Can we take two string from one String in if statement?

I want to print Two String Lexicographical largest and smallest. Largest is printed but smallest give output same as largest. Whats problem in my code?

public class Solution {

public static String getSmallestAndLargest(String s, int k) {
   String sequence = s.substring(0,k);
    String smallest = sequence;
    String largest = sequence;

    for(int i=0;i<=(s.length()-k); i++){
        
        sequence= s.substring(i,(i+k));

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

            }
    // 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;
}




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));
}

}

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

You are comparing sequence with the smallest and largest. And your sequence changes in every iteration of the for loop. And inside the if condition, you are checking if your current sequence is smaller/larger than your smallest and largest and if so, you have to update your smallest and largest values.

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

In Java language, variable assignment is done as follows.

variable = value;