* empty at any point. (The sum ensures accuracy up through at least 1<<31 per-segment
     * modifications before recheck.) Method containsValue() uses similar constructions for
     * stability checks.
     */
    long sum = 0L;
    Segment<K, V>[] segments = this.segments;
    for (Segment<K, V> segment : segments) {
      if (segment.count != 0) {
        return false;
      }