public static String getSmallestAndLargest(String s, int k) {

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

    for (int x=1; x<=s.length()-k; x++) {

        String tempSub = s.substring(x, x+k);

        smallest = (tempSub.compareTo(smallest)<0) ? tempSub : smallest;
        largest = (tempSub.compareTo(largest)>0) ? tempSub : largest;
    }
    return smallest + "\n" + largest;
}