private enum State {
    /** We have computed the next element and haven't returned it yet. */
    READY,

    /** We haven't yet computed or have already returned the element. */
    NOT_READY,

    /** We have reached the end of the data and are finished. */
    DONE,

    /** We've suffered an exception and are kaput. */
    FAILED,
  }

  @CheckForNull private T next;

  /**

    MinMaxPriorityQueue<Integer> mmHeap = MinMaxPriorityQueue.create();
    mmHeap.addAll(Lists.newArrayList(1, 2));
    Iterator<Integer> it = mmHeap.iterator();
    assertTrue("Iterator has reached end prematurely", it.hasNext());
    it.next();
    it.next();
    try {
      it.next();
      fail("No exception thrown when iterating past end of heap");
    } catch (NoSuchElementException expected) {
    }

              <validateOnly>true</validateOnly>
              <sourceLevel>1.8</sourceLevel>
              <!--
                To avoid "java.io.IOException: User limit of inotify watches reached" under JDK10.
                Presumably we don't need inotify, since we don't intend to change files during the
                build?
                -->