I am creating a Map<String,String> from List<Person> using java-8 stream

       .collect(Collectors.toMap(Person::getNationality, Person::getName, (name1, name2) -> name1)

But in above case I want to resolve conflict in name by using person's age. is there any way to pass merge function something around the lines (age1, age2) -> // if age1 is greater than age2 return name1, else return name2 ?


To select a person based on its age, you need the Person instance to query the age. You cannot reconstitute the information after you mapped the Person to a plain name String.

So you have to collect the persons first, to be able to select the oldest, followed by mapping them to their names:

    .collect(Collectors.groupingBy(Person::getNationality, Collectors.collectingAndThen(
        o -> o.get().getName())));

If you don't want to use a helper data structure, it is possible if you first keep your Person info and perform the merge based on it and apply the mapping afterwards:

public void test() {
    final List<Person> persons = new ArrayList<>();

    final BinaryOperator<Person> mergeFunction =
        (lhs, rhs) -> lhs.getAge() > rhs.getAge() ? lhs : rhs;

    final Function<Person, String> mapFunction = Person::getName;

    final Map<String, String> personNamesByNation =
                Collectors.groupingBy(Person::getNation, // KeyMapper Person.getNation: Map<String, List<Person>>
                            Collectors.reducing(mergeFunction), // Merge Persons into single value via merge function: Map<String, Optional<Person>>
                            Optional::get), // unwrap value: Map<String, Person>
                        mapFunction))); // apply map function afterwards: Map<String, String>
  • 1
    You can also use final BinaryOperator<Person> mergeFunction = BinaryOperator.maxBy(Comparator.comparingInt(Person::getAge));, which is what you get when using Collectors.maxBy(…) instead of Collectors.reducing(…), so these solutions are equivalent. – Holger Jun 12 at 12:59
  • Hm, true, when I started your answer wasn't there yet. I guess my answer also works if you want to aggregate Persons into a new Person object, but for the question of the OP yours looks cleaner. – sfiss Jun 12 at 13:03
  • 1
    Indeed, it was overlapping. Nothing wrong with that. I just wanted to mention the equivalence, in case, any reader is wondering whether there are functional differences. – Holger Jun 12 at 13:04

order the elements of stream by age and then just choose first:

       .collect(Collectors.toMap(Person::getNationality, Person::getName, (n1, n2) -> n1));

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