[jira] [Commented] (FLINK-7010) Lamdba expression in flatMap throws InvalidTypesException in DataSet

["Dawid Wysakowicz (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-7010?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16064349#comment-16064349 ] 

Dawid Wysakowicz commented on FLINK-7010:
-----------------------------------------

Lambdas can be used only with Eclipse JDT compiler. See this section in docs: [docs|https://ci.apache.org/projects/flink/flink-docs-release-1.3/dev/java8.html#compiler-limitations]

> Lamdba expression in flatMap throws InvalidTypesException in DataSet
> --------------------------------------------------------------------
>
>                 Key: FLINK-7010
>                 URL: https://issues.apache.org/jira/browse/FLINK-7010
>             Project: Flink
>          Issue Type: Bug
>            Reporter: Fang Yong
>
> When I create an example and use lambda in flatMap as follows
> {noformat}
>     ExecutionEnvironment env = ExecutionEnvironment.getExecutionEnvironment();
>     DataSet<String> source = env.fromCollection(
>         Lists.newArrayList("hello", "flink", "test", "flat", "map", "lambda"));
>     DataSet<Tuple2<Integer, String>> tupled = source.flatMap((word, out) -> {
>       int length = word.length();
>       out.collect(Tuple2.of(length, word));
>     });
>     try {
>       tupled.print();
>     } catch (Exception e) {
>       throw new RuntimeException(e);
>     }
> {noformat}
> InvalidTypesException was throwed and the exception stack is as follows:
> {noformat}
> Caused by: org.apache.flink.api.common.functions.InvalidTypesException: The return type of function 'testFlatMap(FlatMapTest.java:20)' 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.
> 	at org.apache.flink.api.java.DataSet.getType(DataSet.java:178)
> 	at org.apache.flink.api.java.DataSet.collect(DataSet.java:407)
> 	at org.apache.flink.api.java.DataSet.print(DataSet.java:1605)
> Caused by: org.apache.flink.api.common.functions.InvalidTypesException: The generic type parameters of 'Collector' are missing. 
> It seems that your compiler has not stored them into the .class file. 
> Currently, only the Eclipse JDT compiler preserves the type information necessary to use the lambdas feature type-safely. 
> See the documentation for more information about how to compile jobs containing lambda expressions.
> 	at org.apache.flink.api.java.typeutils.TypeExtractor.validateLambdaGenericParameter(TypeExtractor.java:1653)
> 	at org.apache.flink.api.java.typeutils.TypeExtractor.validateLambdaGenericParameters(TypeExtractor.java:1639)
> 	at org.apache.flink.api.java.typeutils.TypeExtractor.getUnaryOperatorReturnType(TypeExtractor.java:573)
> 	at org.apache.flink.api.java.typeutils.TypeExtractor.getFlatMapReturnTypes(TypeExtractor.java:188)
> 	at org.apache.flink.api.java.DataSet.flatMap(DataSet.java:266)
> {noformat}
> The 20th line code is
> {noformat}
>  DataSet<Tuple2<Integer, String>> tupled = source.flatMap((word, out) -> { 
> {noformat}
> When I use FlatMapFunction instead of lambda, it will be all right



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