String smallest = "";
String largest = "";

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

for(int i = 0; ( i + k )<= s.length(); i++){

        String texto = s.substring(i, i + k);

        if (texto.compareTo(smallest)<0 || smallest =="") {
            smallest = texto;
        } 
        if (texto.compareTo(largest)>0 || largest =="") {
            largest = texto;
        } 

    }

Code

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

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

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

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

    return smallest + "\n" + largest;
}

this took too long

import java.io.*;
import java.util.*;
import java.lang.*;
public class Solution {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String smallest = "";
        String largest = "";
        String originalString = scan.nextLine();
        int substringLength = scan.nextInt();
        List<String> wordsList = new ArrayList<>();

        // Iterate through the string, extracting substrings of the desired length
        for (int i = 0; i <= originalString.length() - substringLength; i++) {
            String substring = originalString.substring(i, i + substringLength);
            wordsList.add(substring);
        }
        Collections.sort(wordsList);
        String firstWordOfFirstSentence = wordsList.get(0).split("\\s+")[0];
        String lastSentence = wordsList.get(wordsList.size() - 1);
        String lastWordOfLastSentence = lastSentence.split("\\s+")[lastSentence.split("\\s+").length - 1];   
        System.out.println(firstWordOfFirstSentence); 
        System.out.println(lastWordOfLastSentence);
    }
}

Substring comparisons in Java are powerful but must be used carefully to avoid common pitfalls like index errors and unnecessary memory usage (in older versions). For readability and safety, prefer utility methods like startsWith(), endsWith(), or regionMatches() when appropriate. Betguru ID