[英]How to get list of ids when using groupingBy in java lambda expression
假设我们有一个对象User
:
public class User {
private Integer id;
private Integer age;
private String name;
private String family;
public User() {}
public User(Integer id, Integer age, String name, String family) {
this.id = id;
this.age = age;
this.name = name;
this.family = family;
}
// getters and setters emited for brevity
}
我编写了如下的 lambda 表达式,以根据对某些字段进行分组来获得对象的平均年龄:
public static void main(String[] args) {
List<User> allUsers = new ArrayList<>();
allUsers.add(new User(1, 25, "Rasool", "Ghafari"));
allUsers.add(new User(2, 27, "Ali", "Sohrabi"));
allUsers.add(new User(3, 25, "Ali", "Azizkhani"));
allUsers.add(new User(4, 25, "Rasool", "Asadi"));
allUsers.add(new User(5, 27, "Ali", "Rasooli Sa'd Abad"));
allUsers.add(new User(6, 26, "Rasool", "Ameri"));
Map<List<Object>, Long> groupedResult =
allUsers.stream().collect(
groupingBy(
u -> Arrays.asList(u.getName()),
averagingDouble(User::getAge)
)
);
// result:
// {[Rasool] = 25.33, [Ali] = 26.33}
}
现在,我想在这个 lambda 中添加一些东西,但不知道该怎么做。 我想获取每个组中分组对象的id
列表,可能是这样的:
// {[Rasool, [1, 4, 6]] = 25.33, [Ali, [2, 3, 5]] = 26.33}
一点改变就足够了:
Map<List<Object>, List<User>> groupedResult =
allUsers.stream().collect(
groupingBy(
u -> Arrays.asList(u.getName(), u.getAge()),
Collectors.toList()
)
);
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