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https://issues.apache.org/jira/browse/MAPREDUCE-4507?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13427993#comment-13427993
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Harsh J commented on MAPREDUCE-4507:
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The {{map()}} function is to be properly overriden when using the new API.
Using @Override annotations on map() (and for that matter, reduce() too) will
help you catch your mistake here.
As discussed on http://search-hadoop.com/m/hSxqz1vsQPc, this is a user-side
mistake, but in no way a bug. See
http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/mapreduce/Mapper.html#map(KEYIN,%20VALUEIN,%20org.apache.hadoop.mapreduce.Mapper.Context).
We can add a javadoc improvement (and a tutorial improvement) to state the
right answer to avoiding this issue: Always use @Override annotations when
overriding methods. (Any IDE today provides support for this).
> IdentityMapper is being triggered when the type of the Input Key at class
> level and method level has a conflict
> ---------------------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-4507
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-4507
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv1
> Affects Versions: 1.0.3
> Environment: linux ubuntu
> Reporter: Bejoy KS
>
> If we use the default InputFormat (TextInputFormat) but specify the Key type
> in mapper as IntWritable instead of Long Writable. The framework is supposed
> throw a class cast exception.Such an exception is thrown only if the key
> types at class level and method level are the same (IntWritable). But if we
> provide the Input key type as IntWritable on the class level but LongWritable
> on the method level (map method), instead of throwing a compile time error,
> the code compliles fine . In addition to it on execution the framework
> triggers Identity Mapper instead of the custom mapper provided with the
> configuration. In this case the 'mapreduce.map.class' in job.xml shows mapper
> as Custom Mapper itself , it should show IdentityMapper in cases where
> IdentityMapper is triggered to avoid confusion and easy debugging.
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