}
  }

  AggregateFutureState(int remainingFutures) {
    this.remaining = remainingFutures;
  }

  final Set<Throwable> getOrInitSeenExceptions() {
    /*
     * The initialization of seenExceptions has to be more complicated than we'd like. The simple
     * approach would be for each caller CAS it from null to a Set populated with its exception. But

  private @Nullable Set<Throwable> seenExceptions = null;
  private int remaining;

  AggregateFutureState(int remainingFutures) {
    this.remaining = remainingFutures;
  }

  final Set<Throwable> getOrInitSeenExceptions() {
    if (seenExceptions == null) {
      seenExceptions = newHashSet();
      addInitialException(seenExceptions);
    }
    return seenExceptions;
  }