vowels

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

public class Main {
    private static Set<Character> vowels = new HashSet<>(Arrays.asList('a', 'e', 'i', 'o', 'u'));

    public static void main(String[] args) {
        String input = "abcdefghijklmnopqrstuvwxyz";
        int vowelsCount = countVowels(input);
        System.out.println("There are " + vowelsCount + " vowels in '" + input + "'");
    }

    private static int countVowels(String str) {
        int count = 0;
        for(int i = 0; i < str.length(); ++i) {
            if(vowels.contains(str.charAt(i)))
                ++count;
        }
        return count;
    }
}