Editing Interface (computing) (section)

== Software interfaces ==
{{See also|Application binary interface|Application programming interface}}

A software interface may refer to a wide range of different types of interfaces at different "levels". For example, an operating system may interface with pieces of hardware. [[application software|Application]]s or [[Computer program|program]]s running on the operating system may need to interact via data [[Stream (computing)|streams]], filters, and pipelines.<ref name="BuyyaMastering13">{{cite book |url=https://books.google.com/books?id=VSDZAgAAQBAJ&pg=SA2-PA13 |title=Mastering Cloud Computing |author=Buyya, R. |publisher=Tata McGraw-Hill Education |page=2.13 |year=2013 |isbn=9781259029950}}</ref> In [[Object-oriented programming|object oriented programs]], objects within an application may need to interact via [[Method (computer science)|methods]].<ref name="PooObject08">{{cite book |chapter=Chapter 2: Object, Class, Message and Method |title=Object-Oriented Programming and Java |url=https://archive.org/details/springer_10.1007-978-1-84628-963-7 |author1=Poo, D. |author2=Kiong, D. |author3=Ashok, S. |publisher=Springer-Verlag |pages=7–15 |year=2008 |isbn=9781846289637}}</ref>

=== In practice ===
A key principle of design is to prohibit access to all resources by default, allowing access only through well-defined entry points, i.e., interfaces.<ref>
{{cite web
 |access-date  = 2011-08-03
 |author      = Bill Venners
 |date        = 2005-06-06
 |website= artima developer
 |title       = Leading-Edge Java: Design Principles from Design Patterns: Program to an interface, not an implementation - A Conversation with Erich Gamma, Part III
 |quote       = Once you depend on interfaces only, you're decoupled from the implementation. That means the implementation can vary, and that is a healthy dependency relationship. For example, for testing purposes you can replace a heavy database implementation with a lighter-weight mock implementation. Fortunately, with today's refactoring support you no longer have to come up with an interface up front. You can distill an interface from a concrete class once you have the full insights into a problem. The intended interface is just one 'extract interface' refactoring away. ...
 |url         = http://www.artima.com/lejava/articles/designprinciples.html
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20110805191904/http://www.artima.com/lejava/articles/designprinciples.html
 |archive-date = 2011-08-05
}}
</ref> Software interfaces provide access to computer resources (such as memory, CPU, storage, etc.) of the underlying computer system; direct access (i.e., not through well-designed interfaces) to such resources by software can have major ramifications—sometimes disastrous ones—for functionality and stability.{{citation needed|date=June 2018}}

Interfaces between software components can provide [[constant (computer science)|constant]]s, [[data type]]s, types of [[subroutine|procedure]]s, [[exception handling|exception]] specifications, and [[method signature]]s. Sometimes, public [[Variable (programming)|variables]] are also defined as part of an interface.<ref name="PattersonComputer04">{{cite book |title=Computer Organization and Design: The Hardware/Software Interface |author1=Patterson, D.A. |author2=Hennessy, J.L. |date=7 August 2004 |publisher=Elsevier |edition=3rd |page=656 |isbn=9780080502571}}</ref>

The interface of a software module ''A'' is deliberately defined separately from the [[Implementation (computer science)|implementation]] of that module. The latter contains the actual code of the procedures and methods described in the interface, as well as other "private" variables, procedures, etc. Another software module ''B'', for example the [[Client (computing)|client]] to ''A'', that interacts with ''A'' is forced to do so only through the published interface. One practical advantage of this arrangement is that replacing the implementation of ''A'' with another implementation of the same interface should not cause ''B'' to fail—how ''A'' internally meets the requirements of the interface is not relevant to ''B'', which [[Separation of concerns|is only concerned]] with the specifications of the interface. (See also [[Liskov substitution principle]].){{citation needed|date=June 2018}}

