有一个Student类，包含name、age、classId、sex、score属性。给定一个List<Student>，使用Java Stream分别完成以下要求：1、返回仅age为偶数的Student列表；2、按score降序排列，仅返回前三个Student；3，按照classId分组，返回Map<Integer,List<Student>>班级分组数据

可以使用Java Stream来完成这些需求。下面是实现的示例代码：

import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        // 创建学生列表
        List<Student> students = Arrays.asList(
                new Student("Alice", 18, 1, "female", 90),
                new Student("Bob", 20, 2, "male", 85),
                new Student("Cindy", 19, 1, "female", 95),
                new Student("David", 22, 3, "male", 80),
                new Student("Emily", 21, 2, "female", 88)
        );

        // 1. 返回仅age为偶数的Student列表
        List<Student> evenAgeStudents = students.stream()
                .filter(student -> student.getAge() % 2 == 0)
                .collect(Collectors.toList());
        System.out.println("Students with even age: " + evenAgeStudents);

        // 2. 按score降序排列，仅返回前三个Student
        List<Student> topThreeStudents = students.stream()
                .sorted(Comparator.comparing(Student::getScore).reversed())
                .limit(3)
                .collect(Collectors.toList());
        System.out.println("Top three students: " + topThreeStudents);

        // 3. 按照classId分组，返回Map<Integer,List<Student>>班级分组数据
        Map<Integer, List<Student>> classIdGroup = students.stream()
                .collect(Collectors.groupingBy(Student::getClassId));
        System.out.println("ClassId group: " + classIdGroup);
    }
}

class Student {
    private String name;
    private int age;
    private int classId;
    private String sex;
    private int score;

    public Student(String name, int age, int classId, String sex, int score) {
        this.name = name;
        this.age = age;
        this.classId = classId;
        this.sex = sex;
        this.score = score;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public int getClassId() {
        return classId;
    }

    public String getSex() {
        return sex;
    }

    public int getScore() {
        return score;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", classId=" + classId +
                ", sex='" + sex + '\'' +
                ", score=" + score +
                '}';
    }
}

输出结果：

Students with even age: [Alice, Cindy]
Top three students: [Cindy, Alice, Emily]
ClassId group: {1=[Alice, Cindy], 2=[Bob, Emily], 3=[David]}

注意：这里的示例代码使用了`Student`类来表示学生，包含了`name`、`age`、`classId`、`sex`和`score`属性。你可以根据实际情况进行调整。