集合类题，第二问要求灵活方式解，不要用stream，求写下代码和注释

有如下Student 类
public class Student{
private String name;
private int age;
private double score;
private String classNum;
//需要添加get和set
}
其中，classNum 表示学生的班号，例如“class05”。 有如下List 
List list=new ArrayList();
list.add(new Student("Tom", 18, 100, "class05"));
list.add(new Student("Jerry", 22, 70, "class04"));
list.add(new Student("Owen", 25, 90, "class05"));
list.add(new Student("Jim", 30, 80, "class05"));
list.add(new Student("Steve", 28, 66, "class06"));
list.add(new Student("Kevin", 24, 100, "class04"));
在这个list 的基础上，完成下列要求：
 1） 计算所有学生的平均年龄 
2） 计算各个班级的平均分

import java.util.ArrayList;
import java.util.List;


public class MyTest {
    public static void main(String[] args) {
        List<Student> list=new ArrayList<Student>();
        list.add(new Student("Tom", 18, 100, "class05"));
        list.add(new Student("Jerry", 22, 70, "class04"));
        list.add(new Student("Owen", 25, 90, "class05"));
        list.add(new Student("Jim", 30, 80, "class05"));
        list.add(new Student("Steve", 28, 66, "class06"));
        list.add(new Student("Kevin", 24, 100, "class04"));

        double sum=0 ,ave=0;
        for (Student student : list) {
            double score = student.getScore();
            sum+=score;
        }
        ave=sum/list.size();
        System.out.println(ave);
    }

 
 
    public static class Student {

        private String name;
        private Integer age;
        private double score;
        private String className;

        public Student(String name, Integer age, double score, String className) {
            this.name = name;
            this.age = age;
            this.score = score;
            this.className = className;
        }

        public void setName(String name) {
            this.name = name;
        }

        public void setAge(Integer age) {
            this.age = age;
        }

        public void setScore(double score) {
            this.score = score;
        }

        public void setClassName(String className) {
            this.className = className;
        }

        public String getName() {
            return name;
        }

        public Integer getAge() {
            return age;
        }

        public double getScore() {
            return score;
        }

        public String getClassName() {
            return className;
        }

    }
}