Better way to invert cases of characters in a string in Java

Question

As a novice Java programmer who barely got started in Java programming, I am totally exhausted in trying to find a solution to this issue. A course that I am currently studying gave homework that asked me to create a Java class that has a sort of “reverse” method that returns a new version of the string of the current string where the capitalization is reversed (ie, lowercase to uppercase and uppercase to lowercase) for the alphabetical characters specified in a given condition. Say if I were to reverse “abc, XYZ; 123.” using reverse("bcdxyz@3210."), it must return "aBC, xyz; 123.". (PS: the class ignores numbers and special characters and the variable "myString" is where the "abc, XYZ; 123." goes to.). So far, I've only managed to return out "aBC, XYZ; 123." with the code below. Am I missing something here?

public String reverse(String arg) {
    // TODO Implement method 
    String arg_no_sym = arg.replaceAll("[^a-zA-Z0-9]","");
    String arg_perfect = arg_no_sym.replaceAll("\\d","");
    
    if (myString != null) {
        char[] arrayOfReplaceChars = arg_perfect.toCharArray();
        char[] arrayOfmyString = myString.toCharArray();
        for (int i = 0; i < arg_perfect.length(); i++) {
            myString = myString.replace(String.valueOf((arrayOfReplaceChars[i])), String.valueOf((arrayOfReplaceChars[i])).toUpperCase()); 
        }
        return myString;
    }
    else {
        return "";
    }
}

Answer 1

How about using the methods isUpperCase() and isLowerCase() to check the case of the letters and then use toUpperCase() and toLowerCase() to change the case of them?