When using println() in this case you are invoking the toString() method of an ArrayList. toString() tells an ArrayList to display its contents in the String output you have seen. If we were to check out the source code for the method, I expect there is an Iterator in there somewhere or maybe an enhanced for loop.
Like Marc said, this is only allowing us to view the contents, you are unable to manipulate them at this point. If you wanted to change the output when invoking toString() you could create, for example, MyArrayList, extending ArrayList, and override the toString() method. But that's only if you want to be extra fancy
