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https://issues.apache.org/jira/browse/KAFKA-4481?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15716140#comment-15716140
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Xavier Léauté commented on KAFKA-4481:
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This change should be backwards-compatible and would only affect someone
extending or re-implementing kafka streams interfaces.
> Relax Kafka Streams API type constraints
> ----------------------------------------
>
> Key: KAFKA-4481
> URL: https://issues.apache.org/jira/browse/KAFKA-4481
> Project: Kafka
> Issue Type: Improvement
> 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}}s,
> {{StreamPartitioner}}s, {{KeyValueMapper}}s, {{ValueMapper}}s,
> {{ProcessorSupplier}}s, {{ValueJoiner}}s, etc.
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