code: public <K, V> ConcurrentMap<K, V> makeMap() {
      if (!useCustomMap) {
        return new ConcurrentHashMap<>(getInitialCapacity(), 0.75f, getConcurrencyLevel());
      }
      return MapMakerInternalMap.create(this);
    }
docs: Builds a thread-safe map. This method does not alter the state of this {@code MapMaker}
  instance, so it can be invoked again to create multiple independent maps.
  
  <p>The bulk operations {@code putAll}, {@code equals}, and {@code clear} are not guaranteed to
  be performed atomically on the returned map. Additionally, {@code size} and {@code
  containsValue} are implemented as bulk read operations, and thus may fail to observe concurrent
  writes.
  
  @return a serializable concurrent map having the requested features