Encapsulation (computer programming)
In an object-oriented programming language encapsulation is used to refer to one of two related but distinct notions, and sometimes to the combination[1][2] thereof:
- A language mechanism for restricting access to some of the object's components.[3][4]
- A language construct that facilitates the bundling of data with the methods operating on that data.[5][6]
Programming language researchers and academics generally use the first meaning alone or in combination with the second as a distinguishing feature of object oriented programming. The second definition is motivated by the fact that in many OOP languages hiding of components is not automatic or can be overridden; thus information hiding is defined as a separate notion by those who prefer the second definition.
As information hiding mechanism
Under this definition, encapsulation means that the internal representation of an object is generally hidden from view outside of the object's definition. Typically, only the object's own methods can directly inspect or manipulate its fields. Some languages like Smalltalk and Ruby only allow access via object methods, but most others (e.g. C++ or Java) offer the programmer a degree of control over what is hidden, typically via keywords like public and private.[4] Please note that the ISO C++ standard refers to "private" and public as "access specifiers" and they do not "hide any information". Information hiding is accomplished by furnishing a compiled version of the source code that is interfaced via a header file.
Hiding the internals of the object protects its integrity by preventing users from setting the internal data of the component into an invalid or inconsistent state. A benefit of encapsulation is that it can reduce system complexity and thus increases robustness, by allowing the developer to limit the interdependencies between software components.
Almost always there is a way to override such protection - usually via reflection API (Ruby, Java, C# etc.), sometimes by mechanism like name mangling (Python), or special keyword like friend in C++. This access is often necessary for debuggers, serialization, unit testing, but not uncommonly in normal code when private was overeagerly applied. [1]
This mechanism is not unique to object-oriented programming. Implementations of abstract data types, e.g. modules offer a similar form of encapsulation. This similarity stems from the fact that both notions rely on the same mathematical fundament of an existential type.[7]
As bundling mechanism
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In combination
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An analogy can be made here with the notion of a capsule, which not only encloses its contents, but also protects it from the exterior environment.[2]
References
- ^ Michael Lee Scott, Programming language pragmatics, Edition 2, Morgan Kaufmann, 2006, ISBN 0126339511, p. 481: "Encapsulation mechanisms enable the programmer to group data and the subroutines that operate on them together in one place, and to hide irrelevant details from the users of an abstraction."
- ^ a b Nell B. Dale, Chip Weems, Programming and problem solving with Java, Edition 2, Jones & Bartlett Publishers, 2007, ISBN 0763734020, p. 396
- ^ John C. Mitchell, Concepts in programming languages, Cambridge University Press, 2003, ISBN 0521780985, p.522
- ^ a b Pierce, Benjamin (2002). Types and Programming Languages. MIT Press. ISBN 0-262-16209-1. p. 266
- ^ Wm. Paul Rogers, Encapsulation is not information hiding, JavaWorld.com, 05/18/01
- ^ Thomas M. Connolly, Carolyn E. Begg, Database systems: a practical approach to design, implementation, and management, Edition 4, Pearson Education, 2005, ISBN 0321210255, Chapter 25, "Introduction to Object DMBS", section "Object-oriented concepts", p. 814
- ^ Pierce (2002), Section 24.2 "Data Abstraction with Existentials"