Interface (object-oriented programming)

In object-oriented programming, an interface or protocol type^[a] 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.^[1] A class which provides the methods listed in an interface is said to implement the interface,^[1] or to adopt the protocol.^[2]

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:

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() { ... }
}

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.^[3] Classes may also explicitly subclass an ABC, such as collections.abc.Iterable.

Type classes in languages like Haskell, or module signatures in ML and OCaml, are used for many of the same things as are interfaces.^{[clarification needed]}

In Rust, interfaces are called traits.^[4] In Rust, a struct does not contain methods, but may add methods through separate impl blocks:

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

fn main() {
    let dog = Dog::new("Arlo".to_string());
    dog.speak();
}

See also

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

Notes

1. Use of these terms varies by programming language. Java and languages derived from it tend to use interface, while protocol is generally more popular elsewhere.