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https://issues.apache.org/jira/browse/MAPREDUCE-4827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13506032#comment-13506032
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Doug Cutting commented on MAPREDUCE-4827:
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Integer#hashCode() is documented to be the integer value.
http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html#hashCode()
Similarly, the hashCode() implelementations for String, Double, Float, Long,
etc. are specified and do not change from one JVM to another.
Also, I didn't veto this change. I just observed that it was not
back-compatible. That should be taken into account if/when it is committed.
> Increase hash quality of HashPartitioner
> ----------------------------------------
>
> Key: MAPREDUCE-4827
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4827
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Reporter: Radim Kolar
> Attachments: betterhash1.txt
>
>
> hash partitioner is using object.hashCode() for splitting keys into
> partitions. This results in bad distributions because hashCode() quality is
> poor.
> These hashCode() functions are sometimes written by hand (very poor quality)
> and sometimes generated from by commons lang code (poor quality). Applying
> some transformation on top of hashCode() provides better distribution.
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