Java method to reverse upper-lower case doesn't reverse all letters

Question

I wanted to create a method that reverse upper-lower case letters in a word. The problem I'm having is that the method doesn't reverse all the letters. For example when I type "nIceToMeEtyoU" it prints "NiCETomEETYou". It didn't work for the "o", the second "e" and "t". I just couldn't figure out what's wrong with the code.

public static String reverseCase(String str) {
    char changed;
    String a = str;
    for (int i = 0; i < a.length(); i++) {
        char d = a.charAt(i);
        boolean letter = Character.isUpperCase(d);
        if (letter == true) {
            changed = Character.toLowerCase(d);
        } else {
            changed = Character.toUpperCase(d);
        }
        a = a.replace(d, changed);
    }
    return a;
}

Answer 1

String::replace returns a new String with all occurrences of the character you wanted replaced changed.

Also, Strings in Java are immutable, meaning you cannot replace a character in a string while keeping the same string. In order to replace the character at a specific index, see this post

Answer 2

It didn't work for the "o","second e" and "t".

The replace() method replaces all occurrances of the character in the string.

Instead, use a StringBuffer to append each character as you iterate through the string.

Then when the loop is finished you recreate the String using the toString() method of the StringBuffer.