Imagine you have a list of people, Roberts, Pauls, Richards, etc, these are people grouped by name into Map<String, List<Person>>
. You want to find the oldest Paul, Robert, etc... You can do it like so:
public static void main(String... args) {
List<Person> people = Arrays.asList(
new Person(23, "Paul"),
new Person(24, "Robert"),
new Person(32, "Paul"),
new Person(10, "Robert"),
new Person(4, "Richard"),
new Person(60, "Richard"),
new Person(9, "Robert"),
new Person(26, "Robert")
);
Person dummy = new Person(0, "");
var mapping = people.stream().collect(groupingBy(Person::getName, reducing(dummy, (p1, p2) -> p1.getAge() < p2.getAge() ? p2 : p1)));
mapping.entrySet().forEach(System.out::println);
}
Say, I want to get a mapping in the form of Map<String, Integer>
instead of Map<String, Person>
, I can do it like so:
var mapping = people.stream().collect(groupingBy(Person::getName, mapping(Person::getAge, reducing(0, (p1, p2) -> p1 < p2 ? p2 : p1))));
The steps above are:
Map<String/*Name*/, List<Person>>
List<Person>
into List<Integer>
I was wondering how to do:
Map<String, List<Person>>
Map<String, Person>
Map<String, Person>
into Map<String, Integer>
. And I want to do all that inside that chain of groupingBy's, reducing's and mapping's. This is the "pseudocode":
var mapping = people.stream().collect(groupingBy(Person::getName, reducing(dummy, (p1, p2) -> p1.getAge() < p2.getAge() ? p2 : p1 /*, have to write some other collector factory method here*/)));
How can I achieve this?
It is more straightforward to do this with the 3-argument version of toMap
collector :
people.stream().collect(toMap(
Person::getName,
Person::getAge,
Integer::max
));
If you really want to use groupingBy
as mentioned in the problem statement, you can still do it like this:
final Map<String, Integer> oldestPersonByName = people.stream()
.collect(Collectors.groupingBy(Person::getName, Collectors.reducing(0, Person::getAge, Integer::max)));
For the reduction you have to pass an identity element, mapping function and a binary operator used to reduce, which is max
in this case.
I think it would be like the following, also using maxBy
instead of reducing
.
people.stream().collect(
groupingBy(
Person::getName,
collectingAndThen(
maxBy(Comparator.comparing(Person::getAge)),
(Optional<Person> max) -> max.get().getAge()
)
)
);
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