ArrayList<ArrayList<Integer>> a = new ArrayList<ArrayList<Integer>>(5);
/*create a new arraylists of arraylist so outer arraylist has 5 element which means size of a=5*/
        for(int i = 0 ; i < a.size() ; i++){
            a.set(i, new ArrayList<Integer>(10));
/*loop over 5 elements of a and put an arraylist of 10 integers in the lists.*/
        }
/*at this point you got object named a (arraylist of arraylists of Integer objects) but you haven't put any integer in inner arraylist. so a.get(4).get(9) is not assigned yet.*/ 
        System.out.println(a.get(a.size()-1).get(9));//exception thrown