Github user greghogan commented on the issue:
https://github.com/apache/flink/pull/5047
This change doesn't work since `WordWithCount` needs to be a POJO which
requires a default or no-args constructor and non-final public attributes or
getters/setters. Since the type is not a POJO the `TypeExtractor` falls back to
`GenericType`, which cannot be used as a key type as noted in the error message:
~~~~
Tests run: 2, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 3.949 sec
<<< FAILURE! - in
org.apache.flink.streaming.test.socket.SocketWindowWordCountITCase
testJavaProgram(org.apache.flink.streaming.test.socket.SocketWindowWordCountITCase)
Time elapsed: 0.045 sec <<< ERROR!
org.apache.flink.api.common.InvalidProgramException: This type
(GenericType<org.apache.flink.streaming.examples.socket.SocketWindowWordCount.WordWithCount>)
cannot be used as key.
at
org.apache.flink.api.common.operators.Keys$ExpressionKeys.<init>(Keys.java:330)
at
org.apache.flink.streaming.api.datastream.DataStream.keyBy(DataStream.java:295)
at
org.apache.flink.streaming.examples.socket.SocketWindowWordCount.main(SocketWindowWordCount.java:79)
at
org.apache.flink.streaming.test.socket.SocketWindowWordCountITCase.testJavaProgram(SocketWindowWordCountITCase.java:65)
~~~~
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