Github user twalthr commented on the pull request:
https://github.com/apache/incubator-flink/pull/203#issuecomment-64385776
All generic type serializers are mutable types, but if we only have Object
as type information we cannot create instances of it. Additionally, e.g. in
case of a Tuple we may have the information that we have a Tuple2 but without
generic information. The UnkownTypeSerializer supports both cases.
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