MapReduce Features - Hadoop: The Definitive Guide - page 309

To summarize, there is a recipe here to get the effect of sorting by value:
▪ Make the key a composite of the natural key and the natural value.
▪ The sort comparator should order by the composite key (i.e., the natural key and
natural value).
▪ The partitioner and grouping comparator for the composite key should consider
only the natural key for partitioning and grouping.
Java code
Putting this all together results in the code in Example 9-6 . This program uses the plain-
text input again.
Example 9-6. Application to find the maximum temperature by sorting temperatures in the
key
public class MaxTemperatureUsingSecondarySort
extends Configured implements Tool {
static class MaxTemperatureMapper
extends Mapper < LongWritable , Text , IntPair , NullWritable > {
private NcdcRecordParser parser = new NcdcRecordParser ();
@Override
protected void map ( LongWritable key , Text value ,
Context context ) throws IOException , InterruptedException {
parser . parse ( value );
if ( parser . isValidTemperature ()) {
context . write ( new IntPair ( parser . getYearInt (),
parser . getAirTemperature ()), NullWritable . get ());
}
}
}
static class MaxTemperatureReducer
extends Reducer < IntPair , NullWritable , IntPair , NullWritable > {
@Override
protected void reduce ( IntPair key , Iterable < NullWritable > values ,
Context context ) throws IOException , InterruptedException {
context . write ( key , NullWritable . get ());
}
}