public static void count_vowels(String s) {
        s = s.toLowerCase();
        int count = 0;
        List<Character> vowels = Arrays.asList('a', 'e', 'i', 'o', 'u');
        for (int i = 0; i < s.length(); i++) {
            if (vowels.contains(s.charAt(i))) count++;
        }

        System.out.println(count);
    }