Interface (object-oriented programming)

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. 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.

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

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(); }

In C++, there are multiple ways to resemble interfaces. One such way is the Java-style interface, which is done using abstract classes. The other, which resembles Go interfaces, is using concepts. Unlike inheritance, with concepts any type may satisfy a concept (not just classes) so long as it satisfies all of its requirements.