Java Stream中自定义Collector实现复杂数据收集的方法

List<String> list = Stream.of("a", "b", "c")
.collect(Collectors.toList());

Map<Character, List<String>> grouped = Stream.of("a", "b", "c")
.collect(Collectors.groupingBy(s -> s.charAt(0)));

String joined = Stream.of("a", "b", "c")
.collect(Collectors.joining(", "));

public static class CustomCollector<T> implements Collector<Person, List<Person>, Map<String, List<Person>>> {
    @Override
    public Supplier<List<Person>> supplier() {
        return ArrayList::new;
    }

    @Override
    public BiConsumer<List<Person>, Person> accumulator() {
        return (list, person) -> list.add(person);
    }

    @Override
    public BinaryOperator<List<Person>> combiner() {
        return (list1, list2) -> {
            list1.addAll(list2);
            return list1;
        };
    }

    @Override
    public Function<List<Person>, Map<String, List<Person>>> finisher() {
        return list -> {
            Map<String, List<Person>> result = new HashMap<>();
            // 自定义排序规则
            list.sort((p1, p2) -> {
                if (p1.age != null && p2.age != null) {
                    return p1.age.compareTo(p2.age);
                } else if (p1.age != null) {
                    return -1;
                } else if (p2.age != null) {
                    return 1;
                } else {
                    return p1.name.compareTo(p2.name);
                }
            });

            // 自定义分组规则
            for (Person person : list) {
                if (result.containsKey(person.name.substring(0, 2))) {
                    result.get(person.name.substring(0, 2)).add(person);
                } else {
                    List<Person> group = new ArrayList<>();
                    group.add(person);
                    result.put(person.name.substring(0, 2), group);
                }
            }
            return result;
        };
    }

    @Override
    public Set<Collector.Characteristics> characteristics() {
        return EnumSet.of(Collector.Characteristics.IDENTITY_FINISH);
    }
}

import java.util.*;
public class CustomCollectorDemo {
    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
                new Person("Alice", 25),
                new Person("Bob", 23),
                new Person("Charlie", 28),
                new Person("David", 25),
                new Person("Eva", 23),
                new Person("Frank", 28)
        );
        Map<String, List<Person>> result = people.stream()
                .collect(new CustomCollector<>());
        System.out.println(result);
    }

    static class Person {
        String name;
        Integer age;
        ...
    }
}