buffer[j] = tmp;
  }

  /*
   * While the current implementation returns `this`, that's not something we mean to guarantee.
   * Anyway, the purpose of this method is to implement a BinaryOperator combiner for a Collector,
   * so its return value will get used naturally.
   */
  @SuppressWarnings("CanIgnoreReturnValueSuggester")
  TopKSelector<T> combine(TopKSelector<T> other) {

          .doubleValue();

  /**
   * Returns a stream of a million primitive doubles. The stream is parallel, which should cause
   * {@code collect} calls to run in multithreaded mode, so testing the combiner as well as the
   * supplier and accumulator.
   */
  static DoubleStream megaPrimitiveDoubleStream() {
    return DoubleStream.iterate(0.0, x -> x + 1.0).limit(MEGA_STREAM_COUNT).parallel();
  }