import java.util.Scanner;

public class Solution {

public static String getSmallestAndLargest(String s, int k) {
    String smallest = s.substring(0,k);
    String largest = s.substring(0,k);
    String str;
    int size = s.length()-k+1;
    for(int i=0;i<size;++i){
        str = s.substring(i,i+k);
        if(smallest.compareTo(str) > 0)
        smallest =str;
        if(largest.compareTo(str)<0)
        largest = str;           
    }
    return smallest + "\n" + largest;
}

import java.util.Scanner;

public class Solution {

public static String getSmallestAndLargest(String s, int k) {
    String smallest = s.substring(0,k);
    String largest = s.substring(0,k);
    String str;
    int size=s.length()-k+1;
    for(int i=0;i<size;i++)
    {
        str=s.substring(i,i+k);
        if(smallest.compareTo(str)>0)
        smallest=str;
        if(largest.compareTo(str)<0)
        largest=str;
    }
    // Complete the function
    // 'smallest' must be the lexicographically smallest substring of length 'k'
    // 'largest' must be the lexicographically largest substring of length 'k'

    return smallest + "\n" + largest;
}

i have a doubt in this problem , whether we have create a substring from the baseString?

Here is Java Substring Comparisons solution - https://programmingoneonone.com/hackerrank-java-substring-comparisons-problem-solution.html

    ArrayList<String> substrings = new ArrayList<>();
    for(int i=0;i<=s.length()-k;i++){
        String sub = s.substring(i,i+k);
        substrings.add(sub);  
    }
    smallest = substrings.get(0);
    largest = substrings.get(0);
    for(int j=1;j<substrings.size();j++){
        String currentword = substrings.get(j);
        if(currentword.compareTo(smallest)<0){
            smallest=currentword;
        }
        if(currentword.compareTo(largest)>0){
            largest=currentword;
        }
    }
     return smallest + "\n" + largest;
}