java - 基于城市的 Collection 家分组

Question

我有一个人对象，它有一个名字和一个地址列表作为参数。地址有街道、类型、城市和 personId我想按城市获取分组 map 。我卡住了

到目前为止，这是我的代码:

Map<String,List<Person>> MAP = personRepository.findAll().stream() 
         .collect(Collectors.groupingBy(person->person.getAddresses().stream()
         .map(address -> address.getCity())
         ."some kind of collector I assume"))

Answer 1

您可以使用 flatMap 来做到这一点:

Map<String, List<Person>> finalPersonMap = personRepository.findAll().stream()
            .flatMap(person -> person.getAddresses().stream()
                    .map(address -> new AbstractMap.SimpleEntry<>(address.getCity(), person)))
            .collect(Collectors.groupingBy(Map.Entry::getKey,
                    Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

假设基本模型如下:

static class Person {
    List<Address> addresses;

    List<Address> getAddresses() {
        return addresses;
    }
}

static class Address {
    String city;

    String getCity() {
        return city;
    }
}