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https://issues.apache.org/jira/browse/FLINK-6024?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15922699#comment-15922699
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Luke Hutchison commented on FLINK-6024:
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This was for the batch API (DataSet).
More generally, why should the requirements be different here for batch and
streaming APIs? I have come across other differences too between the APIs that
do not seem to be related to limitations of one of the processing modes.
Shouldn't more be shared between the two APIs?
> Need more fine-grained info for "InvalidProgramException: This type (...)
> cannot be used as key"
> ------------------------------------------------------------------------------------------------
>
> Key: FLINK-6024
> URL: https://issues.apache.org/jira/browse/FLINK-6024
> Project: Flink
> Issue Type: Improvement
> Components: Core
> Affects Versions: 1.2.0
> Reporter: Luke Hutchison
>
> I got this very confusing exception:
> {noformat}
> InvalidProgramException: This type (MyType) cannot be used as key
> {noformat}
> I dug through the code, and could not find what was causing this. The help
> text for type.isKeyType(), in Keys.java:329, right before the exception is
> thrown, says: "Checks whether this type can be used as a key. As a bare
> minimum, types have to be hashable and comparable to be keys." However, this
> didn't solve the problem.
> I discovered that in my case, the error was occurring because I added a new
> constructor to the type, and I didn't have a default constructor. This is
> probably quite a common thing to happen for POJOs, so the error message
> should give some detail saying that this is the problem.
> Other things that can cause this to fail, including that the class is not
> public, or the constructor is not public, or the key field is not public, or
> that the key field is not a serializable type, or the key is not Comparable,
> or the key is not hashable, should be given in the error message instead,
> depending on the actual cause of the problem.
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