public class Sample {
    public static void main(String[] args) {
        String s = "ABCD";
        s = s.replace("B", "X");
        System.out.println(s); // AXCD

        String s2 = "AAAA";
        s2.replaceAll("A", "X");
        System.out.println(s2); // AAAA
    }
}