Adding big numbers using a stack of digits in Java

Add.java

public class Add {
	public static int charToInt(char c) {
		return Integer.parseInt(Character.toString(c));
	}
	
	public static LLStack<Integer> stackDigits(String s) {
		LLStack<Integer> stack = new LLStack<Integer>();
		for (int i = 0; i < s.length(); ++i) {
			char c = s.charAt(i);
			try {
				stack.push(charToInt(c));
			} catch (IllegalArgumentException e) {
				System.err.println("Invalid number: " + s);
				System.exit(2);
			}
		}
		return stack;
	}
	
	public static int getNextDigit(LLStack<Integer> stack) {
		if (stack.isEmpty()) {
			return 0;
		} else {
			return stack.pop();
		}
	}
	
	public static void main(String[] args) {
		if (args.length != 2) {
			System.err.println("usage: java Add <number> <number>");
			System.exit(1);
		}
		
		String result = "";
		LLStack<Integer> left = stackDigits(args[0]);
		LLStack<Integer> right = stackDigits(args[1]);

		int carry = 0;
		while (!left.isEmpty() || !right.isEmpty()) {
			int sum = getNextDigit(left) + getNextDigit(right) + carry;
			result = Integer.toString(sum % 10) + result;
			carry = sum / 10;
		}
		if (carry != 0) {
			result = Integer.toString(carry) + result;
		}
		System.out.println(result);
	}
}

LLStack.java

import java.util.NoSuchElementException;

public class LLStack<T> {
	public LLStack() {
		list = new ForwardList<T>();
	}
	
	public T pop() {
		if (list.isEmpty()) {
			throw new NoSuchElementException("No element to pop! List is empty!");
		}
		T data = list.head.data;
		list.removeAtHead();
		return data;
	}
	
	public void push(T element) {
		list.insertAtHead(element);
	}
	
	public boolean isEmpty() {
		return list.isEmpty();
	}
	
	private ForwardList<T> list;
}