* bufferSize ≥ k, then we can ignore any elements greater than this value.
   */
  private @Nullable T threshold;

  @SuppressWarnings("unchecked") // TODO(cpovirk): Consider storing Object[] instead of T[].
  private TopKSelector(Comparator<? super T> comparator, int k) {
    this.comparator = checkNotNull(comparator, "comparator");
    this.k = k;

    Some tests need reflective access to the internals of these packages. It is only the
    tests themselves and not the code being tested that needs that access, though there's no
    obvious way to ensure that.

    We could consider arranging things so that only the tests we know need this would get
    the add-opens. Right now that doesn't seem worth the effort, though.
    -->
    <test.add.opens>
      --add-opens java.base/java.lang=ALL-UNNAMED

    Some tests need reflective access to the internals of these packages. It is only the
    tests themselves and not the code being tested that needs that access, though there's no
    obvious way to ensure that.

    We could consider arranging things so that only the tests we know need this would get
    the add-opens. Right now that doesn't seem worth the effort, though.
    -->
    <test.add.opens>
      --add-opens java.base/java.lang=ALL-UNNAMED