Code

public static String getSmallestAndLargest(String s, int k) { String smallest = ""; String largest = "";

    smallest = s.substring(0, k);
    largest = s.substring(0, k);

    for (int i = 1; i <= s.length() - k; i++) {
        String sub = s.substring(i, i + k);

        if (sub.compareTo(smallest) < 0) {
            smallest = sub;
        }
        if (sub.compareTo(largest) > 0) {
            largest = sub;
        }
    }

    return smallest + "\n" + largest;
}