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

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

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

   if(sub.compareTo(small)<0) {
            small=sub; } 

             else if(sub.compareTo(largest)>0)

    { largest=sub;  }}
    return small + "\n" + largest;
}