=== In object-oriented languages ===
{{Main article|Interface (object-oriented programming)|Concept (generic programming)}}
In some [[object-oriented]] languages, especially those without full [[multiple inheritance]], the term ''interface'' is used to define an [[Abstract data type|abstract type]] that acts as an [[abstraction]] of a [[Class (computer science)|class]]. It contains no data, but defines behaviours as [[Method (computer science)|method]] signatures. A [[class (computer science)|class]] having code and data for all the methods corresponding to that interface and declaring so is said to ''implement'' that interface.<ref>{{cite web|url=http://docs.oracle.com/javase/tutorial/java/concepts/interface.html|title=What Is an Interface|work=The Java Tutorials|publisher=Oracle|access-date=2012-05-01|url-status=live|archive-url=https://web.archive.org/web/20120412093619/http://docs.oracle.com/javase/tutorial/java/concepts/interface.html|archive-date=2012-04-12}}</ref> Furthermore, even in single-inheritance-languages, one can implement multiple interfaces, and hence can ''be'' of different types at the same time.<ref>{{cite web|url=http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html|title=Interfaces|work=The Java Tutorials|publisher=Oracle|access-date=2012-05-01|url-status=live|archive-url=https://web.archive.org/web/20120526080117/http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html|archive-date=2012-05-26}}</ref>

An interface is thus a [[data type|type]] definition; anywhere an object can be exchanged (for example, in a [[function (computer science)|function]] or [[method (computer science)|method]] call) the ''type'' of the object to be exchanged can be defined in terms of one of its implemented ''interface''s or base-classes rather than specifying the specific [[Class (computer science)|class]]. This approach means that any class that implements that interface can be used.{{citation needed|date=June 2018}} For example, a [[Skeleton (computer programming)|dummy implementation]] may be used to allow development to progress before the final implementation is available. In another case, a [[Test-driven development#Fakes, mocks and integration tests|fake or mock]] implementation may be substituted during testing. Such [[Method stub|stub]] implementations are replaced by real code later in the development process.

Usually, a method defined in an interface contains no code and thus cannot itself be called; it must be implemented by non-abstract code to be run when it is invoked.{{citation needed|date=June 2018}} An interface called "<code>[[Stack (data structure)|Stack]]</code>" might define two methods: <code>push()</code> and <code>pop()</code>. It can be implemented in different ways, for example, <code>FastStack</code> and <code>GenericStack</code>—the first being fast, working with a data structure of fixed size, and the second using a data structure that can be resized, but at the cost of somewhat lower speed.

Though interfaces can contain many methods, they may contain only one or even none at all. For example, the [[Java (programming language)|Java]] language defines the interface {{Java|Readable}} that has the single {{Java|read()}} method; various implementations are used for different purposes, including {{Java|BufferedReader}}, {{Java|FileReader}}, {{Java|InputStreamReader}}, {{Java|PipedReader}}, and {{Java|StringReader}}. [[Marker interface pattern|Marker interfaces]] like {{Java|Serializable}} contain no methods at all and serve to provide run-time information to generic processing using [[Reflection (computer programming)|Reflection]].<ref>
{{cite web
 |access-date  = 2011-08-04
 |publisher   = Precise Java
 |title       = Performance improvement techniques in Serialization
 |quote       = We will talk initially about Serializable interface. This is a marker interface and does not have any methods.
 |url         = http://www.precisejava.com/javaperf/j2se/Serialization.htm
 |url-status  = live
 |archive-url  = https://web.archive.org/web/20110824150046/http://www.precisejava.com/javaperf/j2se/Serialization.htm
 |archive-date = 2011-08-24}}</ref>

=== Programming to the interface ===
The use of interfaces allows for a programming style called ''programming to the interface''. The idea behind this approach is to base programming logic on the interfaces of the objects used, rather than on internal implementation details. Programming to the interface reduces dependency on implementation specifics and makes code more reusable.<ref>{{cite book
 | last1 = Gamma
 | last2 = Helm
 | last3 = Johnson
 | last4 = Vlissides
 | year = 1995
 | title = Design Patterns: Elements of Reusable Object-Oriented Software
 | url = https://archive.org/details/designpatternsel00gamm
 | url-access = registration
 | publisher = Addison Wesley
 | pages = [https://archive.org/details/designpatternsel00gamm/page/17 17–18]| isbn = 9780201633610
 }}</ref>

Pushing this idea to the extreme, [[inversion of control]] leaves the ''context'' to inject the code with the specific implementations of the interface that will be used to perform the work.