Find difference between two Strings

You can use StringUtils.difference(String first, String second).

This is how they implemented it:

public static String difference(String str1, String str2) {
    if (str1 == null) {
        return str2;
    }
    if (str2 == null) {
        return str1;
    }
    int at = indexOfDifference(str1, str2);
    if (at == INDEX_NOT_FOUND) {
        return EMPTY;
    }
    return str2.substring(at);
}

public static int indexOfDifference(CharSequence cs1, CharSequence cs2) {
    if (cs1 == cs2) {
        return INDEX_NOT_FOUND;
    }
    if (cs1 == null || cs2 == null) {
        return 0;
    }
    int i;
    for (i = 0; i < cs1.length() && i < cs2.length(); ++i) {
        if (cs1.charAt(i) != cs2.charAt(i)) {
            break;
        }
    }
    if (i < cs2.length() || i < cs1.length()) {
        return i;
    }
    return INDEX_NOT_FOUND;
}