Interface (object-oriented programming)

Template:Short description In object-oriented programming, an interface or protocol typeTemplate:Efn is a data type that acts as an abstraction of a class. It describes a set of method signatures, the implementations of which may be provided by multiple classes that are otherwise not necessarily related to each other.<ref name="csharp-learn">Template:Cite web</ref> A class which provides the methods listed in an interface is said to implement the interface,<ref name="csharp-learn"/> or to adopt the protocol.<ref name="swift-24h">Template:Cite book</ref>

Interfaces are useful for encapsulation and reducing coupling. For example, in Java, the Comparable interface specifies the method compareTo. Thus, a sorting method only needs to take objects of types which implement Comparable to sort them, without knowing about the inner nature of the class (except that two of these objects can be compared via compareTo()).

Examples

Some programming languages provide explicit language support for interfaces: Ada, C#, D, Dart, Delphi, Go, Java, Logtalk, Object Pascal, Objective-C, OCaml, PHP, Racket, Swift, Python 3.8. In languages supporting multiple inheritance, such as C++, interfaces are abstract classes.

In Java, an implementation of interfaces may look like:

<syntaxhighlight lang="java"> class Animal { ... } class Theropod extends Animal { ... }

interface Flyable {

   void fly();

}

interface Vocal {

   void vocalize();

}

public class Bird extends Theropod implements Flyable, Vocal {

   // ...
   public void fly() { ... }
   public void vocalize() { ... }

} </syntaxhighlight>

In languages without explicit support, interfaces are often still present as conventions; this is known as duck typing. For example, in Python, any class can implement an __iter__ method and be used as an iterable.<ref name="python-iter">Template:Cite web</ref> Classes may also explicitly subclass an ABC, such as Template:Code.

Type classes in languages like Haskell, or module signatures in ML and OCaml, are used for many of the same things as are interfaces.Template:Clarify

In Rust, interfaces are called traits.<ref>Template:Cite web</ref> In Rust, a struct does not contain methods, but may add methods through separate Template:Code blocks:

<syntaxhighlight lang="rust"> trait Pet {

   fn speak(&self);

}

struct Dog {

   // Structs only contain their fields
   name: String

}

impl Dog {

   // Not from a trait
   fn new(name: String) -> Self {
       Dog { name }
   }

}

impl Pet for Dog {

   // From a trait
   fn speak(&self) {
       println!("{} says 'Woof!'", self.name);
   }

}

fn main() {

   let dog = Dog::new(String::from("Arlo"));
   dog.speak();

} </syntaxhighlight>

See also

• Interface (computing)
• Protocols in Objective-C
• Interface (Java)
• Concept (generic programming)
• Delegation (programming)
• Class (computer science)
• Application programming interface