Why are you trying to do arithmetic on
Strings? They represent writing, not numbers.
Did you know that you can divide a
String into an array of
chars?
Did you know that
chars are numbers (not letters), and you can do arithmetic with them? So you can use your
char array to create an
int array.
What is going to happen when you get a 4‑digit number out of two 3‑digit numbers? Will you have enough space in your array?
You should move all that code out of the
main method other methods, and you should think of better names for your arrays. There are much better ways to get the carry than using an if.