Principles Of Object Oriented Design

A suite of eleven principles, conceived by people such as RobertCecilMartin, BertrandMeyer, BarbaraLiskov, etc. and compiled by RobertCecilMartin.

There are six papers that describe all of these principles. They can be found in the resources section of http://www.objectmentor.com/. The first four papers are named for the first four principles, the final papers cover the remaining two principles: "Granularity" and "Stability".


There are five principles of class design (aka SOLID): There are three principles of package cohesion There are three principles of package coupling -- RobertCecilMartin 19990907


Other OoDesignPrinciples:

The NakedObjects book claims that the single most important principle of OO design is BehavioralCompleteness. And what would that be?

I would assert that this principal is covered by SRP, which as well as requiring a Class to exhibit a single responsibility implies that a responsibility should be entirely encapsulated by a single class. -- MartinSpamer.

I believe this principle is valuable, at least so far as its name provides a meaningful key to thinking about design. Relative to the Principle of Essential Representation (PER) - proposed (below) as a single, basic principle for object-oriented design - the principle of BehavioralCompleteness covers that a representation (e.g. a type) should include all its essential representations (e.g. behavior characteristics, or method and events). However, it does not explicitly cover that a representation also should include only essential representations. We could say, to elaborate the principle of BehavioralCompleteness, that a representation should not "over-behave" or be "over-complete". This would beg the question of what is "over", and how we can tell. The concept of an "essential representation"; the interpretation of 'essential' as necessary and original using a simple, intuitive version of "possible world semantics"; and the application of a "method of variation" to explore possible worlds and distinguish essential representations, does cover this: including inessential behaviors - ones that do not vary, across some possible worlds, only with the identity of their types - is "over-behaving" or being "over-complete". -- Carl R. Castro


Are there other principles of object-oriented design than those listed by RobertCecilMartin, PrinciplesOfObjectOrientedDesign?

Yes! In particular see the list of (often conflicting) DefinitionsForOo, including the links (in particular the stuff on paulgraham.com that is linked to was a bit too long to paraphrase on c2.com). I think RobertCecilMartin is interesting, and says worthwhile things about software, but truthfully he really annoys me by redefining words. He has an idiosyncratic definition for "OO" in particular that no one in the world except his followers share, as mentioned on the cited page. His list on PrinciplesOfObjectOrientedDesign, on the other hand, is an excellent list, but again, is about software in general, absolutely not about OO in particular; most of it applies perfectly well to old style structured programming in procedural languages. Which reminds me, see the much-neglected yet not obsolete CouplingAndCohesion. -- dm


"Essential Object-Orientation"

A Single Basic Principle for Object-Oriented Design

I believe there is really just a single basic principle of object-orientation and object-oriented design (OOD), which I would like to call "The Principle of Essential Representation" (PER). Other principles are corollaries, interpretations or applications of this principle; so are the relational normal forms.

The Principle of Essential Representation (PER)

Simply put, this principle states that: Similar Principles, and Essence as Relative in Representations

So stated, the PER seems quite similar to Meilir Page-Jones' principle of "connaissance" - that characteristics which are "born together" should "go together" in a representation. However, this rough statement of that principle might suggest that connaissance: So what is important to note here is that the "essentialness" of a representation is relative to an owning or containing representation -- e.g. a characteristic is essential to a type.

(I would like to acknowledge Mr. Page-Jones, however, not only for all his excellent writings but as being the only person - at least whom I have been able to discover - who has even proposed a single basic principle of object-orientation.)

Meaning of the Term 'Essential' and Possible World Semantics

This principle of course requires an interpretation of 'essential' or of 'essence' as applied to characteristics. There are several aspects to such an interpretation, but basically: This interpretation of 'necessary' is actually a simple, intuitive version of Saul Kripke's "possible world" semantics for modal logic.

Applying the Principle by a "Method of Variation", and the Example of the Derivation of MVC

On this interpretation, necessary characteristics can be discovered by applying a "method of variation" to representations - and to the possible applications, configurations, implementations, surrounding representations, etc. - in which they might occur, in something like OOD "thought-experiments".

A prime example of applying this "method of variation" is the derivation of the famous MVC (Model-View-Controller) pattern (which is a kind of basic pattern whose tripartite scheme can be extended in some way to solve almost every OO design problem): Tripartite Schemes as Exemplifying the Principle in General

A similar derivation applying the method of variation can give us good designs for object-relational mapping, considering the relationship between model objects (the object-level "view") and objects in a relational layer (the data-level "model").

In general, tripartite schemes (with a third partition as an actual type or as virtual, in an interface) are the result of applying the PER. Tripartite schemes generally capture the "appearance" of information (at some level of an application), its "reality" (at another level), and the relationship between them - with "appearance"/"reality" providing a basic way of thinking about transformations of information and interactions with it. This is why MVC can seem so powerful as a basic, extensible pattern and why it has become so popular as an effective one. (Check out the webpage of Trygve Reenskaug, its inventor: http://heim.ifi.uio.no/~trygver/)

The Importance of a Single Basic Principle

Having a single basic principle of object-orientation is important, because: Details of Applying the Method of Variation, and Similarities in Applying the Relational Normal Forms

There are actually some subtleties or details to the application of the "method of variation" to distinguish essential vs. inessential representations. These subtleties or details make this method similar to the method of distinguishing invariant data in applying the relational normal forms. Recall that an essential characteristic is necessary (as well as original), and that a necessary characteristic applies in all possible worlds, despite variation in application, configuration, implementation - in short, despite any variation in surrounding representations. In object-oriented applications, representations or definitions may indeed vary - polymorphism is a quite intentional and notable example of such variation. But in this case, the variation in implementation for polymorphism corresponds to a variation in subtype. So we can consider the identity of a type in the application of the method of variation to be like the primary key of a row in the application of the relational normal forms: an inessential characteristic is one that varies independently of the identity of its type. Such a characteristic should instead be represented in another, related type. Similarly, for example, the name of an employee's department manager should be represented in a department table, not in the employee table, where it varies with something other than the employee's primary key - and these tables should be related by a foreign key.

