CharReplacement.java

package com.example.blogservice;

public class CharReplacement {

    public static void main(String[] args) {
        String s = "AyxRTXra"; // ABBAAABB
        System.out.println(convert(s));
    }

    private static String convert(String input) {
        StringBuilder result = new StringBuilder();

        char[] strings = input.toCharArray();

        for (char c : strings) {
            if (isUpperCase(c)) {
                result.append("A");
            } else {
                result.append("B");
            }
        }

        return result.toString();
    }

    private static boolean isUpperCase(char c) {
        // A - Z = 65 - 90
        // a - z = 97 - 122
        return c >= 65 && c <= 90;
    }
}