It will work for all cases. import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

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 scan=new Scanner(System.in);
    String str=scan.next();
    int k=scan.nextInt();
    SortedSet<String> sets=new TreeSet<String>();
    for(int i=0;i<=str.length()-k;i++){
        sets.add(str.substring(i,i+k));
    }
    System.out.println(sets.first());
    System.out.println(sets.last());
}

}

This is a beginner way of coding, think more about defining your own Algorithms than using predefined methods...

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

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

    // Complete the function

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

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

    return smallest + "\n" + largest;
}

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class n {

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 a=sc.next();
    int k=sc.nextInt();
    String max=a.substring(0,k);
    String min=a.substring(0,k);

    for(int i=0; i+k<=a.length(); i++){           
        if(a.substring(i, i+k).compareTo(min)<0) min=a.substring(i, i+k);
        if(a.substring(i, i+k).compareTo(max)>0) max=a.substring(i, i+k);
    }

    System.out.println(min);
    System.out.println(max);
}

}

anyone having trouble to import ?

Super Lazy Approach

public class Solution {

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);
        scan.useDelimiter("\r\n");
        String string = scan.next();
        int size = scan.nextInt();
        
        List<String> list = new LinkedList<String>();
        
        final int max = string.length() - size;
        for (int i = 0; i <= max; i++) {
            list.add(string.substring(i, i + size));
        }
        
        System.out.println(Collections.min(list));
        System.out.println(Collections.max(list));
        
    }
    
}