How can I add (+) an integer to an element in a 2-dimensional ArrayList?

I'm trying to add 1 to an integer in a 2-dimensional ArrayList.

I'm using the set() method with the element + 1 as the second argument, but the "+ 1" isn't working. When I retrieve the element, it defines it as an object, not an integer. How do I get around this?

Code:

ArrayList<ArrayList> inventoryList = new ArrayList(
    Arrays.asList(new ArrayList<String>(), new ArrayList<Integer>()));

...

(inventoryList.get(1)).set(i, ((inventoryList.get(1)).get(i) + 1));

Error:

Main.java:47: error: bad operand types for binary operator '+'
                (inventoryList.get(1)).set(i, ((inventoryList.get(1)).get(i) + 1));
                                                                             ^

My code is at this ideone page. This code is translated from python and I'm currently debugging it so don't worry about the other errors.