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 declare 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.

Interfaces cannot be [[Instance (computer science)|instantiated]], but rather are implemented. A class that implements an interface must implement all of the 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.

== 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:

<sourcesyntaxhighlight lang="Java">

interface Bounceable {

double pi = 3.1415;

void setBounce(); // Note the semicolon

// Interface methods are public, abstract and never final.

// Think of them as prototypes only; no implementations are allowed.

}

</syntaxhighlight>

<source lang="Java">

interface Bounceable {

double pi=3.1415;

void setBounce(); // Note the semicolon

// Interface methods are public, abstract and never final.

// Think of them as prototypes only; no implementations are allowed.

}

</source>

An interface:

* 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==

Line 35 ⟶ 36:

''constant declarations''

''abstract method declarations''

'' static method declarations''

}

Line 42 ⟶ 44:

Thus, a simple interface may be

<sourcesyntaxhighlight lang="java">

public interface Predator {

boolean chasePrey(Prey p);

void eatPrey(Prey p);

}

</syntaxhighlight>

</source>

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===

Line 56 ⟶ 58:

... implements ''InterfaceName''[, ''another interface'', ''another'', ...] ...

[[Class (software)|Classes]] may implement an interface. For example, :

<sourcesyntaxhighlight lang="java">

public class Lion implements Predator {

@Override

public boolean chasePrey(Prey p) {

// programmingProgramming to chase prey p (specifically for a lion)

}

@Override

public void eatPrey(Prey p) {

// programmingProgramming to eat prey p (specifically for a lion)

}

}

</syntaxhighlight>

</source>

If a class implements an interface and does not implement all its methods, it must be marked as <code>abstract</code>. If a class is abstract, one of its [[Subclass (computer science)|subclasses]] is expected to implement its unimplemented methods., Althoughthough if any of the abstract class' subclasses doesdo not implement all interface methods, the subclass itself must be marked again as <code>abstract</code>.

Classes can implement multiple interfaces:

<sourcesyntaxhighlight lang="Java">

public class Frog implements Predator, Prey { ... }

</syntaxhighlight>

</source>

Interfaces can share common class methods:

<sourcesyntaxhighlight lang="Java">

class Animal implements LikesFood, LikesWater {

{ boolean likes() { return true; }

}

</syntaxhighlight>

</source>

However a given class cannot implement the same or a similar interface multiple times:

<sourcesyntaxhighlight lang="Java">

class Animal implements Shares<Boolean>, Shares<Integer> ...

// errorError: repeated interface

</syntaxhighlight>

</source>

Interfaces are commonly used in the Java language for [[Callback (computer science)|callbacks]],<ref>{{cite web |last1=Mitchell |first1=John D. |date=1996-06-01 |df=mdy |url=httphttps://www.javaworldinfoworld.com/javaworldarticle/javatips2077462/jwjava-javatip10tip-10--implement-callback-routines-in-java.html |title=Java WorldTip 10: Implement callback routines in Java |work=[[JavaWorld]] |access-date=2020-07-14}}</ref> as Java does not allow multiple inheritance of classes, nor does it allow the passing of methods (procedures) as arguments. Therefore, in order to pass a method as a parameter to a target method, current practice is to define and pass a reference to an interface as a means of supplying the signature and address of the parameter method to the target method rather than defining multiple variants of the target method to accommodate each possible calling class.

===Subinterfaces===

Interfaces can extend several other interfaces, using the same formula as described below. For example,

<sourcesyntaxhighlight lang="java">

public interface VenomousPredator extends Predator, Venomous {

//interface Interface body

}

</syntaxhighlight>

</source>

is legal and defines a subinterface. Note how itIt allows multiple inheritance, unlike classes. Note also that <code>Predator</code> and <code>Venomous</code> may possibly define or inherit methods with the same signature, say <code>kill(Prey p)</code>. When a class implements <code>VenomousPredator</code> it will implement both methods simultaneously.

==Examples==

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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==