getting an error here in the 6th & 8th line

Question

1 public static List<Integer> compareTriplets(List<Integer> a, List<Integer> b) {
2        List<Integer> rtnarr = new ArrayList<>();
3        rtnarr.set(0,0);rtnarr.set(1,0);
4        for (int i=0; i<3; i++) {
5            if(a.get(i)>b.get(i))
6                rtnarr.set(0,(++(rtnarr.get(0))));
7            if(b.get(i)>a.get(i))
8                rtnarr.set(1,(++(rtnarr.get(1))));
9        }
10        return rtnarr;
11 }

I was expecting a normal result, but it showed error,

rtnarr.set(0,(++(rtnarr.get(0))));
                                           ^
  required: variable
  found:    value
Solution.java:31: error: unexpected type
                rtnarr.set(1,(++(rtnarr.get(1))));
                                           ^
  required: variable
  found:    value
2 errors