part arraylist to 3 strings part

Question

I am trying to split my arrayList which has the size 3n to 3 part (with size n each),
but I am getting just the first one. How can I fix that?

[1,2,3,4,5,6,7,8,9]  ==> [1,2,3][4,5,6][7,8,9]

I appreciate any help.

List<RootCreator> mainSublist = new ArrayList<RootCreator>();
int number = mainList.size()/3;
for(int i= 0 ; i < mainList.size()/3; i++){
    int index = i*3 ;           
    List<RootCreator> sublist =  mainList.subList(0, index);
    mainSublist.addAll(sublist);
}

Answer 1

    List<RootCreator> mainSublist = mainList.subList(0, mainList.size() / 3);

Answer 2

You can use subList(int fromIndex,int toIndex) like this -

static <T> List<List<T>> split(List<T> list, int n) {

    List<List<T>> parts = new ArrayList<List<T>>();
    int size = list.size();

    for (int index = 0; index < size; index += n) {
        parts.add(new ArrayList<T>(
            list.subList(index, Math.min(size, index + n)))
        );
    }
    return parts;
}

Answer 3

The main problem: you always call mainList.subList() with first argument 0.

    List<RootCreator> mainSublist = new ArrayList<RootCreator>();
    int number = mainList.size()/3;
    int first = 0;
    for(; first < mainList.size() - number; first+=number){
        mainSublist.addAll(mainList.subList(first, first + number));
    }
    mainSublist.addAll(mainList.subList(first));