public boolean iterate(String[] o) {
      if (o != null) {
        StringBuilder sb = new StringBuilder();
        for (String element : o) {
          sb.append(element);
        }
        data.add(sb.toString());
      }
      return true;
    }
Iterate through one row of original data.

This UDF accepts arbitrary number of String arguments, so we use
String[]. If it only accepts a single String, then we should use a single
String argument.

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