Protocol (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,...

, a protocol or interface is a common means for unrelated objects

Object (computer science)

In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

to communicate

Communication

Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...

with each other. These are definitions of methods

Method (computer science)

In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...

and values which the objects agree upon in order to cooperate.

For example, in Java

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

(where protocols are termed interfaces), the Comparable interface specifies a method compareTo which implementing classes should implement. This means that a separate sorting method, for example, can sort any object which implements the Comparable interface, without having to know anything about the inner nature of the class (except that two of these objects can be compared by means of compareTo).

The protocol is a description of:

The messages that are understood by the object.
The arguments that these messages may be supplied with.
The types of results that these messages return.
The invariants
Invariant (computer science)
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...

that are preserved despite modifications to the state of an object.
The exceptional situations that will be required to be handled by clients to the object.

If the objects are fully encapsulated

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....

then the protocol will describe the only way in which objects may be accessed by other objects.

Some programming language

Programming language

A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....

s directly support protocols or interfaces (Ada

Ada (programming language)

Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...

, C#, D

D (programming language)

The D programming language is an object-oriented, imperative, multi-paradigm, system programming language created by Walter Bright of Digital Mars. It originated as a re-engineering of C++, but even though it is mainly influenced by that language, it is not a variant of C++...

, Delphi, Java

Java (programming language)

, Logtalk

Logtalk

Logtalk is an object-oriented logic programming language that extends the Prolog language with a feature set suitable for programming in the large. It provides support for encapsulation and data hiding, separation of concerns and enhanced code reuse...

, Objective-C

Objective-C

Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...

, PHP

PHP

PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...

, Dart

Dart (programming language)

Dart is a Web programming language developed by Google. It was unveiled at the in Aarhus, 2011 October 10-12. The goal of Dart is "ultimately to replace JavaScript as the lingua franca of web development on the open web platform."- Reason for a new language :Dart is intended to solve JavaScript's...

). Older languages may also have features that can support the interface concept, such as abstract base classes with pure virtual functions

Virtual function

In object-oriented programming, a virtual function or virtual method is a function or method whose behaviour can be overridden within an inheriting class by a function with the same signature...

in C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

, or object-oriented features in Perl

Perl

Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...

.

Although the Go programming language

Go (programming language)

Go is a compiled, garbage-collected, concurrent programming language developed by Google Inc.The initial design of Go was started in September 2007 by Robert Griesemer, Rob Pike, and Ken Thompson. Go was officially announced in November 2009. In May 2010, Rob Pike publicly stated that Go was being...

is not generally considered an object-oriented language, it does allow methods to be defined on user-defined types. Go has "interface" types that are compatible with any type that supports a given set of methods (the type does not need to explicitly implement the interface). The empty interface, interface{}, is compatible with all types.

Note that functional programming

Functional programming

In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...

and distributed programming languages have a concept which is also called a protocol, but whose meaning is subtly different (i.e. a specification of allowed exchanges of messages, emphasis on exchanges, not on messages). This difference is due to somewhat different assumptions of functional programming and object-oriented programming paradigms. In particular, the following are also considered as part of a protocol in these languages:

The allowed sequences of messages,
Restrictions placed on either participant in the communication,
Expected effects that will occur as the message is handled.

See also