使用自定义收集器进行Java 8分组？ [英] Java 8 grouping using custom collector?
问题描述
我有以下课程。
class Person {
String name;
LocalDate birthday;
Sex gender;
String emailAddress;
public int getAge() {
return birthday.until(IsoChronology.INSTANCE.dateNow()).getYears();
}
public String getName() {
return name;
}
}
我希望能够按年龄分组然后收集人名列表而不是Person对象本身;所有这些都在一个漂亮的lamba表达式中。
I'd like to be able to group by age and then collect the list of the persons names rather than the Person object itself; all in a single nice lamba expression.
为了简化所有这一切，我将链接我当前的解决方案，该解决方案按年龄存储分组结果，然后迭代它以收集名称。
To simplify all of this I am linking my current solution that store the result of the grouping by age and then iterates over it to collect the names.
ArrayList<OtherPerson> members = new ArrayList<>();
members.add(new OtherPerson("Fred", IsoChronology.INSTANCE.date(1980, 6, 20), OtherPerson.Sex.MALE, "fred@example.com"));
members.add(new OtherPerson("Jane", IsoChronology.INSTANCE.date(1990, 7, 15), OtherPerson.Sex.FEMALE, "jane@example.com"));
members.add(new OtherPerson("Mark", IsoChronology.INSTANCE.date(1990, 7, 15), OtherPerson.Sex.MALE, "mark@example.com"));
members.add(new OtherPerson("George", IsoChronology.INSTANCE.date(1991, 8, 13), OtherPerson.Sex.MALE, "george@example.com"));
members.add(new OtherPerson("Bob", IsoChronology.INSTANCE.date(2000, 9, 12), OtherPerson.Sex.MALE, "bob@example.com"));
Map<Integer, List<Person>> collect = members.stream().collect(groupingBy(Person::getAge));
Map<Integer, List<String>> result = new HashMap<>();
collect.keySet().forEach(key -> {
result.put(key, collect.get(key).stream().map(Person::getName).collect(toList()));
});
不理想，为了学习，我希望有一个更优雅和更好的解决方案。
Not ideal and for the sake of learning I'd like to have a more elegant and performing solution.
推荐答案
使用 Collectors.groupingBy ，你可以指定一个使用自定义收集器对值进行缩减操作。在这里，我们需要使用 Collectors.mapping ，它接受一个函数（映射是什么）和一个收集器（如何收集映射的值）。在这种情况下，映射是 Person :: getName ，即返回Person名称的方法引用，我们将其收集到 List 。
When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns the name of the Person, and we collect that into a List.
Map<Integer, List<String>> collect =
members.stream()
.collect(Collectors.groupingBy(
Person::getAge,
Collectors.mapping(Person::getName, Collectors.toList()))
);
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