字符串的字母往后移动3位，数字不变，脑壳空的！！！！大神，求解

换ASC码能不能实现🤔

package com.demo;
public class T {
static char A = 'A';
static char Z = 'Z';
static char a = 'a';
static char z = 'z';
public static void main(String[] args) {
String str = "abc123";
str = jiaMi(str);
System.out.println(str);
str=jieMi(str);
System.out.println(str);
}
public static String jiaMi(String str) {
char[] charArray = str.toCharArray();
for (int i = 0; i < charArray.length; i++) {
if ((charArray[i]>=A&&charArray[i]<=Z)||(charArray[i]>=a&&charArray[i]<=z)) {
charArray[i]=(char)(charArray[i]
+3
);
}
}
str = new String(charArray);
return str;
}
public static String jieMi(String str) {
char[] charArray = str.toCharArray();
for (int i = 0; i < charArray.length; i++) {
if ((charArray[i]>=A&&charArray[i]<=Z)||(charArray[i]>=a&&charArray[i]<=z)) {
charArray[i]=(char)(charArray[i]-3);
}
}
str = new String(charArray);
return str;
}
}