Question

I have a 2D List of type Integer (ArrayList< List < Integer > >),
There are currently 177147 lists of integers.
I want to separate these into 243 different collections (Each collection having 729 lists of elements)

e.g. Array[0] -> Array[728] go into Collection[0]

…until
Array[176417] -> Array[177146] goes into Collection[242]

Should I rather use ArrayList<ArrayList<List<Integer>>>?
If so, how do I separate these items in this manner?

Answer 1

Iterate on the elements and add by slices to the new structure:

public void run() {
    int SLICE = 729;
    List<List<Integer>> list2d = new ArrayList<List<Integer>>();
    // fill original list
    List<List<List<Integer>>> list3d = new ArrayList<List<List<Integer>>>();
    List<List<Integer>> partial = new ArrayList<List<Integer>>(SLICE);
    for (List<Integer> list : list2d) {
        partial.add(list);
        if (partial.size() == SLICE) {
            list3d.add(partial);
            partial = new ArrayList<List<Integer>>(SLICE);
        }
    }
    if (!partial.isEmpty()) {
        list3d.add(partial);
    }
}