[jira] [Work started] (KAFKA-4481) Relax Kafka Streams API type constraints

["Xavier Léauté (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KAFKA-4481?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Work on KAFKA-4481 started by Xavier Léauté.
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> Relax Kafka Streams API type constraints
> ----------------------------------------
>
>                 Key: KAFKA-4481
>                 URL: https://issues.apache.org/jira/browse/KAFKA-4481
>             Project: Kafka
>          Issue Type: Bug
>          Components: streams
>            Reporter: Xavier Léauté
>            Assignee: Xavier Léauté
>              Labels: usability
>
> Streams API methods that apply transformations to streams are currently invariant in the key and value types, when they should probably be contravariant in those types.
> For instance, {{KStream.filter(Predicate<K, V> predicate)}} should be {{KStream.filter(Predicate<? super K, ? super V> predicate)}} to accept predicates that can act on any supertype of K, or V.
> Same thing applies to method that take {{Aggregator}}, {{StreamPartitioner}}, {{KeyValueMapper}}, {{ValueMapper}}, {{ProcessorSupplier}}, {{ValueJoiner}}, etc.



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