I don't think this null is necessary for this problem.

@davorpa If you read the docs, it shows that has the null check built into it.

if (locale == null) {
throw new NullPointerException();
}

do you know the concept of null-safe methods? null is not same as empty

It's no need to check for empty string, because .toUpperCase contains this condition: https://www.javatpoint.com/java-string-touppercase

good work! Like!