Merging two sorted lists into first one

mergeList.java

    static void merge(List<Long> first, List<Long> second) {
        ListIterator<Long> firstIter = first.listIterator();
        ListIterator<Long> secondIter = second.listIterator();

        Long a = getNext(firstIter);
        Long b = getNext(secondIter);

        while (a != null && b != null) {
            if (a > b) {
                firstIter.previous();
                firstIter.add(b);
                firstIter.next();
                b = getNext(secondIter);
            } else {
                a = getNext(firstIter);
            }
        }

        while (b != null) {
            firstIter.add(b);
            b = getNext(secondIter);
        }
    }

    @Nullable
    private static Long getNext(ListIterator<Long> iter) {
        return iter.hasNext() ? iter.next() : null;
    }