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Interface (Java)
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<p>An interface in the <a href="/facts/Java_/9ScgFyAL">Java programming language</a> is an <a href="/facts/Abstract_type/0XDoDYmf">abstract type</a> that is used to declare a behavior that <a href="/facts/Class_(computer_science)/zvzkSAtE">classes</a> must implement. They are similar to <a href="/facts/Protocol_(object-oriented_programming)/jDMpXoLN">protocols</a>. Interfaces are declared using the interface <a href="/facts/Java_keywords/eX58kdML">keyword</a>, and may only contain <a href="/facts/Method_signature/psEPS0UW">method signature</a> and constant declarations (variable declarations that are declared to be both <a href="/facts/Static_variable/dYIs2E5p">static</a> and <a href="/facts/Final_(Java)/gyV5Jhzz">final</a>). All methods of an Interface do not contain implementation (method bodies) as of all versions below Java 8. Starting with Java 8, default: 99 and static: 7 methods may have implementation in the interface definition. Then, in Java 9, private and private static methods were added. At present,[<i>when?</i>] a Java interface can have up to six different types. </p><p>Interfaces cannot be <a href="/facts/Instance_(computer_science)/5zPBGdX1">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="/facts/Abstract_class/0XDoDYmf">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="/facts/Null_pointer/66zwVEfL">null</a>, or be bound to an object that implements the interface. </p><p>One benefit of using interfaces is that they simulate <a href="/facts/Multiple_inheritance/kFvNb9rY">multiple inheritance</a>. All classes in Java must have exactly one <a href="/facts/Base_class/a7R1xv8n">base class</a>, the only exception being <a href="https://docs.oracle.com/en/java/javase/19/docs/api/java.base/java/lang/Object.html">java.lang.Object</a> (the <a href="/facts/Top_type/zp3mbtdR">root class</a> of the Java <a href="/facts/Type_system/Tz0bAMS9">type system</a>); <a href="/facts/Multiple_inheritance/kFvNb9rY">multiple inheritance</a> of classes is not allowed. However, an interface may inherit multiple interfaces and a class may implement multiple interfaces. </p>