Sunday, 26 October 2014

Covariant return type

“In object-oriented programming, a covariant return type of a method is one that can be replaced by a "narrower" type when the method is overridden in a subclass.”

Since JDK 5.0 it is possible to provide covariant return types in methods of subclasses.[2] Before this release, the Java programming language was invariant with regard to method return types.

Unfortunately, covariance is not possible with method parameters. If you wish to use that, reference [1] has a good explanation of how to do this using Generics.

Example

Scala

In Scala all three are possible, contravariant, covariant and invariant for both method parameters as well as method return types. It is used fairly frequently.

For more information, the blog in [3] has some excellent explanation.

References

[1] Covariant Parameter Types
http://www.java-tips.org/java-se-tips/java.lang/covariant-parameter-types-2.html
[2] Wikipedia - Covariant return type
http://en.wikipedia.org/wiki/Covariant_return_type
[3] Atlassian Blogs - Covariance and Contravariance in Scala
http://blogs.atlassian.com/2013/01/covariance-and-contravariance-in-scala/

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