code: public V fill(K key, V value) {
      return segmentFor(key).fill(key, value);
    }
docs: See {@link OffHeapHashMap#fill(Object, Object)} for a detailed description.
  
  @param key key with which the specified value is to be associated
  @param value value to be associated with the specified key
  @return the previous value associated with <tt>key</tt>, or
  <tt>null</tt> if there was no mapping for <tt>key</tt>
  (irrespective of whether the value was successfully installed).