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https://issues.apache.org/jira/browse/FLINK-6114?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15932576#comment-15932576
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Greg Hogan commented on FLINK-6114:
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[~lukehutch] Flink most certainly allows {{groupBy}}s and {{join}}s on generic
fields, for example any of the Gelly algorithms in
{{org.apache.flink.graph.library}}.
The Flink type system must deduce the type of all fields in order to create the
serializer. Types are defined on sources then tracked through each operation.
If a type cannot be deduced from inputs and outputs then it must be explicitly
defined. We can look at this if you are able to post more code.
> Type checking fails with generics, even when concrete type of field is not
> needed
> ---------------------------------------------------------------------------------
>
> Key: FLINK-6114
> URL: https://issues.apache.org/jira/browse/FLINK-6114
> Project: Flink
> Issue Type: Bug
> Components: Core
> Affects Versions: 1.2.0
> Reporter: Luke Hutchison
>
> The Flink type checker does not allow generic types to be used in any field
> of a tuple when a join is being executed, even if the generic is not in a
> field that is involved in the join.
> I have a type Tuple3<String, K, Float>, which contains a generic type
> parameter K. I am joining using .where(0).equalTo(0). The type of field 0 is
> well-defined as String. However, this gives me the following error:
> {noformat}
> Exception in thread "main"
> org.apache.flink.api.common.functions.InvalidTypesException: Type of
> TypeVariable 'K' in 'public static org.apache.flink.api.java.DataSet
> mypkg.MyClass.method(params)' could not be determined. This is most likely a
> type erasure problem. The type extraction currently supports types with
> generic variables only in cases where all variables in the return type can be
> deduced from the input type(s).
> at
> org.apache.flink.api.java.typeutils.TypeExtractor.createSubTypesInfo(TypeExtractor.java:989)
> {noformat}
> The code compiles fine, however -- the static type system is able to
> correctly resolve the types in the surrounding code.
> Really only the fields that are affected by joins (or groupBy, aggregation
> etc.) should be checked for concrete types in this way.
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