does my code have any problem, it always failed to test case 3, however when i try the input of test case 3 as custom input, the output is correct.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;

public class solution {
    static ArrayList<String> subList = new ArrayList<>();

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        int k = sc.nextInt();
        System.out.println(subAllList(s, k));
    }

    private static String subAllList(String s, int k) {
        for (int i = 0; i <= s.length() - k; i++) {
            String stringList = s.substring(i, i + k);
            subList.add(stringList);
        }
        Collections.sort(subList);
        String smallest = subList.get(0);
        String largest = subList.get(subList.size() - 1);
        if (smallest.equalsIgnoreCase(largest)) {
            return smallest;
        } else {
            return smallest + "\n" + largest;
        }
    }
}

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;
}

}

import java.util.Scanner;

public class Solution {

public static String getSmallestAndLargest(String s, int k) {
    String smallest = "";
    String largest = "";
    java.util.List<String> total = new java.util.ArrayList();

    // Complete the function
    // 'smallest' must be the lexicographically smallest substring of length 'k'
    // 'largest' must be the lexicographically largest substring of length 'k'
    for(int i=0;i<=s.length()-1;i++)
    {
        try{
        String sort = s.substring(i, k+i);
        total.add(sort); 
        }
        catch(Exception e)
        {
            break;
        }
    }
    java.util.Collections.sort(total);
    smallest = total.get(0);
    largest = total.get(total.size()-1);

    return smallest + "\n" + largest;
}


public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    String s = scan.next();
    int k = scan.nextInt();
    scan.close();

    System.out.println(getSmallestAndLargest(s, k));
}

}

works perfectly

public class solution{ public static String getSmallestAndLargest(String s, int k) { String smallest = s.substring(0,k); String largest = s.substring(0,k); int i; for(i = 1 ; i<=s.length() - k ; i++){ String comp = s.substring(i,i+k); if(comp.compareTo(smallest)<0){ smallest = comp; } if(comp.compareTo(largest)>0){ largest = comp; } return smallest +"\n"+largest; } public static void main(String[] args) { Scanner scan = new Scanner(System.in); String s = scan.next(); int k = scan.nextInt(); scan.close();

System.out.println(getSmallestAndLargest(s, k));

}

    return smallest + "\n" + largest;
}

import java.util.Scanner;

public class Solution {

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

    // Complete the function
    // 'smallest' must be the lexicographically smallest substring of length 'k'
    // 'largest' must be the lexicographically largest substring of length 'k'

    int stpsize=1;
    int n=s.length()-k+1;
    String []res=new String[n];
    for(int i=0;i<n;i++)
        res[i]=s.substring(i,i+k);

     for (int i = 0; i < res.length - 1; i++) {
        for (int j = 0; j < res.length - i - 1; j++) {
            if (res[j].compareTo(res[j + 1]) > 0) {
                String temp = res[j];
                res[j] = res[j + 1];
                res[j + 1] = temp;
            }
        }
    }
    smallest=res[0];
    largest=res[res.length - 1];
    return smallest + "\n" + largest;
}


public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    String s = scan.next();
    int k = scan.nextInt();
    scan.close();

    System.out.println(getSmallestAndLargest(s, k));
}

}