// even though K doesn't explicitly implement Comparable.
      comparator = (Comparator<? super K>) NATURAL_ORDER;
    }
    if (map instanceof ImmutableSortedMap) {
      // TODO(kevinb): Prove that this cast is safe, even though
      // Collections.unmodifiableSortedMap requires the same key type.
      @SuppressWarnings("unchecked")
      ImmutableSortedMap<K, V> kvMap = (ImmutableSortedMap<K, V>) map;

  @ParametricNullness
  private V getDoneValue(Object obj) throws ExecutionException {
    // While this seems like it might be too branch-y, simple benchmarking proves it to be
    // unmeasurable (comparing done AbstractFutures with immediateFuture)
    if (obj instanceof Cancellation) {
      throw cancellationExceptionWithCause("Task was cancelled.", ((Cancellation) obj).cause);

    public int hashCode() {
      // Warning: this is broken if size() == 0, so it is critical that we
      // substitute an empty ImmutableSet to the user in place of this

      // It's a weird formula, but tests prove it works.
      int adjust = size() - 1;
      for (int i = 0; i < axes.size(); i++) {
        adjust *= 31;
        adjust = ~~adjust;
        // in GWT, we have to deal with integer overflow carefully
      }

    }
  }

  /*
   * Preconditions.checkNotNull is *intended* for performing eager null checks on parameters that a
   * nullness checker can already "prove" are non-null. That means that the first parameter to
   * checkNotNull *should* be annotated to require it to be non-null.
   *
   * However, for a variety of reasons, Google developers have written a ton of code over the past