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https://issues.apache.org/jira/browse/HADOOP-1230?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12616561#action_12616561
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Doug Cutting commented on HADOOP-1230:
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> If you want to have your own MapContext implementation you'd extend
> MapContextWrapper [ ...]
Sorry if I'm being thick, but I don't understand the point of having the
MapContext interface if no one is to implement it directly. We cannot remove
methods from it without breaking all callers. We could add methods if all
implementations always implement an abstract base class ( MapContextWrapper)
instead, but how can we guarantee that folks don't directly implement the
interface?
> you cannot have a class extending 2 abstract classes, you can do that with
> interfaces
This feature is over-rated. One can easily define two classes that share state.
> convenience methods in your abstract class become part of the contract when
> they should not.
Why should they not? What problems does this cause?
> 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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