Editing Class (computer programming) (section)

===Abstract and concrete===
<span class="anchor" id="Abstract_and_concrete_classes"></span><span class="anchor" id="Abstract_and_Concrete"></span>
{{see also|Abstract type}}

In a language that supports inheritance, an ''abstract class'', or ''abstract base class'' (''ABC''), is a class that cannot be directly instantiated. By contrast, a ''concrete class'' is a class that {{em|can}} be directly instantiated. Instantiation of an abstract class can occur only indirectly, via a concrete {{em|sub}}class.

An abstract class is either labeled as such explicitly or it may simply specify ''[[abstract method]]s'' (or ''[[virtual method]]s''). An abstract class may provide implementations of some methods, and may also specify virtual methods via [[Type signature|signatures]] that are to be implemented by direct or indirect descendants of the abstract class. Before a class derived from an abstract class can be instantiated, all abstract methods of its parent classes must be implemented by some class in the derivation chain.<ref name="cpppoly">{{cite web|url=http://www.cplusplus.com/doc/tutorial/polymorphism/
|title=Polymorphism|work=C++ Language Tutorial|publisher=cplusplus.com|access-date=2012-05-02}}</ref>

Most object-oriented programming languages allow the programmer to specify which classes are considered abstract and will not allow these to be instantiated. For example, in [[Java (programming language)|Java]], [[C Sharp (programming language)|C#]] and [[PHP]], the keyword ''abstract'' is used.<ref>{{cite web| url=http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html|title=Abstract Methods and Classes|work=The Java Tutorials|publisher=Oracle|access-date=2012-05-02}}</ref><ref>{{cite web|url=http://php.net/manual/en/language.oop5.abstract.php|title=Class Abstraction|work=PHP Manual|publisher=The PHP Group|access-date=2012-05-02}}</ref> 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).<ref name=cpppoly/>

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.<ref name=cppinterface/> Other languages, notably Java and C#, support a variant of abstract classes called an [[Interface (Java)|interface]] via a keyword in the language. In these languages, [[multiple inheritance]] is not allowed, but a class can implement multiple interfaces. Such a class can only contain abstract publicly accessible methods.<ref name=javainterface/><ref>{{cite web |url=http://msdn.microsoft.com/en-us/library/ms173156.aspx |title=Interfaces (C# Programming Guide) |work=C# Programming Guide |publisher=Microsoft |access-date=2013-08-15}}</ref><ref>
{{cite web |url=http://msdn.microsoft.com/en-us/library/ms173149.aspx |title=Inheritance (C# Programming Guide) |work=C# Programming Guide |publisher=Microsoft |access-date=2012-05-02}}</ref>
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