Github user NicoK commented on the issue:
https://github.com/apache/flink/pull/3816
Actually, `KeyedStream.aggregate` is protected and thus not accessible.
You may, however, access
`WindowedStream#aggregate(AggregateFunction<T,ACC,R>) ` and
`AllWindowedStream#aggregate(AggregateFunction<T,ACC,R>) ` for windowed streams
and use a stateful map or flatmap for the rest.
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