code: @SuppressWarnings("unchecked")
      @NonNull
      public static Set<String> nonNullSet(@Nullable String[] items) {
          return items == null || items.length == 0
                  ? Collections.<String>emptySet()
                  : new HashSet<String>(asList(items));
      }
docs: Converts array of something to non-null set.
  
  @param items source items to convert, can be {@code null}.
  @return non-null, set of something.