Interface (Java): Difference between revisions

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#REDIRECT [[Interfaces (journal)]]

An '''interface''' in the [[Java (programming language)|Java programming language]] is an [[abstract type]] that is used to specify 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.

An=== '''interface''' in the [[Java (programming language)|Java programming language]] is an [[abstract type]] that is used to specify 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.

=== 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.

=== 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. ===

== Overview ==

Interfaces are used to encode similarities which the classes of various types share, but do not necessarily constitute a class relationship. For instance, a [[human]] and a [[parrot]] can both [[whistle]]; however, it would not make sense to represent <code>Human</code>s and <code>Parrot</code>s as subclasses of a <code>Whistler</code> class. Rather they would most likely be subclasses of an <code>Animal</code> class (likely with intermediate classes), but both would implement the <code>Whistler</code> interface.

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.

For example:

<syntaxhighlight lang="Javajava">

interface Bounceable {

double pi = 3.1415;

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The body of the interface contains [[Method (computer programming)#Abstract methods|abstract methods]], but since all methods in an interface are, by definition, abstract, the <code>abstract</code> keyword is not required. Since the interface specifies a set of exposed behaviors, all methods are implicitly <code>public</code>.

Thus, a simple interface may be

<syntaxhighlight lang="java">

public interface Predator {

boolean chasePrey(Prey p);

void eatPrey(Prey p);

}

</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===