code: @SuppressWarnings("unchecked")
      @NonNull
      public static Set<String> nonNullSet(@NonNull String firstItem, @Nullable String[] otherItems) {
          final HashSet<String> set = new HashSet<String>();
          set.add(firstItem);
          if (otherItems != null) {
              set.addAll(asList(otherItems));
          }
          return set;
      }
docs: Converts array of something to non-null set.
  
  @param firstItem  the first required source item.
  @param otherItems other source items to convert, can be {@code null}.
  @return non-null, set of something.