Interface (Java): Difference between revisions

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{{Short description|Concept in the Java computer programming language}}

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>{{sfn|Bloch|2018}}{{rp|99}} and <code>static</code>{{sfn|Bloch|2018}}{{rp|7}} 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,{{when|date=March 2025}} a Java interface can have up to six different types.{{Clarify|date=March 2025}}

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.

One benefit of using interfaces is that they simulate [[multiple inheritance]]. All classes in Java must have exactly one [[base class]], the only exception being {{Javadoc:SE|package=java.lang|java/lang|Object}} (the [[top type|root class]] of the Java [[type system]]); [[multiple inheritance]] of classes is not allowed. However, an interface may inherit multiple interfaces and a class may implement multiple interfaces.

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Another use of interfaces is being able to use an [[Object (computer science)|object]] without knowing its type of class, but rather only that it implements a certain interface. For instance, if one were annoyed by a whistling noise, one may not know whether it is a human or a parrot, because all that could be determined is that a whistler is whistling. The call <code>whistler.whistle()</code> will call the implemented method <code>whistle</code> of object <code>whistler</code> no matter what class it has, provided it implements <code>Whistler</code>. In a more practical example, a [[sorting algorithm]] may expect an object of type {{Javadoc:SE|java/lang|Comparable}}. Thus, without knowing the specific type, it knows that objects of that type can somehow be sorted.

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For example:

<syntaxhighlight lang="Java">

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* declares only method headers and public constants.

* cannot be instantiated.

* can be implemented by a class.{{sfn|Bloch|2018}}{{rp|75}}

* cannot extend a class.

* can extend several other interfaces.{{sfn|Bloch|2018}}{{rp|87}}

==Usage==

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''constant declarations''

''abstract method declarations''

'' static method declarations''

}

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</syntaxhighlight>

The member type declarations in an interface are implicitly static, final and public, but otherwise they can be any type of class or interface.<ref>{{cite web|url=http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.5|title=The Java Language Specification}}</ref>'''

===Implementing interfaces in a class===

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==See also==

* [[Interface (object-oriented programming)]]

* [[Mixin]]

* [[Trait (computer programming)]]

==ReferencesCitations==

{{reflist}}

==References==

*{{cite book | title= "Effective Java: Programming Language Guide" |last=Bloch| first=Joshua| publisher=Addison-Wesley | edition=third | isbn=978-0134685991| year=2018}}

==External links==