Abstract type

<h2 id="creation">Creation</h2>
<p>Abstract classes can be created, signified, or simulated in several ways:
</p>
<ul><li>By use of the explicit <a href="/facts/Keyword_(computer_programming)/pHDlUUgY">keyword</a> abstract in the class definition, as in <a href="/facts/Java_(programming_language)/9ScgFyAL">Java</a>, <a href="/facts/D_(programming_language)/4qIzRjkF">D</a> or <a href="/facts/C_Sharp_(programming_language)/UC2gxeyb">C#</a>.</li>
<li>By including, in the class definition, one or more <a href="/facts/Abstract_method/8kAXxVYs">abstract methods</a> (called <i>pure <a href="/facts/Virtual_functions/cnxckyHy">virtual functions</a></i> in <a href="/facts/C%252B%252B/Et0F2qn9">C++</a>), which the class is declared to accept as part of its protocol, but for which no implementation is provided.</li>
<li>By <a href="/facts/Inheritance_(object-oriented_programming)/a7R1xv8n">inheriting</a> from an abstract type, and not overriding all missing features necessary to complete the class definition.  In other words, a child type that does not implement all abstract methods from its parent becomes abstract itself.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a></li>
<li>In many dynamically typed languages such as <a href="/facts/Smalltalk/Ru2UPVDE">Smalltalk</a>, any class that sends a particular method to <a href="/facts/This_(computer_science)/DYW7Snl3">this</a>, but does not implement that method, can be considered abstract.  (However, in many such languages, like <a href="/facts/Objective-C/5mM557BW">Objective-C</a>, the error is not detected until the class is used, and the message returns results in an exception error message such as "Does not recognize selector: xxx" as - [NSObject doesNotRecognizeSelector:(SEL)selector] is invoked upon detection of an unimplemented method).</li></ul>
<h2 id="examples">Examples</h2>
<h3>Java</h3>
<p>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.
</p>
//By default, all methods in all classes are concrete, unless the abstract keyword is used.
public abstract class Demo {
    // An abstract class may include abstract methods, which have no implementation.
    public abstract int sum(int x, int y);

// An abstract class may also include concrete methods.
    public int product(int x, int y) { 
        return x*y; 
    }
}

//By default, all methods in all interfaces are abstract, unless the default keyword is used.
interface DemoInterface {
    int getLength(); //The abstract keyword can be used here, though is completely useless
    
    //The default keyword can be used in this context to specify a concrete method in an interface
    default int product(int x, int y) {
        return x * y;
    }
}

<h2 id="usage">Usage</h2>
<p>Abstract types are an important feature in <a href="/facts/Static_typing/Tz0bAMS9">statically typed</a> OOP languages. Many <a href="/facts/Dynamic_typing/Tz0bAMS9">dynamically typed</a> languages have no equivalent feature (although the use of <a href="/facts/Duck_typing/tcuo8Ywq">duck typing</a> makes abstract types unnecessary); however <i><a href="/facts/Trait_(computer_programming)/olRmTbFe">traits</a></i> are found in some modern dynamically-typed languages.
</p><p>Some authors argue that classes should be <a href="/facts/Leaf_class_(programming_language)/438ClNtw">leaf classes</a> (have no subtypes), or else be abstract.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>
</p><p>Abstract types are useful in that they can be used to define and enforce a <i><a href="/facts/Protocol_(object-oriented_programming)/jDMpXoLN">protocol</a></i>; a set of operations that all objects implementing the protocol must support.
</p><p>Abstract types are also an essential part of the <a href="/facts/Template_method_pattern/Wr2X8nLc">Template Method Pattern</a>.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Class_(computer_programming)/zvzkSAtE">Class</a></li>
<li><a href="/facts/Concept_(generic_programming)/fCkxQo4W">Concept</a></li>
<li><a href="/facts/Type_class/MBbIhF4n">Type class</a></li></ul>

<h2 id="further-reading">Further reading</h2>
<ul><li><a href="https://archive.org/details/headfirstjava00sier_0/page/688"><i>Head First Java</i></a>. O'Reilly Media. 2003. pp. <a href="https://archive.org/details/headfirstjava00sier_0/page/688">688</a>. <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-596-00920-8.</li>
<li>Core Java: An Integrated Approach by  R. Nageswara Rao</li></ul>
<h2 id="external-links">External links</h2>
<ul><li>"Abstract or Skeletal Interfaces Explained" <a href="http://10kloc.wordpress.com/2012/12/03/abstract-interfaces-the-mystery-revealed/">[1]</a></li>
<li><i>Types and Programming Languages</i> by Benjamin Pierce (MIT Press 2002) <a href="http://www.cis.upenn.edu/~bcpierce/tapl/main.html">[2]</a></li>
<li><i><a href="http://rosettacode.org/wiki/Abstract_type">Abstract type</a></i> at <a href="/facts/Rosetta_Code/HAN5rwmX">Rosetta Code</a></li></ul>

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

<ol>
<li id="fn:1"><p>Mitchell, John C.; Plotkin, Gordon D.; Abstract Types Have Existential Type, ACM Transactions on Programming Languages and Systems, Vol. 10, No. 3, July 1988, pp. 470–502 <a href="http://theory.stanford.edu/~jcm/papers/mitch-plotkin-88.pdf" target="_blank">http://theory.stanford.edu/~jcm/papers/mitch-plotkin-88.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"Abstract Methods and Classes (The Java Tutorials > Learning the Java Language > Interfaces and Inheritance)". Oracle.com. Retrieved 2019-08-14. <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank">https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"Abstract Methods and Classes (The Java Tutorials > Learning the Java Language > Interfaces and Inheritance)". Oracle.com. Retrieved 2019-08-14. <a href="https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html" target="_blank">https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Pure Virtual Functions and Abstract Classes in C++". GeeksforGeeks.org. 15 July 2014. <a href="https://www.geeksforgeeks.org/pure-virtual-functions-and-abstract-classes/" target="_blank">https://www.geeksforgeeks.org/pure-virtual-functions-and-abstract-classes/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>Riel, Arthur (1996). Object-Oriented Design Heuristics. Addison-Wesley Professional. p. 89. ISBN 0-201-63385-X. <a href="0-201-63385-X" target="_blank">0-201-63385-X</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>Meyers, Scott (1996). More Effective C++. Addison-Wesley Professional. p. 258. ISBN 0-201-63371-X. Make non-leaf classes abstract <a href="0-201-63371-X" target="_blank">0-201-63371-X</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
</ol>

Abstract type open-in-new

Abstract type