code: public static MultimapBuilderWithKeys<Object> hashKeys(final int expectedKeys) {
      checkNonnegative(expectedKeys, "expectedKeys");
      return new MultimapBuilderWithKeys<Object>() {
        @Override
        <K, V> Map<K, Collection<V>> createMap() {
          return Platform.newHashMapWithExpectedSize(expectedKeys);
        }
      };
    }
docs: Uses a hash table to map keys to value collections, initialized to expect the specified number
  of keys.
  
  @throws IllegalArgumentException if {@code expectedKeys < 0}