Java 8 - Collections.groupingBy result order

Question

I'm preparing for a Java exam and have one question that got me lots of tough time. Despite studying it hard I'm not able to find out what determines the order of the result.

Have a look, please:

class Country {

    public enum Continent {
        ASIA, EUROPE
    }
    String name;
    Continent region;

    public Country(String na, Continent reg) {
        name = na;
        region = reg;
    }

    public String getName() {
        return name;
    }

    public Continent getRegion() {
        return region;
    }
}

public class OrderQuestion {

    public static void main(String[] args) {
        List<Country> couList = Arrays.asList(
                new Country("Japan", Country.Continent.ASIA),
                new Country("Italy", Country.Continent.EUROPE),
                new Country("Germany", Country.Continent.EUROPE));
        Map<Country.Continent, List<String>> regionNames = couList.stream()
                .collect(Collectors.groupingBy(Country::getRegion,
                        Collectors.mapping(Country::getName, Collectors.toList())));
        System.out.println(regionNames);
    }
}

What is the result?

A. {EUROPE = [Italy, Germany], ASIA = [Japan]}
B. {ASIA = [Japan], EUROPE = [Italy, Germany]}
C. {EUROPE = [Germany, Italy], ASIA = [Japan]}
D. {EUROPE = [Germany], EUROPE = [Italy], ASIA = [Japan]}

and what most important what determines the specific result and not another?

Answer 1

We can eliminate D because keys in Map need to be unique which fails for EUROPE.

We can eliminate C because of order in [Germany, Italy]. Italy was placed before Germany in list, so it also has to be stored in that order in result list.

But how we should decide if we should eliminate B or A? Well, we cant.

Map doesn't guarantee specific order of key-value pairs. Some maps allow remembering order of placing key-value pairs like LinkedHashMap, some allow to order entries by keys like TreeMap, but this behaviour is not specified for Collectors.groupingBy.

It is confirmed by fact that this method is using HashMap, which orders key-value pairs based on hashCode() of key (Country.Continent enum here) and amount of pairs already held. Implementation of hashCode() for Enum is inherited from Object class which means it is based on memory location which can change for each time when we run JVM, so it is random value which prevents us from assuming any order (which confirms that it is unspecified).

So based on lack of specification about Map returned by groupingBy both orders of entries is possible so both A and B are possible answers.