public static <T> void swap(List<T> items, int index1_, int index2_) {
        if (index1_ == index2_) 
            return;
                    
        T item1 = items.get(index1_);
        T item2 = items.get(index2_);
        if (item1 != null)
            items.set(index2_, item1);
        if (item2 != null)
            items.set(index1_, item2);
    }