Github user andralungu commented on the pull request:
https://github.com/apache/flink/pull/799#issuecomment-111761198
Yup, exactly! The use case was: I modified something in the edge data set,
called groupReduceOnNeighbors on the result and got NPE, exception that could
have been avoided with this check, just like was the case for the Degree NPE.
Making sure that the iterator is not null can save some people lots of
headaches, IMO :) And it doesn't hurt anyone who has a correct data set.
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