public Builder<K, V> comparator(Comparator<JsonField<K, V>> comparator) {
            this.comparator = Objects.requireNonNull(comparator, "comparator");
            return this;
        }
@param nameMapper nameMapper does not guarantee uniqueness.
commons-server/src/main/java/com/navercorp/pinpoint/common/server/util/json/JsonFields.java