Interface (Java)

<h2 id="overview">Overview</h2>
<p>Interfaces are used to encode similarities which the classes of various types share, but do not necessarily constitute a class relationship. For instance, a <a href="/facts/Human/rlcTENsr">human</a> and a <a href="/facts/Parrot/u32QQjMe">parrot</a> can both <a href="/facts/Whistle/UIbmhlwN">whistle</a>; however, it would not make sense to represent Humans and Parrots as subclasses of a Whistler class. Rather they  most likely be subclasses of an Animal class (likely with intermediate classes), but both would  implement the Whistler interface.
</p><p>Another use of interfaces is being able to use an <a href="/facts/Object_(computer_science)/eX99dtAw">object</a> 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 whistler.whistle() will call the implemented method whistle of object whistler no matter what class it has, provided it implements Whistler. In a more practical example, a <a href="/facts/Sorting_algorithm/PwHpj1Gs">sorting algorithm</a> may expect an object of type <a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/lang/Comparable.html">Comparable</a>. Thus, without knowing the specific type, it knows that objects of that type can somehow be sorted.
</p><p>For example:
</p>
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.
}

<p>An interface:
</p>
<ul><li>declares only method headers and public constants.</li>
<li>cannot be instantiated.</li>
<li>can be implemented by a class.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>: 75 </li>
<li>cannot extend a class.</li>
<li>can extend several other interfaces.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>: 87 </li></ul>
<h2 id="usage">Usage</h2>
<h3>Defining an interface</h3>
<p>Interfaces are defined with the following syntax (compare to <a href="/facts/Class_(software)/zvzkSAtE">Java's class definition</a>):
</p>
[<i>visibility</i>] interface <i>InterfaceName</i> [extends <i>other interfaces</i>] {
        <i>constant declarations</i>
        <i>abstract method declarations</i>
        <i> static method declarations</i>
}

<p>Example: public interface Interface1 extends Interface2;
</p><p>The body of the interface contains <a href="/facts/Method_(computer_programming)/8kAXxVYs">abstract methods</a>, but since all methods in an interface are, by definition, abstract, the abstract keyword is not required. Since the interface specifies a set of exposed behaviors, all methods are implicitly public.
</p><p>Thus, a simple interface may be
</p>
public interface Predator {
    boolean chasePrey(Prey p);
    void eatPrey(Prey p);
}

<p>The member type declarations in an interface are implicitly static, final and public, but otherwise they can be any type of class or interface.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p>
<h3>Implementing interfaces in a class</h3>
<p>The syntax for implementing an interface uses this formula:
</p>
... implements <i>InterfaceName</i>[, <i>another interface</i>, <i>another</i>, ...] ...

<p><a href="/facts/Class_(software)/zvzkSAtE">Classes</a> may implement an interface. For example:
</p>
public class Lion implements Predator {

@Override
    public boolean chasePrey(Prey p) {
           // Programming to chase prey p (specifically for a lion)
    }

@Override
    public void eatPrey(Prey p) {
           // Programming to eat prey p (specifically for a lion)
    }
}

<p>If a class implements an interface and does not implement all its methods, it must be marked as abstract. If a class is abstract, one of its <a href="/facts/Subclass_(computer_science)/a7R1xv8n">subclasses</a> is expected to implement its unimplemented methods, though if any of the abstract class' subclasses do not implement all interface methods, the subclass itself must be marked again as abstract.
</p><p>Classes can implement multiple interfaces:
</p>
public class Frog implements Predator, Prey { ... }

<p>Interfaces can share common class methods:
</p>
class Animal implements LikesFood, LikesWater {
    boolean likes() { return true; }
}

<p>However a given class cannot implement the same or a similar interface multiple times:
</p>
class Animal implements Shares<Boolean>, Shares<Integer> ...
// Error: repeated interface

<p>Interfaces are commonly used in the Java language for <a href="/facts/Callback_(computer_science)/vPlrCJXG">callbacks</a>,<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> 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.
</p>
<h3>Subinterfaces</h3>
<p>Interfaces can extend several other interfaces, using the same formula as described below. For example,
</p>
public interface VenomousPredator extends Predator, Venomous {
    // Interface body
}

<p>is legal and defines a subinterface. It allows multiple inheritance, unlike classes. Predator and Venomous may possibly define or inherit methods with the same signature, say kill(Prey p). When a class implements VenomousPredator it will implement both methods simultaneously.
</p>
<h2 id="examples">Examples</h2>
<p>Some common <a href="/facts/Java_(Sun)/fpn6x8WL">Java</a> interfaces are:
</p>
<ul><li><a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/lang/Comparable.html">Comparable</a> has the  method <a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/lang/Comparable.html#compareTo(T)">compareTo</a>, which is used to describe two objects as equal, or to indicate one is greater than the other. <a href="/facts/Generic_programming/kqtqJk7p">Generics</a> allow implementing classes to specify which class instances can be compared to them.</li>
<li><a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/io/Serializable.html">Serializable</a> is a <a href="/facts/Marker_interface/6MzQzCcD">marker interface</a> with no methods or fields - it has an empty body. It is used to indicate that a class can be <a href="/facts/Serialization/T00auFvv">serialized</a>. Its <a href="/facts/Javadoc/pm72Xvlf">Javadoc</a> describes how it should function, although nothing is programmatically enforced</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Interface_(object-oriented_programming)/jDMpXoLN">Interface (object-oriented programming)</a></li>
<li><a href="/facts/Mixin/QHG68mgY">Mixin</a></li>
<li><a href="/facts/Trait_(computer_programming)/olRmTbFe">Trait (computer programming)</a></li></ul>
<h2 id="citations">Citations</h2>

<ul><li>Bloch, Joshua (2018). <i>"Effective Java: Programming Language Guide"</i> (third ed.). Addison-Wesley. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 978-0134685991.</li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="http://java.sun.com/docs/books/tutorial/java/concepts/interface.html">What Is an Interface?</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Bloch 2018. - Bloch, Joshua (2018). "Effective Java: Programming Language Guide" (third ed.). Addison-Wesley. ISBN 978-0134685991. <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Bloch 2018. - Bloch, Joshua (2018). "Effective Java: Programming Language Guide" (third ed.). Addison-Wesley. ISBN 978-0134685991. <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Default Methods". Archived from the original on 2017-05-23. Retrieved 2014-06-30. <a href="https://web.archive.org/web/20170523042436/http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html" target="_blank">https://web.archive.org/web/20170523042436/http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Bloch 2018. - Bloch, Joshua (2018). "Effective Java: Programming Language Guide" (third ed.). Addison-Wesley. ISBN 978-0134685991. <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Bloch 2018. - Bloch, Joshua (2018). "Effective Java: Programming Language Guide" (third ed.). Addison-Wesley. ISBN 978-0134685991. <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>"The Java Language Specification". <a href="http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.5" target="_blank">http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.5</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Mitchell, John D. (June 1, 1996). "Java Tip 10: Implement callback routines in Java". JavaWorld. Retrieved 2020-07-14. <a href="https://www.infoworld.com/article/2077462/java-tip-10--implement-callback-routines-in-java.html" target="_blank">https://www.infoworld.com/article/2077462/java-tip-10--implement-callback-routines-in-java.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
</ol>

Interface (Java) open-in-new

Interface (Java)