Collecting a collection of list based on similar index

Question

I have a collection like : List<List<Object>> firstList

I want to group together a similar list of pattern : List<List<Object>> secondList but grouped by indexes.

say

firstList [1]:
   0 = {Object A}"
   1 = {Object B}"
   2 = {Object C}"

firstList [2]:
   0 = {Object A}"
   1 = {Object B}"
   2 = {Object C}"

I want to group this collection as

secondList [1]:
   0 = {Object A}"
   1 = {Object A}"

secondList [2]:
   0 = {Object B}"
   1 = {Object B}"

secondList [3]:
   0 = {Object C}"
   1 = {Object C}"

What I have tried so far is

for (int i = 0; i <firstList.size() ; i++) {
    List<Object> list = firstList.get(i);
    List<Object> rlPr = new ArrayList<>();

    for (int j = 0; j <list.size()-1; j++) {
        rlPr.add(list.get(i));
    }
    secondList.add(rlPr);
}

But I am not getting what is expected. I am using java 8.

EDIT : ALL THE LIST ARE OF SAME SIZES

Answer 1

You can use a Map to group the values by index:

Map<Integer, List<Object>> map = new TreeMap<>();
for (List<Object> objects : firstList) {
    for (int i = 0, l = objects.size(); i < l; i++) {
        map.computeIfAbsent(i, k -> new ArrayList<>()).add(objects.get(i));
    }
}

And then to get the List back:

List<List<Object>> secondList = new ArrayList<>(map.values());