return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
  }

  /**
   * This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
   * referenced by emptyModifiableIterator().
   */
  private enum EmptyModifiableIterator implements Iterator<Object> {
    INSTANCE;

    @Override
    public boolean hasNext() {
      return false;
    }

    @Override
    public Object next() {

    Iterator<V> valueIterator;

    Itr() {
      keyIterator = map.entrySet().iterator();
      key = null;
      collection = null;
      valueIterator = Iterators.emptyModifiableIterator();
    }

    abstract T output(@ParametricNullness K key, @ParametricNullness V value);

    @Override
    public boolean hasNext() {

    assertThrows(UnsupportedOperationException.class, () -> iterator.add("a"));
  }

  public void testEmptyModifiableIterator() {
    Iterator<String> iterator = Iterators.emptyModifiableIterator();
    assertFalse(iterator.hasNext());
    assertThrows(NoSuchElementException.class, () -> iterator.next());
    assertThrows(IllegalStateException.class, () -> iterator.remove());
  }