public void addStatisticField(final StatisticField statisticField) {
        if (fields == null) {
            fields = new ArrayList<>();
        }

        // prevent duplicates
        if (!fields.contains(statisticField)) {
            fields.add(statisticField);
            sortFields();
            fieldPositionMap = null;
        }
    }
Held in a set to prevent duplicates.
<p>
XMLTransient to force JAXB to use the setter
stroom-core-shared/src/main/java/stroom/statistics/impl/sql/shared/StatisticsDataSourceData.java