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https://issues.apache.org/jira/browse/HADOOP-1230?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12616351#action_12616351
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Alejandro Abdelnur commented on HADOOP-1230:
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Maybe I have not explained things clearly.
You could do custom {{MapContext}} even it it is an interface. You would have a
{{MapContext}} interface and a {{MapContextWrapper}} class that implements
{{MapContext}} and delegates all its methods to the {{MapContext}} instance
provided to its constructor. If you want to have your own {{MapContext}}
implementation you'd extend {{MapContextWrapper}} in a {{MyMapContext}} class
and override the necessary methods. If Hadoop later adds a new method to the
Interface {{MapContext}} it will implemented also in {{MapContextWrapper}},
thus your {{MyMapContext}} will continue to work without breaking. This is the
pattern the Servlet API uses with the request/response interfaces and their
corresponding wrapper classes.
Using abstract classes to define contracts create some problems in certain
situations:
* you cannot have a class extending 2 abstract classes, you can do that with
interfaces
* convenience methods in your abstract class become part of the contract when
they should not.
> Replace parameters with context objects in Mapper, Reducer, Partitioner,
> InputFormat, and OutputFormat classes
> --------------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-1230
> URL: https://issues.apache.org/jira/browse/HADOOP-1230
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Reporter: Owen O'Malley
> Assignee: Owen O'Malley
> Attachments: context-objs-2.patch, context-objs-3.patch,
> context-objs.patch
>
>
> This is a big change, but it will future-proof our API's. To maintain
> backwards compatibility, I'd suggest that we move over to a new package name
> (org.apache.hadoop.mapreduce) and deprecate the old interfaces and package.
> Basically, it will replace:
> package org.apache.hadoop.mapred;
> public interface Mapper extends JobConfigurable, Closeable {
> void map(WritableComparable key, Writable value, OutputCollector output,
> Reporter reporter) throws IOException;
> }
> with:
> package org.apache.hadoop.mapreduce;
> public interface Mapper extends Closable {
> void map(MapContext context) throws IOException;
> }
> where MapContext has the methods like getKey(), getValue(), collect(Key,
> Value), progress(), etc.
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