public class MyReducer extends Reducer<Text, Text, Text, Text> {

    @Override
    public void reduce(Text key, Interable<Text> values, Context context)
        throws IOException, InterruptedException {
        Text outputKey = new Text();
        Text outputValue = new Text();
        for (Text value: values) {
            // ...
        }
        context.write(outputKey, outputValue);
    }
}