code: @Override
      public V replace(K key, V value) {
          int hash = hash(key);
          if (value == null) {
            throw new NullPointerException();
          }
          Segment<K,V> s = segmentForHash(hash);
          return s == null ? null : s.replace(key, hash, value);
      }
docs: {@inheritDoc}
  
  @return the previous value associated with the specified key,
  or <tt>null</tt> if there was no mapping for the key
  @throws NullPointerException if the specified key or value is null