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

    for(int i=0; i<s.length()-k+1; i++){
        String subString = s.substring(i,(i+k));

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

    return smallest + "\n" + largest;
}