    public static class MasteringHadoopMapSideJoinReduce extends Reducer<Text, LongWritable, Text, LongWritable>{
 
         private static LongWritable populationValue = new LongWritable(0);
         @Override
         protected void reduce(Text key, Iterable<LongWritable> values, Context context) throws IOException, InterruptedException {
 
              long populationTotal = 0;
 
              for(LongWritable population : values){
                 populationTotal += population.get();
              }
             populationValue.set(populationTotal);
             context.write(key, populationValue);
         }
     }
 
 
