src/java.base/share/classes/java/time/format/DateTimeFormatter.java

*** 1683,1692 ****
--- 1683,1694 ----
       * @return a formatter based on this formatter with the requested resolver style, not null
       */
      public DateTimeFormatter withResolverFields(TemporalField... resolverFields) {
          Set<TemporalField> fields = null;
          if (resolverFields != null) {
+             // An unmodifiable set wrapper is used instead of Set.of because resolverFields may
+             // contain null and Set.of is null-hostile.
              fields = Collections.unmodifiableSet(new HashSet<>(Arrays.asList(resolverFields)));
          }
          if (Objects.equals(this.resolverFields, fields)) {
              return this;
          }