public Builder<T> addLongComparator(final String fieldId,
                                                final ToLongFunction<T> extractor) {
                Objects.requireNonNull(fieldId);
                Objects.requireNonNull(extractor);
                comparatorMap.put(
                        fieldId,
                        new CaseAgnosticComparator<>(Comparator.comparingLong(extractor)));
                return this;
            }
Adds a single comparator for fieldId, using extractor to provide the value.
stroom-util-shared/src/main/java/stroom/util/shared/CompareUtil.java