FrequencyCountingReducer.java

package uk.bl.wa.hadoop.outlinks;

import java.io.IOException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;

import uk.bl.wa.hadoop.mapreduce.MutableInt;

@SuppressWarnings( { "deprecation" } )
/**
 * 
 */
public class FrequencyCountingReducer extends MapReduceBase implements Reducer<Text, Text, Text, Text> {

	@Override
	public void reduce( Text key, Iterator<Text> iterator, OutputCollector<Text, Text> output, Reporter reporter ) throws IOException {

		// Use a simple set to collect only distinct results for this key:
		Map<String, MutableInt> matches = new HashMap<String, MutableInt>();
		while( iterator.hasNext() ) {
			String m = iterator.next().toString();
			// Get or set up the counter:
			MutableInt value = matches.get( m );
			if( value == null ) {
				value = new MutableInt();
				matches.put( m, value );
			}
			// Increment the counter for this match:
			value.inc();
		}

		// Loop through and collect all distinct matches:
		for( String match : matches.keySet() ) {
			// This ignores the count:
			// output.collect( key, new Text( match ) );
			// This also outputs the count:
			output.collect( key, new Text( match + "\t" + matches.get( match ).get() ) );
		}

	}
}