public boolean merge(ArrayList<String> o) {
      if (o != null) {
        data.addAll(o);
      }
      return true;
    }
Merge with a partial aggregation.

This function should always have a single argument which has the same
type as the return value of terminatePartial().

This function should always return true.
contrib/src/java/org/apache/hadoop/hive/contrib/udaf/example/UDAFExampleGroupConcat.java