public Builder<T> addStringComparator(final String fieldId,
                                                  final Function<T, String> extractor) {
                Objects.requireNonNull(fieldId);
                Objects.requireNonNull(extractor);
                comparatorMap.put(
                        fieldId,
                        new CaseAwareComparatorImpl<>(
                                CompareUtil.getNullSafeComparator(extractor),
                                CompareUtil.getNullSafeCaseInsensitiveComparator(extractor)));
                return this;
            }
Adds a pair of null-safe comparators for fieldId, using extractor to provide the value.
stroom-util-shared/src/main/java/stroom/util/shared/CompareUtil.java