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From Robert Metzger <rmetz...@apache.org>
Subject Re: Extract type information from SortedMap
Date Fri, 08 Jul 2016 16:25:07 GMT
Hi Yukun,

can you also post the code how you are invoking the GenericFlatMapper on
the mailing list?

The Java compiler is usually dropping the generic types during compilation
("type erasure"), that's why we can not infer the types.



On Fri, Jul 8, 2016 at 12:27 PM, Yukun Guo <gyk.net@gmail.com> wrote:

> Hi,
> When I run the code implementing a generic FlatMapFunction, Flink
> complained about InvalidTypesException:
>
> public class GenericFlatMapper<T> implements FlatMapFunction<SortedMap<T,
Long>, Tuple2<T, Long>> {
>     @Override
>     public void flatMap(SortedMap<T, Long> m, Collector<Tuple2<T, Long>>
out) throws Exception {
>         for (Map.Entry<T, Long> entry : m.entrySet()) {
>             out.collect(Tuple2.of(entry.getKey(), entry.getValue()));
>         }
>     }
> }
>
>
> *Exception in thread "main"
> org.apache.flink.api.common.functions.InvalidTypesException: The return
> type of function could not be determined automatically, due to type
> erasure. You can give type information hints by using the returns(...)
> method on the result of the transformation call, or by letting your
> function implement the 'ResultTypeQueryable' interface.*
>
> *...*
> *Caused by: org.apache.flink.api.common.functions.InvalidTypesException:
> Type of TypeVariable 'T' in 'class GenericFlatMapper' 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).*
>
> This puzzles me as Flink should be able to infer the type from arguments.
> I know returns(...) or other workarounds to give type hint, but they are
> kind of verbose. Any suggestions?
>
>

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