Adding Mechanism For Strings

Basically, there is a class "InfiniteInteger" which implements Comparable and can be used to store extremely long numbers as strings (i.e. "37493274839247923749234"). It has a couple functions such as "toString()", "compareTo()" and "isNegative()". Then there's the method I can't quite push through, "add(InfiniteInteger anInfiniteInteger)".Somewhere along the way, it just won't carry numbers correctly... It should work for positive and negative numbers but doesn't quite.

public InfiniteInteger plus(final InfiniteInteger anInfiniteInteger)
{
// TO DO
String number1 = new String(string).replaceAll("-", "");
String number2 = new String(anInfiniteInteger.string).replaceAll("-", "");
int[] firstDigits = new int[number1.length()];
int[] secondDigits = new int[number2.length()];
int place = 0;
for(int i = firstDigits.length - 1; i >= 0; i--) {
firstDigits[place] = Integer.parseInt(Character.toString(number1.charAt(i)));
place++;
}
place = 0;
for(int i = secondDigits.length - 1; i >= 0; i--) {
secondDigits[place] = Integer.parseInt(Character.toString(number2.charAt(i)));

[code]...

I can't use any external packages (i.e. Arrays, BigDecimal, ArrayList) and can't use long as a datatype either.