String objects are immutable, they cannot be changed, in this case we are talking about the replace method which returns a new
String object resulting from replacing all occurrences of
oldChar in this string with
newChar.
b.replace(char oldChar, char newChar);
But since this is only a temporary String it must either be put to use straight away i.e.
System.out.println(b.replace('a','d'));
Or a new variable must be assigned its value i.e.
String c = b.replace('a','d');
