Java ArrayList of ArrayList

You are adding a reference to the same inner ArrayList twice to the outer list. Therefore, when you are changing the inner list (by adding 300), you see it in “both” inner lists (when actually there’s just one inner list for which two references are stored in the outer list).

To get your desired result, you should create a new inner list :

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);
}