Tryout.java

public class Tryout {
    public static void main(String[] args) {
        String s = "Tere, TUDENG, 1234!";
        String t = asenda(s); // "xxxx, xxxxxx, 1234!"
        System.out.println(t);
    }
    public static String asenda(String s) {
        String result = "";

        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);

            if (Character.isLetter(c)) {
                result += "x";
            } else {
                result += c;
            }
        }
        return result;
    }
}