@Override
    public void parentEnd() {
      parent.set(index, new ArrayWritable(
          Writable.class, list.toArray(new Writable[list.size()])));
    }
Stands in for a HiveGroupConverter and accumulates multiple values as an
ArrayWritable.
ql/src/java/org/apache/hadoop/hive/ql/io/parquet/convert/Repeated.java