Information hiding
Encyclopedia
In computer science
, information hiding is the principle of segregation of the design
decisions in a computer program
that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed. The protection involves providing a stable interface
which protects the remainder of the program from the implementation (the details that are most likely to change).
Written another way, information hiding is the ability to prevent certain aspects of a class
or software component from being accessible to its client
s, through an explicit exporting policy and through reliance on the short form as the primary vehicle for class documentation.
A common use of information hiding is to hide the physical storage layout for data so that if it is changed, the change is restricted to a small subset of the total program. For example, if a three-dimensional point (x,y,z) is represented in a program with three floating point
scalar
variables and later, the representation is changed to a single array variable of size three, a module designed with information hiding in mind would protect the remainder of the program from such a change.
In object-oriented programming
, information hiding (by way of nesting
of types) reduces software development risk by shifting the code's dependency on an uncertain implementation (design decision) onto a well-defined interface
. Clients of the interface perform operations purely through it so if the implementation changes, the clients do not have to change.
defined encapsulation as "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." This formulation is cited by a number of books as an authoritative definition of encapsulation.
The purpose is to achieve potential for change: the internal mechanisms of the component can be improved without impact on other components, or the component can be replaced with a different one that supports the same public interface. Encapsulation also protects the integrity of the component, by preventing users from setting the internal data of the component into an invalid or inconsistent state. Another benefit of encapsulation is that it reduces system complexity and thus increases robustness, by limiting the interdependencies between software components.
In this sense, the idea of encapsulation is more general than how it is applied in OOP: for example, a relational database is encapsulated in the sense that its only public interface is a Query language
(SQL
for example), which hides all the internal machinery and data structures of the database management system. As such, encapsulation is a core principle of good software architecture, at every level of granularity.
Encapsulating software behind an interface allows the construction of objects that mimic the behavior and interactions of objects in the real world. For example, a simple digital alarm clock is a real-world object that a lay person can use and understand. They can understand what the alarm clock does, and how to use it through the provided interface (buttons and screen), without having to understand every part inside of the clock. Similarly, if you replaced the clock with a different model, the lay person could continue to use it in the same way, provided that the interface works the same.
In the more concrete setting of an object-oriented programming language, the notion is used to mean either an information hiding mechanism, a bundling mechanism, or the combination of the two. (See Encapsulation (object-oriented programming)
for details.)
, "On the Criteria to Be Used in Decomposing Systems Into Modules" published in the Communications of the ACM
in December 1972. Before then, modularity was discussed by Richard Gauthier and Stephen Pont in their 1970 book titled Designing Systems Programs although modular programming
itself had been used at many commercial sites for many years previously - especially in I/O
sub-systems and software libraries- without acquiring the 'information hiding' tag - but for similar reasons (as well as the more obvious code reuse
reason).
For instance, a car manufacturer may have a luxury version of the car as well as a standard version. The luxury version comes with a more powerful engine than the standard version. The engineer
s designing the two different car engines, one for the luxury version and one for the standard version, provide the same interface for both engines. Both engines fit into the engine bay of the car which is the same between both versions. Both engines fit the same transmission, the same engine mounts, and the same controls. The differences in the engines are that the more powerful luxury version has a larger displacement with a fuel injection system that is programmed to provide the fuel air mixture that the larger displacement engine requires.
In addition to the more powerful engine, the luxury version may also offer other options such as a better radio with CD player, more comfortable seats, a better suspension system with wider tires, and different paint colors. With all of these changes, most of the car is the same between the standard version and the luxury version. The radio with CD player is a module which replaces the standard radio, also a module, in the luxury model. The more comfortable seats are installed into the same seat mounts as the standard types of seats. Whether the seats are leather or plastic, or offer lumbar support or not, doesn't matter.
The engineers design the car by dividing the task up into pieces of work which are assigned to teams. Each team then designs their component to a particular standard or interface which allows the sub-team flexibility in the design of the component while at the same time ensuring that all of the components will fit together.
Motor vehicle manufacturers frequently use the same core structure for several different models, in part as a cost-control measure. Such a "platform
" also provides an example of information hiding, since the floorpan can be built without knowing whether it is to be used in a sedan or a hatchback.
As can be seen by this example, information hiding provides flexibility. This flexibility allows a programmer to modify functionality of a computer program during normal evolution as the computer program is changed to better fit the needs of users. When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes.
