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https://issues.apache.org/jira/browse/FLINK-17083?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17083040#comment-17083040
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Aljoscha Krettek commented on FLINK-17083:
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Yes, I think that restriction makes sense.
> Restrict users of accumulators not to return null in Accumulator#getLocalValue
> ------------------------------------------------------------------------------
>
> Key: FLINK-17083
> URL: https://issues.apache.org/jira/browse/FLINK-17083
> Project: Flink
> Issue Type: Improvement
> Components: API / Core
> Affects Versions: 1.11.0
> Reporter: Caizhi Weng
> Priority: Major
>
> [This discussion|https://issues.apache.org/jira/browse/FLINK-13880] raises a
> problem that, if a user returns a null value in
> {{Accumulator#getLocalValue}}, he will be notified that the {{failureCause}}
> should not be null when fetching the accumulator result, which seems to be
> really wired.
> The problem is that we're not explicitly restricting the users not to return
> null values in {{Accumulator#getLocalValue}}.
> [~aljoscha] is it legal to return null values? If not, we should explicitly
> throw a related exception to the users instead of giving the users a somewhat
> unrelated exception.
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