That's very easy task for all Java's users. I think, that it task is easely than other task's.

Please use spoiler flag next time.

Thanks a lot!

It works because a new object is returned. The variable 'x' does not change.
public String replace(char oldChar, char newChar)
Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.

Thanks for your guys' explanation!
Correct me if I'm wrong. So, if my understanding is correct, the reason "final" is used is to ensure that String x does not change in the hands of outsiders, and the reason "x.replace" works is because String x is an object...

Warning: a mutable object declared as final will still be mutable. The only thing that final locks is the binding between the variable name and the reference of the object in memory, so final "protects the code" only when used on immutable data structures or basic data types:

final List<String> lst = new ArrayList<>();
lst.add("stuff");                              // => works like a charm!