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https://issues.apache.org/jira/browse/HADOOP-8619?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13424779#comment-13424779
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Radim Kolar commented on HADOOP-8619:
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Java serialization do not capture constructor arguments into output stream.
More detail about protocol is here
http://docs.oracle.com/javase/6/docs/platform/serialization/spec/protocol.html
If you create own constructor, java will not create default zero arg
constructor http://xahlee.info/java-a-day/constructer.html - section "Java
Technicality: Default Constructors"
> WritableComparator must implement no-arg constructor
> ----------------------------------------------------
>
> Key: HADOOP-8619
> URL: https://issues.apache.org/jira/browse/HADOOP-8619
> Project: Hadoop Common
> Issue Type: Improvement
> Components: io
> Affects Versions: 3.0.0
> Reporter: Radim Kolar
> Fix For: 0.23.0, 2.0.0-alpha, 3.0.0
>
> Attachments: writable-comparator.txt
>
>
> Because of reasons listed here:
> http://findbugs.sourceforge.net/bugDescriptions.html#SE_COMPARATOR_SHOULD_BE_SERIALIZABLE
> comparators should be serializable. To make deserialization work, it is
> required that all superclasses have no-arg constructor.
> http://findbugs.sourceforge.net/bugDescriptions.html#SE_NO_SUITABLE_CONSTRUCTOR
> Simply add no=arg constructor to WritableComparator.
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