Editing Class (computer programming) (section)

===Sealed===
<span class="anchor" id="Non-subclassable"></span><span class="anchor" id="Sealed"></span>
A sealed class cannot be subclassed. It is basically the opposite of an ''abstract'' class, which must be derived to be used. A sealed class is implicitly ''concrete''.

A class is declared as sealed via the keyword {{code|sealed|lang=csharp}} in C# or {{code|final|lang=java}} in Java or PHP.<ref>{{cite web |url=http://msdn.microsoft.com/en-us/library/ms173149.aspx |title=sealed (C# Reference) |work=C# Reference |publisher=Microsoft |access-date=2012-05-08}}</ref><ref>
{{cite web |url=http://docs.oracle.com/javase/tutorial/java/IandI/final.html |title=Writing Final Classes and Methods |work=The Java Tutorials |publisher=Oracle |access-date=2012-05-08}}</ref><ref>
{{cite web |url=http://php.net/manual/en/language.oop5.final.php |title=PHP: Final Keyword |work=PHP Manual |publisher=The PHP Group |access-date=2014-08-21}}</ref>

For example, Java's {{Java|String}} class is marked as ''final''.<ref>{{cite web |url=http://docs.oracle.com/javase/7/docs/api/java/lang/String.html |title=String (Java Platform SE 7) |work=Java Platform, Standard Edition 7: API Specification |publisher=Oracle |access-date=2012-05-08}}</ref>

Sealed classes may allow a compiler to perform optimizations that are not available for classes that can be subclassed.<ref>{{cite web |last1=Brand |first1=Sy |title=The Performance Benefits of Final Classes |url=https://devblogs.microsoft.com/cppblog/the-performance-benefits-of-final-classes/ |website=Microsoft C++ team blog |date=2 March 2020 |publisher=Microsoft |access-date=4 April 2020}}</ref>
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<!-- It is also possible not to declare the whole class as such, but only the [[Override (object-oriented programming)|override]] as sealed. This classes are used because of efficiency concerns (can be called like static classes) and security (avoids inadvertent modification of the class semantics).<ref>{{cite web |access-date=2011-08-03 |date=2002-03-25 |first=Hanspeter |last=Mössenböck |page=17 |publisher=Institut für Systemsoftware, Johannes Kepler Universität Linz, Fachbereich Informatik |title=Advanced C#: Overriding of Methods |url=http://ssw.jku.at/Teaching/Lectures/CSharp/Tutorial/Part2.pdf}}
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