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https://issues.apache.org/jira/browse/KAFKA-13641?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17486584#comment-17486584
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Matthias J. Sax commented on KAFKA-13641:
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The Java API faces the same issue – it's hard from a backward compatibility POV
to change the signature.
Also the input `null`-value question is tricky: atm, to avoid ambiguity, the
runtime drops all join input records with `null`-value (also with `null`-key)
as malformed. Thus, if you get a `null` you always know that it's a left/outer
join call with no matching join record from the other side.
> Kafka Streams Scala: Add `Option` to `ValueJoiner` parameters
> -------------------------------------------------------------
>
> Key: KAFKA-13641
> URL: https://issues.apache.org/jira/browse/KAFKA-13641
> Project: Kafka
> Issue Type: Improvement
> Components: streams
> Reporter: Mohammad Yousuf Minhaj Zia
> Priority: Minor
>
> Since `ValueJoiner` right parameter in `leftJoins`, `outerJoins` can be
> nullable, I am wondering if can wrap them around Scala `Option`.
> However, there is also the concern that the left hand side value can be null
> in the case of tombstone messages, in which the `Option` semantics can be
> misleading. I still feel this could be a useful feature in reducing the
> number of `NullPointerExceptions`.
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