A DESCRIPTION OF THE PROBLEM :
The provided example does not provide an identity value to the reducing Collector factory method

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
For example, given a stream of Person, to calculate the longest last name of residents in each city:


     Comparator<String> byLength = Comparator.comparing(String::length);
     Map<City, String> longestLastNameByCity
         = people.stream().collect(groupingBy(Person::getCity,
                                              reducing("", Person::getLastName, BinaryOperator.maxBy(byLength))));
ACTUAL -
For example, given a stream of Person, to calculate the longest last name of residents in each city:


     Comparator<String> byLength = Comparator.comparing(String::length);
     Map<City, String> longestLastNameByCity
         = people.stream().collect(groupingBy(Person::getCity,
                                              reducing(Person::getLastName, BinaryOperator.maxBy(byLength))));

URL OF FAULTY DOCUMENTATION :
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#reducing-U-java.util.function.Function-java.util.function.BinaryOperator-