such as the law of demeter and programming guidelines. It is widely known among developers with the following concise formulation. What they found was that when methods were written in a form which complied with the law of demeter, the resulting codebase was easier to maintain and evolve. The law of demeter or lod as it is commonly called, is really more precisely the law of demeter for functionsmethods lodf.

In the application of lod to objectoriented design and programming we have. Loose coupling states that each given object should only be aware and interact with the components closest to it, in other words the object shouldnt talk to strangers. What does the law of demeter have to do with demeter. The demeters law is known as dont talk to strangers because any method of. Read this essay on fluent programming vs law of demeter. The are no real laws in programming, and thats sort of the only law. The law of demeter helps you reduce coupling in your code. In this video you are going to learn about the law of demeter to help you write better ruby code. The project was named in honor of demeter, distributionmother and the greek goddess of agriculture, to signify a bottomup philosophy of programming which is also embodied in the law itself.

The law of demeter design pattern and how to use it in. During the development of the system they realized that the code that fulfilled a series of rules was less coupled. May 22, 2007 kola oyedeji just wrote a very nice article for the coldfusion developers journal on applying the law of demeter in coldfusion object oriented programming oop. A unit should have only limited knowledge about other units. The law of demeter involves modularity, encapsulation, cohesion, coupling, responsibility stratification, and information hiding. The wellknown law of demeter is seriously misunderstood by objectoriented programmersit isnt equal to the law of one dot. More precisely, the method of a particular class should call. O itself ms parameters any objects createdinstantiated within m os direct component objects a global variable. It is a designstyle rule for objectoriented programs. Law of demeter lod the law of demeter was originally formulated as a style rule for designing objectoriented systems. Demeter origins the classic law of demeter refers to a paper that was. Loose coupling is nearly always a virtue but is just one component of design and must be balanced against competing. When do you violate the law of demeter in coldfusion oop.

The law of demeter lod is a set of coding rules which result in loosely coupled objects. However, we can run the program and check at runtime whether violations occur this is a nice exercise in aspectoriented programming. Feb 28, 2015 the law of demeter is an interesting programming principle. First discussed at the northeastern university in 1987, this principle states.

For example, demeterf is an exciting java tool to follow the law of demeter in java. Holland and colleagues were programming a system called demeter using oriented object programming. Kola oyedeji just wrote a very nice article for the coldfusion developers journal on applying the law of demeter in coldfusion object oriented programming oop. The law of demeter is engineered to help programmers using objectoriented languages gain a lot of clarity of code for a relatively small price. Introducing demeter and its laws by brad appleton if you hang around any of the objectoriented mailing lists, chat groups, or usenet newgroups, youll notice that the name demeter keeps popping up every few months or so on a regular basis. A method should have limited knowledge of an object model. In this article we will explain the heuristic called the law of demeter lod and give an example of how it works in java the law of demeter or principle of least knowledge says that a module should not know about the innards of the objects it manipulates. These long chains of queries, called train wrecks, violate something called the law of demeter. Come browse our large digital warehouse of free sample essays.

The law of demeter formulates the ruleofthumb that modules in objectoriented program code should only talk to their immediate friends. During the development of the system, they realized. Law of demeter and how to work with it programming ideas. It might also be one of the most often ignored things in. As to the law of demeter, there are multiple concerns here. Before we delve into the solid arena, its useful to explore a less wellknown principle, known as lod, or the principle of least knowledge. Get the knowledge you need in order to pass your classes and more. As the original paper itself calls it, it is the law of good style. The law of demeter design pattern and how to use it in rails with the delegate method the law of demeter, or also known as the principle of least knowledge, is a very interesting design pattern that you should use in your rails projects. The law of demeter is not a dot counting exercise youve. First, your constructor takes its own builder as a parameter, then extracts elements from it. An object a can request a service call a method of an object instance b, but. Lod can be regarded as the principle of assuming least structural knowledge something its creator calls structureshy programming. The demeter project develops tools that make it easier to follow the law of demeter.

The law of demeter creates more problems than it solves january 22, 2020 join my mailing list most developers, when invoking the law of demeter or when pointing out a demeter violation, do so when a line of code has more than one dot. The law of demeter or the principle of least knowledge is a design guideline for developing software applications. Despite its name, violating the law of demeter will not get you on an episode of cops nor is it some inviolable law of nature. Your classes are less subject to changes in other classes. The law of demeter was refined from only talk to your friends to only talk to your friends who share your concerns and this refined form is called the law of demeter for concerns lodc. The law of demeter is a good practice to follow, and has the following positive benefits on the classes you create. Aug 19, 2019 in this article we will explain the heuristic called the law of demeter lod and give an example of how it works in java the law of demeter or principle of least knowledge says that a module should not know about the innards of the objects it manipulates.

It re stricts the number of types the programmer has to be aware of when writing a method. Principle of least knowledge the law of demeter lod is a simple style rule for designing objectoriented systems. This practice uses encapsulation in order to reduce coupling between your components, and therefore it helps you improve your code quality. Other times it is used in passing as an obscure reference. During the development of the system they realized that the code that fulfilled a. The style rule was discovered at northeastern university in the fall of 1987 by ian holland. Law of demeter learning and improving as a craftsman. We called it law of demeter because we discovered it while working on demeter but it is a general style rule for structureshy programming. It doesnt have anything to do with mutability, so i feel the conclusion by the author is somewhat arbitrary. Sep 26, 2019 this law was proposed by ian holland in 1987. This is a clear violation of demeter, and also creates a superfluous dependency.

Demystifying the law of demeter principle infoworld. Software programming is a balanced mix of art sometimes a euphemism for improvisation and a bunch of. As the original paper points out, it was developed during design and implementation of the demeter system hence the name and was held to be a law for the developers of that system. Following the law, or refactoring based on it, leads to much improved, readable and more maintainable code. 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. Any method m of an object o may only invoke the methods of the following kinds of objects. Like the nearpointless solid principles, demeter, too, suffers from making a vague claim that drives developers to. In that sense modules, packages and microservices are just new types of classes we invented since the law of demeter was formulated in 87. Formally, the law states that each component of a software design should only communicate with nearby components. Worse yet, the builder is acting as a mini service locator, masking the real dependencies of the class. Law of demeterlod is more of a guideline than a principle to help reduce coupling between components. This law was proposed by ian holland in 1987 when he and his colleagues were programming a system called demeter using oriented object programming. The law of demeter creates more problems than it solves.