💬 "В философии существенными называют свойства, которыми объект должен обладать, чтобы быть именно этим объектом. Автомобиль должен иметь двигатель, колеса и двери — если объект не обладает каким-нибудь из этих существенных свойств, это не автомобиль."
-- "Code Complete" by Steve McConnell

💬 "Несущественными (акцидентными, побочными) свойствами называют свойства, которыми объект обладает в силу случайности, — свойства, не влияющие на его суть. Так, автомобиль может иметь четырехцилиндровый двигатель с турбонаддувом, восьмицилиндровый или любой другой и все же являться автомобилем. Тип двигателя и колес, число дверей — все это несущественные свойства. Можете также думать о них как о второстепенных, произвольных, необязательных и случайных."
-- "Code Complete" by Steve McConnell

💬 "Following Aristotle, I divide them [difficulties] into essence - the difficulties inherent in the nature of the software - and accidents - those difficulties that today attend its production but that are not inherent.
<...>
The complexity of software is in essential property, not an accidental one. Hence descriptions of a software entity that abstract away its complexity often abstract away its essence. Mathematics and the physical sciences made great strides for three centuries by constructing simplified models of complex phenomena, deriving properties from the models, and verifying those properties experimentally. This worked because the complexities ignored in the models were not the essential properties of the phenomena. It does not work when the complexities are the essence.”
—“No Silver Bullet - Essence and Accident in Software Engineering” by Frederick P. Brooks, Jr.

💬 encapsulation, the fact that one cannot see, much less design, the inner structure of the pieces.
—"The Mythical Man-Month Essays on Software Engineering Anniversary Edition" by Frederick P. Brooks, Jr.

💬 "OO makes code understandable by encapsulating moving parts. FP makes code understandable by minimizing moving parts."
– Michael Feathers

💬 "Ответ прост. Это из-за привязанности ООП к сложным языкам. Вместо объяснения людям, что ООП является видом проектирования, и вооружения их принципами проектирования, их учили, что ООП — это использование определенного инструмента. Любыми срeдствами можно писать и плохие, и хорошие программы. Если не учить людей проектированию, то языки не имеют большого значения. Результатом будут плохие разработки с помощью этих языков и малая выгода. А если выгода невелика, то ООП не войдет в моду.

The answer is simple. It is because [O-O] has been tied to a variety of complex languages. Instead of teaching people that O-O is a type of design, and giving them design principles, people have taught that O-O is the use of a particular tool. We can write good or bad programs with any tool. Unless we teach people how to design, the languages matter very little. The result is that people do bad de-signs with these languages and get very little value from them. If the value is small, it won't catch on."
—"The Mythical Man-Month Essays on Software Engineering Anniversary Edition" by Frederick P. Brooks, Jr.

💬 Когда абстракция нас покидает, на помощь приходит инкапсуляция. Абстракция говорит: «Вы можете рассмотреть объект с общей точки зрения». Инкапсуляция добавляет: «Более того, вы не можете рассмотреть объект с иной точки зрения».

Продолжим нашу аналогию: инкапсуляция позволяет вам смотреть на дом, но не дает подойти достаточно близко, чтобы узнать, из чего сделана дверь. Инкапсуляция позволяет вам знать о существовании двери, о том, открыта она или заперта, но при этом вы не можете узнать, из чего она сделана (из дерева, стекловолокна, стали или другого материала), и уж никак не сможете рассмотреть отдельные волокна древесины.

Encapsulation picks up where abstraction leaves off. Abstraction says, "You're allowed to look at an object at a high level of detail." Encapsulation says, "Furthermore, you aren't allowed to look at an object at any other level of detail."

Continuing with the housing-materials analogy: encapsulation is a way of saying that you can look at the outside of the house but you can't get close enough to make out the door's details. You are allowed to know that there's a door, and you're allowed to know whether the door is open or closed, but you're not allowed to know whether the door is made of wood, fiberglass, steel, or some other material, and you're certainly not allowed to look at each individual wood fiber.
—"Code Complete" 2nd edition by Steve McConnell, перевод: Издательско-торговый дом "Русская Редакция"

💬 "Впервые сокрытие информации было представлено на суд общественности в 1972 г. Дэвидом Парнасом (David Parnas) в статье «On the Criteria to Be Used in Decomposing Systems Into Modules (О критериях, используемых при декомпозиции систем на модули)». С сокрытием информации тесно связана идея «секретов» — аспектов проектирования и реализации, которые разработчик ПО решает скрыть в каком- то месте от остальной части программы.

Information hiding first came to public attention in a paper published by David Parnas in 1972 called "On the Criteria to Be Used in Decomposing Systems Into Modules." Information hiding is characterized by the idea of "secrets," design and implementation decisions that a software developer hides in one place from the rest of a program."
—"Code Complete" 2nd edition by Steve McConnell, перевод: Издательско-торговый дом "Русская Редакция"

💬 Абстракция позволяет выделить существенные аспекты поведения, а инкапсуляция является тем инструментом, который позволяет спрятать ненужные подробности и детали реализации с глаз долой.

💬 "Abstraction and encapsulation are complementary concepts: Abstraction focuses on the observable behavior of an object, whereas encapsulation focuses on the implementation that gives rise to this behavior.

Encapsulation is most often achieved through information hiding (not just data hiding), which is the process of hiding all the secrets of an object that do not contribute to its essential characteristics; typically, the structure of an object is hidden, as well as the implementation of its methods. “No part of a complex system should depend on the internal details of any other part” [50]. Whereas abstraction “helps people to think about what they are doing,” encapsulation “allows program changes to be reliably made with limited effort” [51].
Encapsulation hides the details of the implementation of an object. Encapsulation provides explicit barriers among different abstractions and thus leads to a clear separation of concerns. For example, consider again the structure of a plant. To understand how photosynthesis works at a high level of abstraction, we can ignore details such as the responsibilities of plant roots or the chemistry of cell walls. Similarly, in designing a database application, it is standard practice to write programs so that they don’t care about the physical representation of data but depend only on a schema that denotes the data’s logical view [52]. In both of these cases, objects at one level of abstraction are shielded from implementation details at lower levels of abstraction.

