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--- # this is so called 'Yaml Front Matter', read up on it here: http://jekyllrb.com/docs/frontmatter/ layout: pattern title: Filterer Pattern folder: filterer permalink: /patterns/filterer/ description: Design pattern that helps container-like objects to return filtered version of themselves.# short meta description that shows in Google search results categories:
- Functional tags:
- Extensibility
Name / classification
Filterer Pattern
Intent
The intent of this design pattern is to to introduce a functional interface that will add a functionality for container-like objects to easily return filtered versions of themselves.
Explanation
The container-like object needs to have a method that returns an instance of Filterer. This helper interface gives
ability to covariantly specify a lower bound of contravariant Predicate in the subinterfaces of interfaces representing the container-like objects.
Class diagram
Applicability
Pattern can be used when working with container-like objects that use subtyping, instead of parametrizing(generics) for extensible class structure. It enables you to easily extend filtering ability of container-like objects as business requirements change.
Tutorials
- Article about Filterer pattern posted on it's author's blog
- Application of Filterer pattern in domain of text analysis
Known uses
One of the uses is present on the blog presented in this link. It presents how to use Filterer pattern to create text issue anaylyzer with support for test cases used for unit testing.
Consequences (the good and the bad, add criticism here)
Good :
- you can easily introduce new subtypes for container-like objects and subtypes for objects that are contained within them and still be able to filter easily be new properties of those new subtypes.
Bad :
- covariant return types mixed with generics can be sometimes tricky
Credits
- Author of the pattern : Tomasz Linkowski