{{Merge to|Interface (object-oriented programming)|discuss=Talk:Protocol (object-oriented programming)#Proposed merge of Interface (Java) into Protocol (object-oriented programming)|date=November 2022}}
An '''interface''' in the [[Java (programming language)|Java programming language]] is an [[abstract type]] that is used to describedeclare a behavior that [[class (computer science)|classes]] must implement. They are similar to [[Protocol (object-oriented programming)|protocol]]s. Interfaces are declared using the <code>interface</code> [[Java keywords|keyword]], and may only contain [[method signature]] and constant declarations (variable declarations that are declared to be both <code>[[Static_variable#Static_Variables_as_Class_Variables|static]]</code> and <code>[[Final (Java)|final]]</code>). All methods of an Interface do not contain implementation (method bodies) as of all versions below Java 8. Starting with Java 8, <code>default</code> and <code>static</code> methods may have implementation in the <code>interface</code> definition.<ref>{{cite web|url=http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html|title=Default Methods|access-date=2014-06-30|archive-url=https://web.archive.org/web/20170523042436/http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html|archive-date=2017-05-23|url-status=dead}}</ref> Then, in Java 9, <code>private</code> and <code>private static</code> methods were added. At present, a Java interface can have up to six different types.
Interfaces cannot be [[Instance (computer science)|instantiated]], but rather are implemented. A class that implements an interface must implement all of the non-default methods described in the interface, or be an [[abstract class]]. Object references in Java may be specified to be of an interface type; in each case, they must either be [[null pointer|null]], or be bound to an object that implements the interface.
