UpperLowerInverse.java

public static void main(String[] args) {
    String a = "Charlie123";
    StringBuilder sb = new StringBuilder();
    for (char c : a.toCharArray()) {
        if ('a' <= c && c <= 'z') {
            sb.append(String.valueOf(c).toUpperCase());
        } else if ('A' <= c && c <= 'Z') {
            sb.append(String.valueOf(c).toLowerCase());
        } else {
            sb.append(c);
        }
    }
    System.out.println(sb.toString());
}