You should change that signature, yes, but not the ArrayList part. It's the part inside the ArrayList you should change.
Applying coding against interfaces as well, you get the following signature:
Instead of just the number, you create a Customer object and add that to the list.
That is, unless the customer data is the same in each row. Then you can use a Map<
String,Customer> but the Customer object must have a collection of IDs.
On a side note, in the above code one line is repeated. You can move that out of the if-else:
Or, if you use a Map<String,Customer>: