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

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

Xavier Léauté created KAFKA-4481:
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             Summary: 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


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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