@Override
    public long getLength() {
      return 0;
    }
Input format that creates a configurable number of map tasks each provided with a single row of
NullWritables. This can be useful when trying to write mappers which don't have any real input
(eg when the mapper is simply producing random data as output)
hbase-mapreduce/src/test/java/org/apache/hadoop/hbase/mapreduce/NMapInputFormat.java