code: @SuppressWarnings("unchecked")
      static final <K,V> Segment<K,V> segmentAt(Segment<K,V>[] ss, int j) {
          long u = (j << SSHIFT) + SBASE;
          return ss == null ? null :
              (Segment<K,V>) UNSAFE.getObjectVolatile(ss, u);
      }
docs: Gets the jth element of given segment array (if nonnull) with
  volatile element access semantics via Unsafe. (The null check
  can trigger harmlessly only during deserialization.) Note:
  because each element of segments array is set only once (using
  fully ordered writes), some performance-sensitive methods rely
  on this method only as a recheck upon null reads.