Encapsulation (object-oriented programming)

In a programming language encapsulation is used to refer to one of two related but distinct notions, and sometimes to the combination[1][2] thereof:

Some programming language researchers and academics use the first meaning alone or in combination with the second as a distinguishing feature of object oriented programming, while other programming languages which provide lexical closures view encapsulation as a feature of the language orthogonal to object orientation.

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.

Contents

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] It should be noted that the ISO C++ standard refers to private and public as "access specifiers" and that 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 usage like friend in C++.

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]

In combination

With regards to combination (or bundling) data, this is prevalent in any object that is created. An object's state will depend on its methods that do work on or with the object's internal data.

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

  1. ^ 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."
  2. ^ a b Nell B. Dale, Chip Weems, Programming and problem solving with Java, Edition 2, Jones & Bartlett Publishers, 2007, ISBN 0763734020, p. 396
  3. ^ John C. Mitchell, Concepts in programming languages, Cambridge University Press, 2003, ISBN 0521780985, p.522
  4. ^ a b Pierce, Benjamin (2002). Types and Programming Languages. MIT Press. ISBN 0-262-16209-1.  p. 266
  5. ^ Wm. Paul Rogers, Encapsulation is not information hiding, JavaWorld.com, 05/18/01
  6. ^ 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
  7. ^ Pierce (2002), Section 24.2 "Data Abstraction with Existentials"

External links

SOA Patterns.org
Encapsulation
Encapsulation