使用 Java Stream API 进行多级分组和求和

Question

我有课

public class Person {

    private String name;
    private String country;
    private String city;
    private String pet;
    private int totalCountryToCityCount;
    private int petCount;

    public Person(String name, String country, String city, String pet, int total, int petCount) {
        this.name = name;
        this.country = country;
        this.city = city;
        this.pet = pet;
        this.totalCountryToCityCount = total;
        this.petCount = petCount;
    }

    public String getName() {
        return name;
    }

    public String getCountry() {
        return country;
    }

    public String getCity() {
        return city;
    }

    public String getPet() {
        return pet;
    }

    public int getPetCount() {
        return petCount;
    }

    public int getTotalCountryToCityCount() {
        return totalCountryToCityCount;

    }

}

并且给定一个 Person 类的列表，我已经根据类的不同属性进行聚合。 例如 -

Person person1 = new Person("John", "USA", "NYC", "Max", 1, 2);
        Person person2 = new Person("Steve", "UK", "London", "Lucy", 2, 8);
        Person person3 = new Person("Anna", "USA", "NYC", "Max", 4, 32);
        Person person4 = new Person("Mike", "USA", "Chicago", "Duke", 5, 1);
        Person person5 = new Person("Test", "INDIA", "HYD", "Tommy", 4, 32);
        Person person6 = new Person("Test1", "INDIA", "HYD", "Tommy", 4, 65);
        Person person7 = new Person("Tim", "USA", "Chicago", "Duke", 5, 111);
        Person person8 = new Person("Tim", "USA", "Chicago", "Puke", 5, 111);
        Person person9 = new Person("Test1", "INDIA", "DELHI", "Tommy", 4, 65);
List<Person> persons = Arrays
            .asList(person1, person2, person3, person4, person5, person6, person7, person8,
                person9);

现在我需要得到一个结果，即我应该根据国家和城市的组合获得总“totalCountryToCityCount”，并且我应该根据国家、城市和宠物的组合获得总“petCount”。我可以使用 groupingBy 和 summingint 分别获取它们

private Map<String, Map<String, Integer>> getTotalCountForCountry(List<Person> persons) {
        return persons.stream().collect(groupingBy(Person::getCountry, getCityCount()));
    }

    public Collector<Person, ?, Map<String, Integer>> getCityCount() {
        return groupingBy(Person::getCity, summingInt(Person::getTotal));
    }

    public Map<String, Map<String, Map<String, Integer>>> threeLevelGrouping(List<Person> persons) {
       return  persons
            .stream().collect(
                groupingBy(Person::getCountry, groupByCityAndPetName()
                )
            );
    }

    private Collector<Person, ?, Map<String, Map<String, Integer>>> groupByCityAndPetName() {
        return groupingBy(Person::getCity, groupByPetName());
    }

    private Collector<Person, ?, Map<String, Integer>> groupByPetName() {
        return groupingBy(Person::getPet, summingInt(Person::getPetCount));
    }

给出结果

{USA={Chicago={Puke=111, Duke=112}, NYC={Max=34}}, UK={London={Lucy=8}}, INDIA={DELHI={Tommy=65}, HYD={Tommy=97}}}
{USA={Chicago=15, NYC=5}, UK={London=2}, INDIA={DELHI=4, HYD=8}}

但我想要的实际结果是：-

{USA={Chicago={15,{Puke=111, Duke=112}}, NYC={5,{Max=34} }, UK={London={2, {Lucy=8}}, INDIA={DELHI={4, {Tommy=65}}, , HYD={8,{Tommy=97}}}}

有没有办法使用 Java 流 API 实现相同的功能

我也尝试过使用代码 -

 personList.stream().collect(groupingBy(person -> person.getCountry(), collectingAndThen(reducing(
            (a, b) -> new Person(a.getName(), a.getCountry(), a.getCity(), a.getPet(),
                a.getTotal() + b.getTotal(), a.getPetCount() + b.getPetCount())),
            Optional::get)))
            .forEach((country, person) -> System.out.println(country + person));

但是得到了结果 -

USAPerson{name='John', country='USA', city='NYC'}
UKPerson{name='Steve', country='UK', city='London'}
INDIAPerson{name='Test', country='INDIA', city='HYD'}

令人惊讶地删除了计数

Answer 1

您真正要找的是Collectors::teeing，但仅在java-12 中可用：

System.out.println(
        persons.stream()
               .collect(Collectors.groupingBy(
                   Person::getCountry,
                   Collectors.groupingBy(
                       Person::getCity,
                       Collectors.teeing(
                           Collectors.summingInt(Person::getTotalCountryToCityCount),
                           Collectors.groupingBy(
                               Person::getPet,
                               Collectors.summingInt(Person::getPetCount)
                           ),
                           SimpleEntry::new
                       )
                   ))));

java-8 的后端端口可用here。