public class JavaSubstringComparisons {
    public static void main(String[] args) {
        System.out.println(getSmallestAndLargest("welcometojava", 14));
    }

    // Gets the smallest and largest substring with length k from a string s
    public static String getSmallestAndLargest(String s, int k) {
        if (k <= s.length()) {
            String smallestSubstring = s.substring(0, k);
            String largestSubstring = "";

            for (int i = 0; i <= s.length() - k; i++) {
                String substring = s.substring(i, i + k);
                if (substring.compareTo(smallestSubstring) < 0) {
                    smallestSubstring = substring;
                }
                if (substring.compareTo(largestSubstring) > 0) {
                    largestSubstring = substring;
                }
            }

            return smallestSubstring + "\n" + largestSubstring;
        } else {
            return "Substring length cannot be greater than string length";
        }
    }
}