如何在倒数后追加一个字母？

public class AppendLetter {
    public static String appendLetterToEnd(String str, char letter) {
        char[] charArray = str.toCharArray();
        int lastIndex = charArray.length - 1;
        charArray[lastIndex] = charArray[lastIndex] + "" + letter;
        return new String(charArray);
    }

    public static void main(String[] args) {
        String originalString = "Hello";
        char letterToAppend = 'W';
        String modifiedString = appendLetterToEnd(originalString, letterToAppend);
        System.out.println(modifiedString);
    }
}