code: @Override
      public ChronicleMapBuilder<K, V> averageKey(K averageKey) {
          Objects.requireNonNull(averageKey);
          checkSizeIsStaticallyKnown(keyBuilder, "Key");
          this.averageKey = averageKey;
          sampleKey = null;
          averageKeySize = UNDEFINED_DOUBLE_CONFIG;
          return this;
      }
docs: {@inheritDoc}
  
  @param averageKey the average (by footprint in serialized form) key, is going to be put
  into the hash containers, created by this builder
  @throws NullPointerException {@inheritDoc}
  @see #averageKeySize(double)
  @see #constantKeySizeBySample(Object)
  @see #averageValue(Object)
  @see #actualChunkSize(int)