public void reduce(Key key, Iterable<LongWritable> values, Context context)
      throws IOException, InterruptedException {
      long sum = 0;
      for (LongWritable val : values) {
        sum += val.get();
      }
      result.set(sum);
      context.write(key, result);
    }
Maps the data.
@param row     The current table row key.
@param values  The columns.
@param context The current context.
@throws IOException When something is broken with the data.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/CellCounter.java