Adding two big integers represented as strings

class DemoJavaA {

    static Integer carry = 0;

    public static void main(String[] args) {
        String s1 = "1231823742198742198429492183492184792";
        String s2 = "5873294573295872345872352387523984584";
        add(s1, s2);
    }

    public static void add(String a, String b) {
        String s = "";
        int i = a.length() - 1;
        int j = b.length() - 1;
        while (true) {
            int i1 = Integer.parseInt(Character.toString(a.charAt(i)));
            int i2 = Integer.parseInt(Character.toString(b.charAt(j)));
            Integer i3 = i1 + i2 + carry;
            if (i3 > 9) {
                carry = 1;
                i3 = i3 - 10;
            } else {
                carry = 0;
            }
            s = s + i3;
            i--;
            j--;
            if (i < 0) {
                if (carry > 0) {
                    s = s + carry;
                }
                break;
            }
        }
        System.out.println(" r = " + (new StringBuilder(s)).reverse());

    }
}

class DemoJavaA {

static Integer carry = 0;

public static void main(String[] args) {

String s1 = "1231823742198742198429492183492184792";

String s2 = "5873294573295872345872352387523984584";

add(s1, s2);

}

public static void add(String a, String b) {

String s = "";

int i = a.length() - 1;

int j = b.length() - 1;

while (true) {

int i1 = Integer.parseInt(Character.toString(a.charAt(i)));

int i2 = Integer.parseInt(Character.toString(b.charAt(j)));

Integer i3 = i1 + i2 + carry;

if (i3 > 9) {

carry = 1;

i3 = i3 - 10;

} else {

carry = 0;

}

s = s + i3;

i--;

j--;

if (i < 0) {

if (carry > 0) {

s = s + carry;

}

break;

}

System.out.println(" r = " + (new StringBuilder(s)).reverse());

}