Wildcard (Java): Difference between revisions

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The '''wildcard''' <code>?</code> in [[Java (programming language)|Java]] is a special [[type parameter]] that controls the type safety of the use of [[Generics in Java|generic]] (parameterized) types. It can be used in variable declarations and instantiations<!--in special circumstances, see section below)--> as well as in method definitions, but not in the definition of a generic type.<ref name="gilad2004">{{citecitation|title=Generics in the Java Programming Language|url= http://www.oracle.com/technetwork/java/javase/generics-tutorial-159168.pdf|author=Gilad Bracha|date=June 2004|section=4. Wildcards|accessdate=6 March 2016}}</ref><ref>{{citecitation|url=http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.1.2|title=Java Language Specification|section=8.1.2 Generic Classes and Type Parameters|publisher=Oracle|accessdate=6 March 2016}}</ref> This is a form of ''use-site'' [[variance annotation]], in contrast with the ''definition-site'' variance annotations found in [[C Sharp (programming language)|C#]] and [[Scala (programming language)|Scala]].

== Covariance for generic types ==

Unlike arrays (which are [[Covariance and contravariance (computer science)#Covariant arrays in Java and C#|covariant]] in Java), different instantiations of a generic type are not compatible with each other, not even explicitly: With the declaration <code>Generic<Supertype> superGeneric; Generic<Subtype> subGeneric;</code> the compiler would report a conversion error for both castings <code>(Generic<Subtype>)superGeneric</code> and <code>(Generic<Supertype>)subGeneric</code>.

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== References ==

{{Reflist}}

* The Java Language Specification, Third Edition (Sun), ISBN 978-0-321-24678-3 http://java.sun.com/docs/books/jls/

* Java Tutorials, Lesson Generics http://download.oracle.com/javase/tutorial/java/generics/index.html

* Capturing Wildcards, http://bayou.io/draft/Capturing_Wildcards.html

* Typkompatibilität in Java http://public.beuth-hochschule.de/~solymosi/veroeff/typkompatibilitaet/Typkompatibilitaet.html#Joker (in German)

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