Cars provide another example of this in how they interface with drivers. They present a standard interface (pedals, wheel, shifter, signals, gauges, etc.) on which people are trained and licensed. Thus, people only have to learn to drive a car; they don't need to learn a completely different way of driving every time they drive a new model. (Granted, there are manual and automatic transmissions and other such differences, but on the whole cars maintain a unified interface.)
and encapsulation at length and state that in their experience, designers overuse inheritance (Gang of Four 1995:20). The danger is stated as follows:
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, information hiding is the principle of segregation of the design
Design
Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...
decisions in a computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed. The protection involves providing a stable interface
Interface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
which protects the remainder of the program from the implementation (the details that are most likely to change).
Written another way, information hiding is the ability to prevent certain aspects of a class
Class (computer science)
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
or software component from being accessible to its client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....
s, through an explicit exporting policy and through reliance on the short form as the primary vehicle for class documentation.
Overview
The term encapsulation is often used interchangeably with information hiding. Not all agree on the distinctions between the two though; one may think of information hiding as being the principle and encapsulation being the technique. A software module hides information by encapsulating the information into a module or other construct which presents an interface.A common use of information hiding is to hide the physical storage layout for data so that if it is changed, the change is restricted to a small subset of the total program. For example, if a three-dimensional point (x,y,z) is represented in a program with three floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
scalar
Scalar (computing)
In computing, a scalar variable or field is one that can hold only one value at a time; as opposed to composite variables like array, list, hash, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable...
variables and later, the representation is changed to a single array variable of size three, a module designed with information hiding in mind would protect the remainder of the program from such a change.
In object-oriented programming
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
, information hiding (by way of nesting
Nesting (computing)
In computing science and informatics, the word nesting may denote several different constructions and activities where information is organized in layers or objects contain other similar objects. The rather general term is thus used in quite specific ways for various situations and concepts...
of types) reduces software development risk by shifting the code's dependency on an uncertain implementation (design decision) onto a well-defined interface
Interface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
. Clients of the interface perform operations purely through it so if the implementation changes, the clients do not have to change.
Encapsulation
In his influential book on object-oriented design, Grady BoochGrady Booch
Grady Booch is an American software engineer. Booch is best known for developing the Unified Modeling Language with Ivar Jacobson and James Rumbaugh. Grady is recognized internationally for his innovative work in software architecture, software engineering, and collaborative development environments...
defined encapsulation as "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." This formulation is cited by a number of books as an authoritative definition of encapsulation.
The purpose is to achieve potential for change: the internal mechanisms of the component can be improved without impact on other components, or the component can be replaced with a different one that supports the same public interface. Encapsulation also protects the integrity of the component, by preventing users from setting the internal data of the component into an invalid or inconsistent state. Another benefit of encapsulation is that it reduces system complexity and thus increases robustness, by limiting the interdependencies between software components.
In this sense, the idea of encapsulation is more general than how it is applied in OOP: for example, a relational database is encapsulated in the sense that its only public interface is a Query language
Query language
Query languages are computer languages used to make queries into databases and information systems.Broadly, query languages can be classified according to whether they are database query languages or information retrieval query languages...
(SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
for example), which hides all the internal machinery and data structures of the database management system. As such, encapsulation is a core principle of good software architecture, at every level of granularity.
Encapsulating software behind an interface allows the construction of objects that mimic the behavior and interactions of objects in the real world. For example, a simple digital alarm clock is a real-world object that a lay person can use and understand. They can understand what the alarm clock does, and how to use it through the provided interface (buttons and screen), without having to understand every part inside of the clock. Similarly, if you replaced the clock with a different model, the lay person could continue to use it in the same way, provided that the interface works the same.
In the more concrete setting of an object-oriented programming language, the notion is used to mean either an information hiding mechanism, a bundling mechanism, or the combination of the two. (See Encapsulation (object-oriented programming)
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 thereof:* A language mechanism for restricting access to some of the object's components....
for details.)
History
The concept of information hiding was first documented in a paper by David ParnasDavid Parnas
David Lorge Parnas is a Canadian early pioneer of software engineering, who developed the concept of information hiding in modular programming, which is an important element of object-oriented programming today. He is also noted for his advocacy of precise documentation.- Biography :Parnas earned...
