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An interface in the Java programming language is an abstract type that is used to declare a behavior that classes must implement. They are similar to protocols.Interfaces are declared using the interface keyword, and may only contain method signature and constant declarations (variable declarations that are declared to be both static and final).
In object-oriented programming, an interface or protocol type [a] is a data type that acts as an abstraction of a class. It describes a set of method signatures , the implementations of which may be provided by multiple classes that are otherwise not necessarily related to each other. [ 1 ]
Client interface: This is the public non-virtual interface; Subclass interface: This is the private interface, which can have any combination virtual and non-virtual methods. With such a structure, the fragile base class interface problem is mitigated. The only detriment is that the code is enlarged a little. [2]
By default, all methods in all classes are concrete, unless the abstract keyword is used. An abstract class may include abstract methods, which have no implementation. By default, all methods in all interfaces are abstract, unless the default keyword is used. The default keyword can be used to specify a concrete method in an interface.
The use of interfaces to allow disparate teams to collaborate raises the question of how interface changes happen in interface-based programming. The problem is that if an interface is changed, e.g. by adding a new method, old code written to implement the interface will no longer compile – and in the case of dynamically loaded or linked plugins, will either fail to load or link, or crash at ...
[26] [27] In C++, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). [25] A class consisting of only pure virtual methods is called a pure abstract base class (or pure ABC) in C++ and is also known as an interface by users of the language. [13]
An abstract method is one with only a signature and no implementation body. It is often used to specify that a subclass must provide an implementation of the method, as in an abstract class . Abstract methods are used to specify interfaces in some programming languages.
Prior to Java 8, an interface could contain only abstract methods. Java 17 allows an interface to have static, private, and default methods that can also contain implementation. However, interfaces cannot have non-final variables. Interfaces let you define a relationship between unrelated classes through the abstract concept. [3]