如何在不使用replace()方法、StringBuilder、StringBuffer或数组的情况下替换字符串中的所有字符？

public class StringReplacement {
    public static String replaceAllChars(String str, char targetChar, char replacementChar) {
        if (str == null || str.isEmpty()) {
            return "";
        }
        
        if (str.charAt(0) == targetChar) {
            return replacementChar + replaceAllChars(str.substring(1), targetChar, replacementChar);
        } else {
            return str.charAt(0) + replaceAllChars(str.substring(1), targetChar, replacementChar);
        }
    }
    
    public static void main(String[] args) {
        String str = "Hello, World!";
        char targetChar = 'o';
        char replacementChar = '*';
        
        String replacedStr = replaceAllChars(str, targetChar, replacementChar);
        System.out.println(replacedStr);
    }
}