Sorting of arraylist of multiple arraylists

Question

I have an arraylist of multiple arraylists like-

ArrayList<ArrayList<String>> al1=new ArrayList<ArrayList<String>>();

the arraylist contains the elements:

[[Total for all Journals, IOP, IOPscience, , , , , 86, 16, 70, 17, 8, 14, 6, 17, 19, 5], [2D Materials, IOP, IOPscience, 10.1088/issn.2053-1583, 2053-1583, , 2053-1583, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [Acta Physica Sinica (Overseas Edition), IOP, IOPscience, 10.1088/issn.1004-423X, 1004-423X, 1004-423X, , 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [Advances in Natural Sciences: Nanoscience and Nanotechnology, IOP, IOPscience, 10.1088/issn.2043-6262, 2043-6262, , 2043-6262, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], [Applied Physics Express, IOP, IOPscience, , 1882-0786, 1882-0778, 1882-0786, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]]

Now i want to sort the main arraylist. Main arraylist contains 5 inner arraylists.Now the sorting must be like the 7th element of every inner arraylist is compared which is integer and inner arraylists are arranged accorting to the value of their 7th element.

Collections.sort(al1, new Comparator<ArrayList<String>>() {
  @Override public int compare(ArrayList<String> o1, ArrayList<String> o2) {
    return o1.get(7).compareTo(o2.get(7));
  }
});

Answer 1

Indexes in Java begin at 0. The 7th element has the index 6. Moreover, you have to convert the String to int in order to be properly compared.

Try this :

Comparator<ArrayList<String>> cmp = new Comparator<ArrayList<String>>()
{
    public int compare(ArrayList<String> a1, ArrayList<String> a2)
    {
        // TODO check for null value
        return new Integer(a1.get(6)).compareTo(new Integer(a2.get(6));
    }
};

Collections.sort(yourList, cmp);