Perfectly working, Logic is just that you need to keep checking all the substrings untill compateTo() function filters all the substrings lexographically.

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

    for (int i=0; i<=s.length()-k; i++){
        dummyString = s.substring(i, i+k);
        if(dummyString.compareTo(largest)>0){
            largest = dummyString;
        }            
        if(dummyString.compareTo(smallest)<0){
            smallest = dummyString;
        }            
    }
    return smallest + "\n" + largest;
}

}