“For abstraction to work, implementations must be encapsulated” [53]. In practice, this means that each class must have two parts: an interface and an implementation. The interface of a class captures only its outside view, encompassing our abstraction of the behavior common to all instances of the class. The implementation of a class comprises the representation of the abstraction as well as the mechanisms that achieve the desired behavior. The interface of a class is the one place where we assert all of the assumptions that a client may make about any instances of the class; the implementation encapsulates details about which no client may make assumptions.

To summarize, we define encapsulation as follows:

Encapsulation is the process of compartmentalizing the elements of an abstraction that constitute its structure and behavior; encapsulation serves to separate the contractual interface of an abstraction and its implementation.

Britton and Parnas call these encapsulated elements the “secrets” of an abstraction [54].

<...>

Hiding is a relative concept: What is hidden at one level of abstraction may represent the outside view at another level of abstraction. The underlying representation of an object can be revealed, but in most cases only if the creator of the abstraction explicitly exposes the implementation, and then only if the client is willing to accept the resulting additional complexity. Thus, encapsulation cannot stop a developer from doing stupid things; as Stroustrup points out, “Hiding is for the prevention of accidents, not the prevention of fraud” [56]. Of course, no programming language prevents a human from literally seeing the implementation of a class, although an operating system might deny access to a particular file that contains the implementation of a class.

-- "Object-Oriented Analysis and Design with Applications" 3rd edition by Grady Booch, Robert A. Maksimchuk, Michael W. Engle, Bobbi J. Young Ph.D., Jim Conallen, Kelli A. Houston

💬 Information-hiding objects can be created in any language; my first experiments used primitive versions of FORTRAN.

💬 When it is applied properly it is usually successful. Abstractions such as Unix pipes, postscript or X-win-dows achieve the goal of allowing several implementations of the same interface. However, although the principle was published 30 years ago, today’s software is full of places where information is not hidden, interfaces are complex, and changes are unreasonably difficult.

💬 While the basic idea is to hide information that is likely to change, one can often profit by hiding other information as well because it allows the re-use of algorithms or the use of more efficient algorithms.

💬 The terms "module" and "component" are often used interchangeably, which causes confusion. As I mentioned earlier, any system is composed of components. Therefore, a module is a component. However, not every component is a module. To design a flexible system it’s not enough to decompose a system into an arbitrary set of components. Instead, a modular design should enable you to alter the system by combining, reconfiguring, or replacing its components — its modules.
-- "Balancing Coupling in Software Design: Universal design principles for architecting modular software systems" by Vlad Khononov

💬 Encapsulation is most often achieved through information hiding (not just data hiding).
-- "Object-Oriented Analysis and Design with Applications" 3rd edition by Grady Booch, Robert A. Maksimchuk, Michael W. Engle, Bobbi J. Young Ph.D., Jim Conallen, Kelli A. Houston

💬 InformationHiding should inform the way you encapsulate things
-- https://wiki.c2.com/?EncapsulationIsNotInformationHiding

💬 Encapsulation provides explicit barriers among different abstractions and thus leads to a clear separation of concerns.
-- "Object-Oriented Analysis and Design with Applications" 3rd edition by Grady Booch, Robert A. Maksimchuk, Michael W. Engle, Bobbi J. Young Ph.D., Jim Conallen, Kelli A. Houston

💬 Encapsulation means that access to the implementation part of a module is not possible from outside the module, i.e. the implementation of a module is hidden. This means that the usage of a module cannot depend upon internal details of its implementation, and thus it is possible to change the implementation of a module without affecting its usage.
-- "Object-Oriented Programming in the BETA Programming Language" by Ole Lehrmann Madsen, Birger Møller-Pedersen, Kristen Nygaard

💬 Abstraction is one of the fundamental ways that we as humans cope with complexity. Dahl, Dijkstra, and Hoare suggest that “abstraction arises from a recognition of similarities between certain objects, situations, or processes in the real world, and the decision to concentrate upon these similarities and to ignore for the time being the differences” [42]. Shaw defines an abstraction as “a simplified description, or specification, of a system that emphasizes some of the system’s details or properties while suppressing others. A good abstraction is one that emphasizes details that are significant to the reader or user and suppresses details that are, at least for the moment, immaterial or diversionary” [43]. Berzins, Gray, and Naumann recommend that “a concept qualifies as an abstraction only if it can be described, understood, and analyzed independently of the mechanism that will eventually be used to realize it” [44]. Combining these different viewpoints, we define an abstraction as follows:

An abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provide crisply defined conceptual boundaries, relative to the perspective of the viewer.

An abstraction focuses on the outside view of an object and so serves to separate an object’s essential behavior from its implementation. Abelson and Sussman call this behavior/implementation division an abstraction barrier [45] achieved by applying the principle of least commitment, through which the interface of an object provides its essential behavior, and nothing more [46]. We like to use an additional principle that we call the principle of least astonishment, through which an abstraction captures the entire behavior of some object, no more and no less, and offers no surprises or side effects that go beyond the scope of the abstraction.

-- "Object-Oriented Analysis and Design with Applications" 3rd edition by Grady Booch, Robert A. Maksimchuk, Michael W. Engle, Bobbi J. Young Ph.D., Jim Conallen, Kelli A. Houston