如何根据属性使用RxJava2对ArrayList进行分组？

import io.reactivex.Observable;
import io.reactivex.Observer;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Function;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class RxJavaGroupByExample {
    public static void main(String[] args) {
        List<Person> personList = new ArrayList<>();
        personList.add(new Person("Alice", 20));
        personList.add(new Person("Bob", 25));
        personList.add(new Person("Charlie", 20));
        personList.add(new Person("David", 30));
        personList.add(new Person("Eve", 25));

        Observable.fromIterable(personList)
                .groupBy(new Function<Person, Integer>() {
                    @Override
                    public Integer apply(Person person) throws Exception {
                        return person.getAge();
                    }
                })
                .subscribe(new Observer<GroupedObservable<Integer, Person>>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                    }

                    @Override
                    public void onNext(GroupedObservable<Integer, Person> group) {
                        group.subscribe(new Observer<Person>() {
                            @Override
                            public void onSubscribe(Disposable d) {
                            }

                            @Override
                            public void onNext(Person person) {
                                System.out.println("Age: " + group.getKey() + ", Person: " + person.getName());
                            }

                            @Override
                            public void onError(Throwable e) {
                            }

                            @Override
                            public void onComplete() {
                            }
                        });
                    }

                    @Override
                    public void onError(Throwable e) {
                    }

                    @Override
                    public void onComplete() {
                    }
                });
    }

    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;
        }
    }
}

Age: 20, Person: Alice
Age: 20, Person: Charlie
Age: 25, Person: Bob
Age: 25, Person: Eve
Age: 30, Person: David