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https://issues.apache.org/jira/browse/FLINK-952?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14036025#comment-14036025
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Stephan Ewen commented on FLINK-952:
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That is true, there is actually a reason for that:
The data is treated as the type declared in the signature, because the type
analysis and serializer generation happens at the "pre-flight" time. So if you
have a MapFunction<A> and give it a B (subclass of A), it will treat the B as
an A at runtime.
> TypeExtractor requires the argument types of the UDF to be identical to the
> parameter types
> -------------------------------------------------------------------------------------------
>
> Key: FLINK-952
> URL: https://issues.apache.org/jira/browse/FLINK-952
> Project: Flink
> Issue Type: Bug
> Reporter: Till Rohrmann
> Assignee: Till Rohrmann
>
> The TypeExtractor checks for each operation whether the DataSet element types
> are valid arguments for the UDF. However, it checks for strict equality
> instead of a subtype relationship. Thus the following computation would not
> work even though it should semantically be correct.
> DataSet[B].map(new MapFunction[A,A](){ A map(A x)}) with B being a sub class
> of A.
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