使用Java Stream高效处理列表去重：按ID保留最新记录

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

public class Student {
    private String id;
    private LocalDateTime startDatetime;

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

    public String getId() {
        return id;
    }

    public LocalDateTime getStartDatetime() {
        return startDatetime;
    }

    // 为了更好的演示和调试，建议重写toString方法
    @Override
    public String toString() {
        return "Student{" +
               "id='" + id + '\'' +
               ", startDatetime=" + startDatetime +
               '}';
    }

    // 重写equals和hashCode方法，虽然在此特定场景不直接用于去重，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.Map;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.stream.Collectors;

public class StudentDeduplicator {

    public static void main(String[] args) {
        // 示例数据
        List<Student> students = new ArrayList<>(List.of(
            new Student("1", LocalDateTime.now()), // 最新的id为1的学生
            new Student("1", LocalDateTime.of(2000, 2, 1, 1, 1)),
            new Student("1", LocalDateTime.of(1990, 2, 1, 1, 1)),
            new Student("2", LocalDateTime.of(1990, 2, 1, 1, 1)),
            new Student("3", LocalDateTime.of(2020, 5, 10, 10, 0)),
            new Student("3", LocalDateTime.of(2021, 1, 1, 0, 0)) // 最新的id为3的学生
        ));

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

        // 使用Stream进行去重，保留最新记录
        List<Student> uniqueStudents = students.stream()
            .collect(Collectors.toMap(
                Student::getId, // KeyMapper: 使用Student的ID作为Map的键
                Function.identity(), // ValueMapper: 将整个Student对象作为Map的值
                BinaryOperator.maxBy(Comparator.comparing(Student::getStartDatetime)) // MergeFunction: 当ID冲突时，保留startDatetime最大的那个
            ))
            .values() // 获取Map中所有的值（即去重后的Student对象）
            .stream() // 将值集合转换为新的Stream
            // 结果通常需要按某个顺序排列，例如按startDatetime排序
            .sorted(Comparator.comparing(Student::getStartDatetime))
            .toList(); // Java 16+ 使用.toList()，Java 8-15使用.collect(Collectors.toList())

        System.out.println("去重并排序后的学生列表:");
        uniqueStudents.forEach(System.out::println);
    }
}