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Interface (Java)

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An interface in the <a href="Java_(programming_language)" title="Java (programming language)">Java programming language</a> is an <a href="Abstract_type" title="Abstract type">abstract type</a> that is used to declare a behavior that <a href="Class_(computer_science)" class="mw-redirect" title="Class (computer science)">classes</a> must implement. They are similar to <a href="Protocol_(object-oriented_programming)" class="mw-redirect" title="Protocol (object-oriented programming)">protocols</a>. Interfaces are declared using the <code>interface</code> <a href="Java_keywords" class="mw-redirect" title="Java keywords">keyword</a>, and may only contain <a href="Method_signature" class="mw-redirect" title="Method signature">method signature</a> and constant declarations (variable declarations that are declared to be both <code><a href="Static_variable#Static_Variables_as_Class_Variables" title="Static variable">static</a></code> and <code><a href="Final_(Java)" title="Final (Java)">final</a></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><a href="#cite_note-FOOTNOTEBloch2018-1">[1]</a>: 99 and <code>static</code><a href="#cite_note-FOOTNOTEBloch2018-1">[1]</a>: 7 methods may have implementation in the <code>interface</code> definition.<a href="#cite_note-2">[2]</a> 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 <a href="Instance_(computer_science)" title="Instance (computer science)">instantiated</a>, 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 <a href="Abstract_class" class="mw-redirect" title="Abstract class">abstract class</a>. Object references in Java may be specified to be of an interface type; in each case, they must either be <a href="Null_pointer" title="Null pointer">null</a>, or be bound to an object that implements the interface.

One benefit of using interfaces is that they simulate <a href="Multiple_inheritance" title="Multiple inheritance">multiple inheritance</a>. All classes in Java must have exactly one <a href="Base_class" class="mw-redirect" title="Base class">base class</a>, the only exception being <code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/Object.html">java.lang.Object</a></code> (the <a href="Top_type" class="mw-redirect" title="Top type">root class</a> of the Java <a href="Type_system" title="Type system">type system</a>); <a href="Multiple_inheritance" title="Multiple inheritance">multiple inheritance</a> of classes is not allowed. However, an interface may inherit multiple interfaces and a class may implement multiple interfaces.

<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

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="Human" title="Human">human</a> and a <a href="Parrot" title="Parrot">parrot</a> can both <a href="Whistle" title="Whistle">whistle</a>; 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 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 <a href="Object_(computer_science)" title="Object (computer science)">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 <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 <a href="Sorting_algorithm" title="Sorting algorithm">sorting algorithm</a> may expect an object of type <code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/Comparable.html">Comparable</a></code>. Thus, without knowing the specific type, it knows that objects of that type can somehow be sorted.

For example:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>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.

}

</pre></div>

An interface:

<ul><li>declares only method headers and public constants.</li>

<li>cannot be instantiated.</li>

<li>can be implemented by a class.<a href="#cite_note-FOOTNOTEBloch2018-1">[1]</a>: 75 </li>

<li>cannot extend a class.</li>

<li>can extend several other interfaces.<a href="#cite_note-FOOTNOTEBloch2018-1">[1]</a>: 87 </li></ul>

<div class="mw-heading mw-heading2"><h2 id="Usage">Usage</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Defining_an_interface">Defining an interface</h3></div>

Interfaces are defined with the following syntax (compare to <a href="Class_(software)" class="mw-redirect" title="Class (software)">Java's class definition</a>):

<pre>[visibility] interface InterfaceName [extends other interfaces] {

constant declarations

abstract method declarations

static method declarations

}

</pre>

Example: public interface Interface1 extends Interface2;

The body of the interface contains <a href="Method_(computer_programming)#Abstract_methods" title="Method (computer programming)">abstract methods</a>, 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

boolean chasePrey(Prey p);

void eatPrey(Prey p);

}

</pre></div>

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 href="#cite_note-3">[3]</a>

<div class="mw-heading mw-heading3"><h3 id="Implementing_interfaces_in_a_class">Implementing interfaces in a class</h3></div>

The syntax for implementing an interface uses this formula:

<pre>... implements InterfaceName[, another interface, another, ...] ...

