stream groupby 排序

如果你想在对流对象进行`group by`操作后对每个组进行排序，可以在`groupingBy`方法的第二个参数中指定`Collectors.toList()`来收集元素，并在`toCollection`方法中传入一个自定义的排序比较器。以下是一个示例代码：

import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<Person> persons = List.of(
                new Person("Alice", 25),
                new Person("Bob", 30),
                new Person("Charlie", 20),
                new Person("Dave", 25),
                new Person("Eve", 30)
        );

        Map<Integer, List<Person>> groupedByAgeSorted = persons.stream()
                .collect(Collectors.groupingBy(
                        Person::getAge,
                        Collectors.toList()
                ));

        groupedByAgeSorted.forEach((age, group) -> {
            System.out.println("Age: " + age);
            System.out.println("Group: " + group);
            List<Person> sortedGroup = group.stream()
                    .sorted(Comparator.comparing(Person::getName))
                    .collect(Collectors.toList());
            System.out.println("Sorted Group: " + sortedGroup);
        });
    }

    static class Person {
        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public int getAge() {
            return age;
        }

        @Override
        public String toString() {