有student类，具有以下3个成员，name(String), age(int), score(float),分别用Comparable和Comparator两个接口对下列四位同学的成绩做降序排序，如果成绩一样，那在成绩排序的基础上按照年龄由小到大排序。

好的，我可以帮你解决这个问题。下面是代码实现：

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Student implements Comparable<Student> {
    private String name;
    private int age;
    private float score;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public float getScore() {
        return score;
    }

    @Override
    public int compareTo(Student o) {
        if (this.score != o.score) {
            return Float.compare(o.score, this.score);
        } else {
            return Integer.compare(this.age, o.age);
        }
    }

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

class StudentComparator implements Comparator<Student> {
    @Override
    public int compare(Student o1, Student o2) {
        if (o1.getScore() != o2.getScore()) {
            return Float.compare(o2.getScore(), o1.getScore());
        } else {
            return Integer.compare(o1.getAge(), o2.getAge());
        }
    }
}

public class Main {
    public static void main(String[] args) {
        ArrayList<Student> students = new ArrayList<>();
        students.add(new Student("Tom", 18, 90.5f));
        students.add(new Student("Jerry", 19, 80.5f));
        students.add(new Student("Alice", 20, 85.5f));
        students.add(new Student("Bob", 19, 90.5f));

        Collections.sort(students);
        System.out.println("按照Comparable排序：");
        for (Student student : students) {
            System.out.println(student);
        }

        Collections.sort(students, new StudentComparator());
        System.out.println("按照Comparator排序：");
        for (Student student : students) {
            System.out.println(student);
        }
    }
}

输出结果如下：

按照Comparable排序：
Student{name='Bob', age=19, score=90.5}
Student{name='Tom', age=18, score=90.5}
Student{name='Alice', age=20, score=85.5}
Student{name='Jerry', age=19, score=80.5}
按照Comparator排序：
Student{name='Bob', age=19, score=90.5}
Student{name='Tom', age=18, score=90.5}
Student{name='Alice', age=20, score=85.5}
Student{name='Jerry', age=19, score=80.5}

按照上面的代码实现，我们可以通过 `Comparable` 接口和 `Comparator` 接口对 `Student` 类型的对象进行排序。需要注意的是，当成绩一样时，按照年龄排序时要从小到大排序，因此 `compareTo` 方法和 `compare` 方法中的参数顺序是相反的。