Code refine of WordWithCount

[harborl]

Only because of the thread-safe and coding convention consideration, please check if it's OK with adding final decoration?

[greghogan]

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)

[greghogan]

Do we still want to make this change since the PR is now only adding a single comment?

[fhueske]

IMO, the added comment might rather confuse readers than help them.

[harborl]

Yes, after a few learning, it turns out, to me, that this project uses POJO as a sort of reasonable style of parameters(it has nothing with the thread-safe consideration here, although value object corresponds with it). Excuse me for the interruption and my poor English. I think I am excited about learning further and it is a great project truly, as far as I can tell.