The consideration of relationship when inessential characteristics are "moved" to other types in the design of object-oriented applications is a bit different than the introduction of foreign keys in relational modeling; I would hazard to say it is a bit more thorough-going than the conventions of relational design, which do not explicitly consider partitioning (perhaps just because there are not the two explicit aspects of definition - declaration and implementation - in relational models). Consider the example of the MVC pattern: recognizing that the implementation of view methods can vary independently of model types, we separately encapsulate model and view, achieving at least a bi-partite scheme. A designer might be tempted to stick with a bi-partite scheme, providing a "standard interface" in her architecture for passing values from the view directly to the model - perhaps a name/value pair object that is passed to a save method of the model object whenever the clickOkay event of the view object occurs. However, if the application varies so that the save method of the model object no longer exists, or is no longer named "save", or no longer takes an argument of the same type for passing values (even if it is a name/value pair object of another type) - that is, if the declaration of the model object varies, then the event code of the view object must vary, even though there has been no variation in the type of the view object. In this design, the implementation (a definition or representation) of the view object varies independently of its type, and so is not essential. Another design, without the assumptions of such a "standard interface", is required to eliminate this variation and so to make the definition the event an essential one. For this, the value information from the view object is passed to an object that encapsulates (represents) the relationship between model and view; such an object (i.e. a controller) can perform whatever translation of this information may be needed for, and invoke the appropriate method of, the model object. If there are changes to the save method of the model object - its name, its argument types, etc. - these changes propagate only as far as the controller, and not to the view (unless the semantics of the application change, and more information is needed).

Thank You!

I hope these last few paragraphs firm up and clarify the PER, the concept of "essential representation" as necessary and original, and the application of these by a "method of variation". Really, this principle and its concepts are innate to the ways good designers basically reason about their designs, and to good basic patterns such as MVC. I believe the principle has not yet been articulate due to the combined requirements of modal concepts (such as "necessity"); general but intuitive ways of applying them (such as "possible world semantics"); and interpretations of these for the constructs of object-orientation (the "method of variation" as specifically described above).

Thanks for reading,

-- Carl R. Castro

Could you elaborate on "go together"? I tend to view "go together" as circumstantial (EverythingIsRelative), and thus I am a RelationalWeenie who wants "togetherness" to be ad-hoc for different viewpoints or situations involving the same info. Also, it might be interesting to see how MVC allegedly naturally comes about by your principles in a step-by-step fashion. Perhaps this should be moved to GroupRelatedInformation. -- top

Thank you for these questions.

So far as "go together": the use of this phrase is to summarize Page-Jones; really, he should be the canonical reference on its meaning. I would say that it means "are characteristics of the same representation" - which of course begs the question of relativity which you posed. So far as relativity or circumstantialness is concerned, I think that there is a distinction between a "natural representation" and an "artifactual representation"; the latter kind of representation being what is called a "role" in OO. Many apparently natural representations - e.g. "customer" - are actually artifactual: a customer as a person does not necessarily have an email address, but our company may require an email address for a person as a customer. This involves some kind of stipulative definition, or specification of a "role" - an artifact of a way of doing business, etc. (Trygve Reenskaug, of MVC fame, has done lots of work on roles and collaborations, too: http://heim.ifi.uio.no/~trygver/themes/roles/roles-index.html) Note that natural vs. artifactual has to do with the source of the necessity - but not with its mode or nature; the same possible world semantics can apply. Becoming philosophical, I suppose one could say that this does mean that necessity is relative (to role and its stipulative definition or specification); Kripke actually covered such issues in his semantics for modal logic using "accessibility" relations between possible worlds. I would say that ultimately, everything is relative, but that relatively (given we are in the relative world), there can be a single basic principle, as long as we don't seek after a kind of absolute meaning for its terms and phrases, one that can completely and independently determine its application. (For more on this, read Kripke's discussion of Wittgenstein's supposed argument RE "following a rule" in the Philosophical Investigations; I feel that Kripke did not get to the bottom of Wittgenstein's approach, but did provide many useful insights on it.) I tried to review and comment on EverythingIsRelative, but I got lost in the reference to SoftwareHippy. ;)

So far as the steps of the derivation of MVC - I've recently edited and elaborated on these for clarity above, but in other words, they are roughly as follows. So far as moving to GroupRelatedInformation - I believe this would obscure a role as a basic principle, and also would emphasize a relationship between characteristics themselves, which is not always the basis for their inclusion together in a representation, which is a point of this principle.

-- To discuss this further offline - carlRcastro@yahoo.com.


Related:
CategoryModellingLawsAndPrinciples, CategoryObjectOrientation CategoryDesign

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