This is what @coderator is talking about

public static String getSmallestAndLargest(String s, int k) {
    String smallest = "";
    String largest = "";
    smallest = largest = s.substring(0, k);

    for (int i=1; i<s.length()-k+1; i++) {
        String substr = s.substring(i, i+k);
        if (smallest.compareTo(substr) > 0)
            smallest = substr;
        if (largest.compareTo(substr) < 0)
            largest = substr;
    }

    return smallest + "\n" + largest;
}