Java Substring Comparisons

/*We define the following terms:

Lexicographical Order, also known as alphabetic or dictionary order, orders characters as follows:
Sample Input 0

welcometojava
3
Sample Output 0

ava
wel
 code
 
 import java.util.Scanner;

public class Solution {

    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=0; i<=s.length()-k; i++ ){
            String str = s.substring(i,k+i);
            if (smallest.compareTo(str)>0){
                smallest = str;
            }
            if(largest.compareTo(str)<0){
                largest=str;
            }
        }  
        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));
    }
}