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() {