private class Itr implements Iterator<E> {
    /** Index of element to be returned by next, or a negative number if no such. */
    private int nextIndex;

    /**
     * nextItem holds on to item fields because once we claim that an element exists in hasNext(),
     * we must return it in the following next() call even if it was in the process of being removed
     * when hasNext() was called.
     */
    private @Nullable E nextItem;

  private class Itr implements Iterator<E> {
    /** Index of element to be returned by next, or a negative number if no such. */
    private int nextIndex;

    /**
     * nextItem holds on to item fields because once we claim that an element exists in hasNext(),
     * we must return it in the following next() call even if it was in the process of being removed
     * when hasNext() was called.
     */
    private @Nullable E nextItem;