我可以在Java中向String类添加新方法吗？

public class CustomString {
    public static String reverse(String str) {
        if (str == null || str.isEmpty()) {
            return str;
        }
        StringBuilder sb = new StringBuilder(str);
        return sb.reverse().toString();
    }
}

public class Main {
    public static void main(String[] args) {
        String str = "Hello, World!";
        String reversedStr = CustomString.reverse(str);
        System.out.println(reversedStr); // 输出：!dlroW ,olleH
    }
}