Java Stream实战：高效处理对象列表去重，保留最新记录

import java.time.LocalDateTime;
import java.util.Objects; // 导入Objects类用于hashCode和equals

class Student {
    private String id;
    private LocalDateTime startDatetime;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public LocalDateTime getStartDatetime() {
        return startDatetime;
    }

    public void setStartDatetime(LocalDateTime startDatetime) {
        this.startDatetime = startDatetime;
    }

    public Student(String id, LocalDateTime startDatetime) {
        this.id = id;
        this.startDatetime = startDatetime;
    }

    @Override
    public String toString() {
        return "Student{id='" + id + "', startDatetime=" + startDatetime + "}";
    }

    // 建议重写equals和hashCode，尽管在此特定Stream操作中非必需，但良好的实践
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(id, student.id) && Objects.equals(startDatetime, student.startDatetime);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, startDatetime);
    }
}

import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;

public class StudentProcessor {

    public static void main(String[] args) {
        // 示例数据
        List<Student> students = new ArrayList<>() {{
            add(new Student("1", LocalDateTime.now())); // 最新时间
            add(new Student("1", LocalDateTime.of(2000, 2, 1, 1, 1))); // 较旧时间
            add(new Student("1", LocalDateTime.of(1990, 2, 1, 1, 1))); // 最旧时间
            add(new Student("2", LocalDateTime.of(1990, 2, 1, 1, 1))); // 唯一ID
            add(new Student("3", LocalDateTime.of(2023, 1, 1, 1, 1))); // 唯一ID
            add(new Student("3", LocalDateTime.of(2022, 1, 1, 1, 1))); // 较旧时间
        }};

        System.out.println("原始学生列表:");
        students.forEach(System.out::println);
        System.out.println("\n---");

        // 使用Stream进行去重和筛选
        List<Student> uniqueStudents = students.stream()
            .collect(Collectors.toMap(
                Student::getId, // keyMapper: 以id作为键
                Function.identity(), // valueMapper: 以Student对象本身作为值
                BinaryOperator.maxBy(Comparator.comparing(Student::getStartDatetime)) // mergeFunction: 冲突时保留startDatetime最新的
            ))
            .values() // 获取Map中所有的值（即去重后的Student对象）
            .stream() // 将Map的值再次转换为Stream
            .sorted(Comparator.comparing(Student::getStartDatetime)) // 可选：根据startDatetime排序
            .toList(); // 将Stream收集为List (Java 16+) 或 Collectors.toList() (Java 8-15)

        System.out.println("去重并保留最新记录后的学生列表 (按日期排序):");
        uniqueStudents.forEach(System.out::println);
    }
}

原始学生列表:
Student{id='1', startDatetime=2023-10-27T10:30:00.123456}
Student{id='1', startDatetime=2000-02-01T01:01}
Student{id='1', startDatetime=1990-02-01T01:01}
Student{id='2', startDatetime=1990-02-01T01:01}
Student{id='3', startDatetime=2023-01-01T01:01}
Student{id='3', startDatetime=2022-01-01T01:01}

---
去重并保留最新记录后的学生列表 (按日期排序):
Student{id='2', startDatetime=1990-02-01T01:01}
Student{id='1', startDatetime=2023-10-27T10:30:00.123456} // 注意：实际运行时的LocalDateTime.now()会有毫秒级差异
Student{id='3', startDatetime=2023-01-01T01:01}