It will work for all cases. import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner scan=new Scanner(System.in);
    String str=scan.next();
    int k=scan.nextInt();
    SortedSet<String> sets=new TreeSet<String>();
    for(int i=0;i<=str.length()-k;i++){
        sets.add(str.substring(i,i+k));
    }
    System.out.println(sets.first());
    System.out.println(sets.last());
}

}

This is a beginner way of coding, think more about defining your own Algorithms than using predefined methods...

public static String getSmallestAndLargest(String s, int k) {

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

    // Complete the function

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

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

    return smallest + "\n" + largest;
}

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class n {

public static void main(String[] args) {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
    Scanner sc=new Scanner(System.in);
    String a=sc.next();
    int k=sc.nextInt();
    String max=a.substring(0,k);
    String min=a.substring(0,k);

    for(int i=0; i+k<=a.length(); i++){           
        if(a.substring(i, i+k).compareTo(min)<0) min=a.substring(i, i+k);
        if(a.substring(i, i+k).compareTo(max)>0) max=a.substring(i, i+k);
    }

    System.out.println(min);
    System.out.println(max);
}

}

Try this. Simple and Straight. Passes all test cases. DO UPVOTE if you like the solution. Thank You.

 public static String getSmallestAndLargest(String s, int k) {
        String smallest =s.substring(0,k);
        String largest = s.substring(0,k);
        int i=0;
        int j;
        while(i+k<=s.length())
        {
            if(s.substring(i,i+k).compareTo(smallest) < 0 )
            smallest = s.substring(i,i+k);
            else if(s.substring(i,i+k).compareTo(largest) > 0)
            largest = s.substring(i,i+k);
            i++;
        }
        
        return smallest + "\n" + largest;
    }

anyone having trouble to import ?