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https://issues.apache.org/jira/browse/MAPREDUCE-6837?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Daniel Templeton reassigned MAPREDUCE-6837:
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Assignee: Peter Cseh
> Add an equivalent to Crunch's Pair class
> ----------------------------------------
>
> Key: MAPREDUCE-6837
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-6837
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: mrv2
> Reporter: Daniel Templeton
> Assignee: Peter Cseh
> Labels: newbie++
>
> Crunch has this great {{Pair}} class
> (https://crunch.apache.org/apidocs/0.14.0/org/apache/crunch/Pair.html) that
> saves one from constantly implementing composite writables. It seems silly
> that we still don't have an equivalent in MR.
> I would like to see a new class with the following API:
> {code}
> package org.apache.hadoop.io;
> public class CompositeWritable<P extends WritableComparable, S extends
> WritableComparable> implements WritableComparable<CompositeWritable> {
> public CompositeWritable(P primary, S secondary);
> public P getPrimary();
> public void setPrimary(P primary);
> public S getSecondary();
> public void setSecondary(S secondary);
> // Return true if both primaries and both secondaries are equal
> public boolean equals(CompositeWritable o);
> // Return the primary's hash code
> public long hashCode();
> // Sort first by primary and then by secondary
> public int compareTo(CompositeWritable o);
> public void readFields(DataInput in);
> public void write(DataOutput out);
> }
> {code}
> With such a class, implementing a secondary sort would mean just implementing
> a custom grouping comparator. That comparator could also be implemented as
> part of this JIRA:
> {code}
> package org.apache.hadoop.io;
> public class CompositeGroupingComparator extends WritableComparator {
> ...
> }
> {code}
> Or some such.
> Crunch also provides {{Tuple3}}, {{Tuple4}}, and {{TupleN}} classes, but I
> don't think we need to add equivalents. If someone really wants that
> capability, they can nest composite keys.
> Don't forget to add unit tests!
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