Divisions using Arraylists
At first, I thought that you were going to implement division of two integers, each represented as a list of digits.  But then, you simply concatenate all of the elements of each operand to be parsed as a double.  If you're going to cheat on the arithmetic using the built-in floating-point support, then what's the point of the exercise?  In fact, it doesn't even care whether you pass in an ArrayList of individual digits {1, 2, 3, 4, 5} or an ArrayList of one element {12345} — either way, it all gets stringified and parsed as 12345.0d.