public List<CustomRollUpMask> getMasks() {
            final List<CustomRollUpMask> list = new ArrayList<>();
            for (final MaskHolder holder : this) {
                list.add(holder.getMask());
            }
            return list;
        }
Extension of ArrayList that adds an ID value to each
{@link CustomRollUpMask} object added to it
stroom-statistics/stroom-statistics-client/src/main/java/stroom/statistics/impl/sql/client/presenter/StatisticsCustomMaskListPresenter.java