, "On the Criteria to Be Used in Decomposing Systems Into Modules" published in the Communications of the ACM
Communications of the ACM
Communications of the ACM is the flagship monthly journal of the Association for Computing Machinery . First published in 1957, CACM is sent to all ACM members, currently numbering about 80,000. The articles are intended for readers with backgrounds in all areas of computer science and information...
in December 1972. Before then, modularity was discussed by Richard Gauthier and Stephen Pont in their 1970 book titled Designing Systems Programs although modular programming
Modular programming
Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
itself had been used at many commercial sites for many years previously - especially in I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
sub-systems and software libraries- without acquiring the 'information hiding' tag - but for similar reasons (as well as the more obvious code reuse
Code reuse
Code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software.-Overview:Ad hoc code reuse has been practiced from the earliest days of programming. Programmers have always reused sections of code, templates, functions, and procedures...
reason).
Example of information hiding
Information hiding serves as an effective criterion for dividing any piece of equipment, software or hardware, into modules of functionality. For instance a car is a complex piece of equipment. In order to make the design, manufacturing, and maintenance of a car reasonable, the complex piece of equipment is divided into modules with particular interfaces hiding design decisions. By designing a car in this fashion, a car manufacturer can also offer various options while still having a vehicle which is economical to manufacture.For instance, a car manufacturer may have a luxury version of the car as well as a standard version. The luxury version comes with a more powerful engine than the standard version. The engineer
Engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
s designing the two different car engines, one for the luxury version and one for the standard version, provide the same interface for both engines. Both engines fit into the engine bay of the car which is the same between both versions. Both engines fit the same transmission, the same engine mounts, and the same controls. The differences in the engines are that the more powerful luxury version has a larger displacement with a fuel injection system that is programmed to provide the fuel air mixture that the larger displacement engine requires.
In addition to the more powerful engine, the luxury version may also offer other options such as a better radio with CD player, more comfortable seats, a better suspension system with wider tires, and different paint colors. With all of these changes, most of the car is the same between the standard version and the luxury version. The radio with CD player is a module which replaces the standard radio, also a module, in the luxury model. The more comfortable seats are installed into the same seat mounts as the standard types of seats. Whether the seats are leather or plastic, or offer lumbar support or not, doesn't matter.
The engineers design the car by dividing the task up into pieces of work which are assigned to teams. Each team then designs their component to a particular standard or interface which allows the sub-team flexibility in the design of the component while at the same time ensuring that all of the components will fit together.
Motor vehicle manufacturers frequently use the same core structure for several different models, in part as a cost-control measure. Such a "platform
Automobile platform
An automobile platform is a shared set of common design, engineering, and production efforts, as well as major components over a number of outwardly distinct models and even types of automobiles, often from different, but related marques...
" also provides an example of information hiding, since the floorpan can be built without knowing whether it is to be used in a sedan or a hatchback.
As can be seen by this example, information hiding provides flexibility. This flexibility allows a programmer to modify functionality of a computer program during normal evolution as the computer program is changed to better fit the needs of users. When a computer program is well designed decomposing the source code solution into modules using the principle of information hiding, evolutionary changes are much easier because the changes typically are local rather than global changes.
Cars provide another example of this in how they interface with drivers. They present a standard interface (pedals, wheel, shifter, signals, gauges, etc.) on which people are trained and licensed. Thus, people only have to learn to drive a car; they don't need to learn a completely different way of driving every time they drive a new model. (Granted, there are manual and automatic transmissions and other such differences, but on the whole cars maintain a unified interface.)
Relation to object-oriented programming
The authors of Design Patterns discuss the tension between inheritanceInheritance (computer science)
In object-oriented programming , inheritance is a way to reuse code of existing objects, establish a subtype from an existing object, or both, depending upon programming language support...
and encapsulation at length and state that in their experience, designers overuse inheritance (Gang of Four 1995:20). The danger is stated as follows:
- "Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation'". (Gang of Four 1995:19)
See also
- Implementation inheritance
- Inheritance semantics
- Modularity (programming)Modularity (programming)Modular programming is a software design technique that increases the extent to which software is composed of separate, interchangeable components called modules by breaking down program functions into modules, each of which accomplishes one function and contains everything necessary to accomplish...
- Virtual inheritanceVirtual inheritanceVirtual inheritance is a topic of object-oriented programming. It is a kind of inheritance in which the part of the object that belongs to the virtual base class becomes common direct base for the derived class and any next class that derives from it...
- Transparency (computing)Transparency (computing)Any change in a computing system, such as new feature or new component, is transparent if the system after change adheres to previous external interface as much as possible while changing its internal behaviour. The purpose is to shield from change all systems on the other end of the interface...
- Scope (programming)Scope (programming)In computer programming, scope is an enclosing context where values and expressions are associated. Various programming languages have various types of scopes. The type of scope determines what kind of entities it can contain and how it affects them—or semantics...
- Compartmentalization (information security)