Solution in java 15. I had to import ArrayList and Comparator classes.

public static void main(String[] args) {

    Scanner scan = new Scanner(System.in);
    String s = scan.next();
    int k = scan.nextInt();

    ArrayList<String> lista = new ArrayList<String>();
    for (int i= 0; i < s.length()-(k-1);i++){
        lista.add(s.substring(i,k+i));
    }

    lista.sort(Comparator.naturalOrder());
    System.out.println(lista.get(0));
    System.out.println(lista.get(lista.size()-1));
}

}

import java.io.; import java.util.;

public class Solution {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner sc = new Scanner(System.in);
    String str = sc.nextLine();
    int k = sc.nextInt();
    System.out.println(smallestSubString(str,k)+"\n"+largestSubString(str,k));
}
static String smallestSubString(String str,int k)
{
    List<String> list = getSubStrings(str,k);
    //int len = list.length;
    return list.get(0);
}
static String largestSubString(String str,int k)
{
    List<String> list = getSubStrings(str,k);
    int len = list.size();
    return list.get(len-1);
}
static List<String> getSubStrings(String str, int k)
{
    List<String> subStr = new ArrayList<String>();
    int l = str.length();
    for(int i=0;i<l;i++)
    {
        String sub = "";
        boolean flag = false;
        for(int j=i;j<i+k;j++)
        {
            if(j>=l)
            {
                //subStr.add(sub);
                flag = true;
                break;
            }
            else
            {
                 sub+=str.charAt(j);  
            }
        }
        if(flag)
        {
            break;
        }
        subStr.add(sub);
    }
    Collections.sort(subStr);
    return subStr;
}

}

public static String getSmallestAndLargest(String str, int number) { String smallest = ""; String largest = "";

    for(int i=0;i<=str.length()-number;i++){
        int range = i + number;
        String subStringI = str.substring(i,range);
        if(subStringI.compareTo(largest)>0){
            largest=subStringI;
        }
        if(subStringI.compareTo(smallest)<0){
            smallest=subStringI;
        }
        if(i==0){
            smallest=subStringI;
        }
    }
    return smallest + "\n" + largest;
}