Problem with analog pins' name when trying to connect Arduino board and Processing
(as suggested in comments to bump it down)

your code works ok with me. I've shorted the 3.3v to analog A0 and your code fluctuates between 679~680 in processing 

 

If it helps, this is the software versions I'm using: Processing - 3.0.1 Arduino IDE - 1.6.6 Firmata - 2.5.0