How can I invert the case of a String in Java?

Question

I want to change a String so that all the uppercase characters become lowercase, and all the lower case characters become uppercase. Number characters are just ignored.

Answer 1

I think the simplest solution to understand would be something like:

public static String reverseCase(String text) {
    StringBuilder sb = new StringBuilder();
    for (char c : text.toCharArray())
        sb.append((Character.isUpperCase(c)) ? 
                Character.toLowerCase(c) : 
                (Character.isLowerCase(c) ? Character.isUpperCase(c) : c));
    return sb.toString();
}

I think you can read through this easier and StringBuilder has an append(char) method anyways + Character.toUpperCase and toLowerCase are both just static methods. I just felt bad that the only StringBuilder example had ascii index arithmetics included as well.

For those who don't like ternary expressions, here's the equivalent:

public static String reverseCase(String text) {
    StringBuilder sb = new StringBuilder();
    for (char c : text.toCharArray())
        if (Character.isUpperCase(c)) 
            c = Character.toLowerCase(c);
        else if (Character.isLowerCase(c))
            c = Character.toUpperCase(c);
        sb.append(c);
    return sb.toString();
}