arraylist of arraylist
public static void main(String[] args) {
ArrayList<ArrayList<Integer>> outer = new ArrayList<ArrayList<Integer>>();
ArrayList<Integer> inner = new ArrayList<Integer>();
inner.add(100);
inner.add(200);
outer.add(inner); // add first list
inner = new ArrayList<Integer>(inner); // create a new inner list that has the same content as
// the original inner list
outer.add(inner); // add second list
outer.get(0).add(300); // changes only the first inner list
System.out.println(outer);
}