@ParametricNullness
    abstract T result(int entryIndex);

    private void checkForConcurrentModification() {
      if (backingMap.modCount != expectedModCount) {
        throw new ConcurrentModificationException();
      }
    }

    @Override
    public boolean hasNext() {
      checkForConcurrentModification();
      return entryIndex >= 0;
    }

    @Override
    @ParametricNullness