Java Stream API：实现多条件过滤、按月分组及聚合计数

import java.time.LocalDate;
import java.util.Objects;

// 事件类型枚举
enum State {
    JOIN, EXIT, OTHER
}

// 原始Person数据模型
class Person {
    private String id;
    private String name;
    private String surname;
    private State event; // JOIN, EXIT
    private Object value; // 示例中未使用，可根据实际需求扩展
    private LocalDate eventDate;

    public Person(String id, State event, LocalDate eventDate) {
        this.id = id;
        this.event = event;
        this.eventDate = eventDate;
    }

    // Getters
    public String getId() { return id; }
    public State getEvent() { return event; }
    public LocalDate getEventDate() { return eventDate; }

    @Override
    public String toString() {
        return "Person{" +
               "id='" + id + '\'' +
               ", event=" + event +
               ", eventDate=" + eventDate +
               '}';
    }
}

// 聚合结果DTO
class DTO {
    private int month;
    private State info;
    private int totalEmployees;

    public DTO(int month, State info, int totalEmployees) {
        this.month = month;
        this.info = info;
        this.totalEmployees = totalEmployees;
    }

    // Getters
    public int getMonth() { return month; }
    public State getInfo() { return info; }
    public int getTotalEmployees() { return totalEmployees; }

    @Override
    public String toString() {
        return "DTO{" +
               "month=" + month +
               ", info=" + info +
               ", totalEmployees=" + totalEmployees +
               '}';
    }
}

// 分组键辅助类 (Java 16+ 可以使用 record)
// 对于Java 8-15，需要手动实现 equals() 和 hashCode()
record MonthState(int month, State info) {}

/*
// Java 8-15 版本的 MonthState 类
class MonthState {
    private final int month;
    private final State info;

    public MonthState(int month, State info) {
        this.month = month;
        this.info = info;
    }

    public int getMonth() { return month; }
    public State getInfo() { return info; }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        MonthState that = (MonthState) o;
        return month == that.month && info == that.info;
    }

    @Override
    public int hashCode() {
        return Objects.hash(month, info);
    }
}
*/

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

public class StreamAggregationTutorial {

    public static void main(String[] args) {
        // 模拟原始数据
        Map<String, List<Person>> personListById = Map.of(
            "per1", List.of(new Person("per1", State.JOIN, LocalDate.of(2022, 1, 10))),
            "per2", List.of(new Person("per2", State.JOIN, LocalDate.of(2022, 1, 10))),
            "per3", List.of(
                new Person("per3", State.EXIT, LocalDate.of(2022, 1, 10)),
                new Person("per3", State.EXIT, LocalDate.of(2022, 2, 10))
            ),
            "per4", List.of(new Person("per4", State.JOIN, LocalDate.of(2022, 3, 10))),
            "per5", List.of(new Person("per5", State.OTHER, LocalDate.of(2022, 4, 10))) // 应该被过滤掉
        );

        List<DTO> result = personListById.values().stream()
            // 1. 扁平化处理：将Map中List<Person>的值转换为单个Person对象的Stream
            .flatMap(List::stream)
            // 2. 多条件过滤：只保留事件类型为JOIN或EXIT的Person
            .filter(person -> person.getEvent() == State.JOIN || person.getEvent() == State.EXIT)
            // 3. 分组与计数：
            //    - 使用Collectors.groupingBy()进行分组。
            //    - 分组键是一个MonthState对象，结合了月份和事件类型。
            //    - 下游收集器Collectors.counting()用于统计每个分组的元素数量。
            .collect(Collectors.groupingBy(
                p -> new MonthState(p.getEventDate().getMonthValue(), p.getEvent()),
                Collectors.counting()
            ))
            // 4. 将Map的entrySet转换为Stream，以便进行后续的映射操作
            .entrySet().stream()
            // 5. 映射：将Map.Entry<MonthState, Long>转换为DTO对象
            //    - e.getKey()获取MonthState对象（分组键）
            //    - e.getValue()获取计数结果（Long类型，需要转换为int）
            .map(e -> new DTO(e.getKey().month(), e.getKey().info(), e.getValue().intValue()))
            // 6. 排序：根据DTO的月份进行排序
            .sorted(Comparator.comparing(DTO::getMonth))
            // 7. 收集：将Stream中的DTO对象收集到List中
            .toList(); // Java 16+，等同于 .collect(Collectors.toList())

        // 打印结果
        result.forEach(System.out::println);
    }
}