Java Stream API：按共享属性聚合对象并合并日期范围

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Objects;

public class Entities {
    private final Date start_dt;
    private final Date stop_dt;
    private final int groupNum;

    // 构造函数，接受字符串日期并解析
    public Entities(String start_dt_str, String stop_dt_str, int groupNum) throws ParseException {
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
        this.start_dt = formatter.parse(start_dt_str);
        this.stop_dt = formatter.parse(stop_dt_str);
        this.groupNum = groupNum;
    }

    // 构造函数，接受Date对象
    public Entities(Date start_dt, Date stop_dt, int groupNum) {
        this.start_dt = start_dt;
        this.stop_dt = stop_dt;
        this.groupNum = groupNum;
    }

    // Getter方法
    public Date getStart_dt() {
        return start_dt;
    }

    public Date getStop_dt() {
        return stop_dt;
    }

    public int getGroupNum() {
        return groupNum;
    }

    @Override
    public String toString() {
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
        return "Entities [start_dt=" + formatter.format(start_dt) +
               ", stop_dt=" + formatter.format(stop_dt) +
               ", groupNum=" + groupNum + "]";
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Entities entities = (Entities) o;
        return groupNum == entities.groupNum &&
               Objects.equals(start_dt, entities.start_dt) &&
               Objects.equals(stop_dt, entities.stop_dt);
    }

    @Override
    public int hashCode() {
        return Objects.hash(start_dt, stop_dt, groupNum);
    }
}

import java.text.ParseException;
import java.util.List;
import java.util.stream.Collectors;

public class AggregationExample {

    public static void main(String[] args) throws ParseException {
        // 1. 创建初始列表
        List<Entities> baseList = List.of(
            new Entities("2018-11-13", "2019-01-13", 1),
            new Entities("2019-01-14", "2019-03-06", 1),
            new Entities("2019-03-07", "2019-11-18", 1),
            new Entities("2020-08-23", "2020-08-23", 2),
            new Entities("2021-11-19", "2022-12-23", 2));

        // 2. 使用Stream API进行聚合
        List<Entities> result = baseList.stream()
            // 2.1. 按groupNum分组，结果为Map<Integer, List<Entities>>
            .collect(Collectors.groupingBy(Entities::getGroupNum))
            // 2.2. 将Map转换为Stream<Map.Entry<Integer, List<Entities>>>
            .entrySet().stream()
            // 2.3. 对每个分组进行映射，生成新的Entities对象
            .map(entry -> {
                List<Entities> groupEntities = entry.getValue();
                // 2.4. 提取最早的start_dt（列表第一个元素的start_dt）
                // 2.5. 提取最晚的stop_dt（列表最后一个元素的stop_dt）
                // 注意：这里假设groupEntities列表内部已按日期排序
                return new Entities(
                    groupEntities.get(0).getStart_dt(), // 获取组内第一个元素的开始日期
                    groupEntities.get(groupEntities.size() - 1).getStop_dt(), // 获取组内最后一个元素的结束日期
                    entry.getKey() // 获取当前组的groupNum
                );
            })
            // 2.6. 将结果收集为List<Entities>
            .toList(); // Java 16+，等同于.collect(Collectors.toList())

        // 打印聚合结果
        result.forEach(System.out::println);
    }
}

Entities [start_dt=2018-11-13, stop_dt=2019-11-18, groupNum=1]
Entities [start_dt=2020-08-23, stop_dt=2022-12-23, groupNum=2]