</pre>

<a href="Class_(software)" class="mw-redirect" title="Class (software)">Classes</a> may implement an interface. For example:

@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)

}

}

</pre></div>

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 <a href="Subclass_(computer_science)" class="mw-redirect" title="Subclass (computer science)">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 <code>abstract</code>.

Classes can implement multiple interfaces:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public class Frog implements Predator, Prey { ... }

</pre></div>

Interfaces can share common class methods:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>class Animal implements LikesFood, LikesWater {

boolean likes() { return true; }

}

</pre></div>

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

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>class Animal implements Shares<Boolean>, Shares<Integer> ...

// Error: repeated interface

</pre></div>

Interfaces are commonly used in the Java language for <a href="Callback_(computer_science)" class="mw-redirect" title="Callback (computer science)">callbacks</a>,<a href="#cite_note-4">[4]</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.

<div class="mw-heading mw-heading3"><h3 id="Subinterfaces">Subinterfaces</h3></div>

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

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>public interface VenomousPredator extends Predator, Venomous {

// Interface body

}

</pre></div>

is legal and defines a subinterface. It allows multiple inheritance, unlike classes. <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.

<div class="mw-heading mw-heading2"><h2 id="Examples">Examples</h2></div>

Some common <a href="Java_(Sun)" class="mw-redirect" title="Java (Sun)">Java</a> interfaces are:

<ul><li><code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/Comparable.html">Comparable</a></code> has the method <code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/lang/Comparable.html#compareTo(T)">compareTo</a></code>, which is used to describe two objects as equal, or to indicate one is greater than the other. <a href="Generic_programming" title="Generic programming">Generics</a> allow implementing classes to specify which class instances can be compared to them.</li>

<li><code><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/io/Serializable.html">Serializable</a></code> is a <a href="Marker_interface" class="mw-redirect" title="Marker interface">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="Serialization" title="Serialization">serialized</a>. Its <a href="Javadoc" title="Javadoc">Javadoc</a> describes how it should function, although nothing is programmatically enforced</li></ul>

<ul><li><a href="Interface_(object-oriented_programming)" title="Interface (object-oriented programming)">Interface (object-oriented programming)</a></li>

<li><a href="Mixin" title="Mixin">Mixin</a></li>

<li><a href="Trait_(computer_programming)" title="Trait (computer programming)">Trait (computer programming)</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="Citations">Citations</h2></div>

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<li id="cite_note-FOOTNOTEBloch2018-1">^ <a href="#cite_ref-FOOTNOTEBloch2018_1-0">a</a> <a href="#cite_ref-FOOTNOTEBloch2018_1-1">b</a> <a href="#cite_ref-FOOTNOTEBloch2018_1-2">c</a> <a href="#cite_ref-FOOTNOTEBloch2018_1-3">d</a> <a href="#CITEREFBloch2018">Bloch 2018</a>.

</li>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20170523042436/http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html">"Default Methods"</a>. Archived from <a rel="nofollow" class="external text" href="http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html">the original</a> on 2017-05-23. Retrieved 2014-06-30.</cite>

</li>

<li id="cite_note-3"><a href="#cite_ref-3">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html#9.5">"The Java Language Specification"</a>.</cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite id="CITEREFMitchell1996" class="citation web cs1">Mitchell, John D. (June 1, 1996). <a rel="nofollow" class="external text" href="https://www.infoworld.com/article/2077462/java-tip-10--implement-callback-routines-in-java.html">"Java Tip 10: Implement callback routines in Java"</a>. <a href="JavaWorld" class="mw-redirect" title="JavaWorld">JavaWorld</a>. Retrieved 2020-07-14.</cite>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

<ul><li><cite id="CITEREFBloch2018" class="citation book cs1">Bloch, Joshua (2018). "Effective Java: Programming Language Guide" (third ed.). Addison-Wesley. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0134685991</bdi>.</cite></li></ul>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://java.sun.com/docs/books/tutorial/java/concepts/interface.html">What Is an Interface?</a></li></ul></div><div><div class="zim-footer">

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