What will be the output of the program?

String a = 'ABCD'; 
String b = a.toLowerCase(); 
b.replace('a','d'); 
b.replace('b','c'); 
System.out.println(b);

ANS:A - abcd

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');