append() doesn't modify the existing array, it returns a new array with the additional element.  In your example, the rows variable contains the unmodified array, and newData contains the updated array.

Well, the overall aim is to append the new item, and save the contents of the new array to a file. 

i don't know anything about java memory management but you might be okay just appending the array.  the reason why i hypothesize that despite my lack of memory management knowledge is that if you look at Ben Fry's zipcode example, he's loading 43,000 zip codes into an array and the program loads up quickly and works flawlessly.  you can find it in the examples under visualizing data.  his visualization only loads the array once at the beginning.  now, whether that will hold true for a program that continuously runs, i cannot say.

There is no reason to re-read the file every time, but you can keep updating the file for future reference.