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https://issues.apache.org/jira/browse/FLINK-4275?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15404186#comment-15404186
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Aljoscha Krettek commented on FLINK-4275:
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This was changed to make it possible in the {{WindowOperator}} to check whether
a state is empty.
> Generic Folding, Reducing and List states behave differently from other state
> backends
> --------------------------------------------------------------------------------------
>
> Key: FLINK-4275
> URL: https://issues.apache.org/jira/browse/FLINK-4275
> Project: Flink
> Issue Type: Bug
> Components: State Backends, Checkpointing, Streaming
> Reporter: Gyula Fora
> Priority: Critical
>
> In
> https://github.com/apache/flink/commit/12bf7c1a0b81d199085fe874c64763c51a93b3bf
> the expected behaviour of Folding/Reducing/List states have been changed to
> return null instead of empty collections/default values.
> This was adapted for the included state backends (Memory, FS, Rocks) but not
> for the Generic state wrappers. As there are no tests for the Generic backend
> using the StateBackendTestBase this didnt show.
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