Generate random letters to sort in positive order

public class Test {
    
    
    private static String[] a = {
    
    "a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};

    public static void main(String[] args) {
    
    
        //生成随机字母
        StringJoiner sj = new StringJoiner(",");
        Random random = new Random();
        for (int i = 0; i < 20; i++) {
    
    
            sj.add(a[random.nextInt(a.length)]);
        }
        System.out.println("生成的随机数:" + sj.toString());
        String s1 = sj.toString().replaceAll(",", "");
        char[] chars = s1.toCharArray();
        //排序
        Arrays.sort(chars);
        //去重
        Set<String> strings = new HashSet<>();
        for (int i = 0; i < chars.length; i++) {
    
    
            strings.add(chars[i] + "");
        }
        System.out.println(strings);

    }
}