This can be done without consuming extra space by maintaining an array/list

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

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

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

        if (smallest.compareTo(newSubstring) > 0) {
            smallest = newSubstring;
        }

        if (largest.compareTo(newSubstring) < 0) {
            largest = newSubstring;
        }
    }

    return smallest + "\n" + largest;
}