Java 有关类的问题

问题遇到的现象和发生背景

问题相关代码，请勿粘贴截图

import java.util.Scanner;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int sum = 0;
        Cla cc = new Cla();
        cc.num = 0;
        while(sc.hasNext()){
            Student s = new Student();
            s.name = sc.next();
            s.id = sc.next();
            s.sex = sc.next();
            s.age = sc.nextInt();
            cc.stu[cc.num] = s;
            sum +=cc.stu[cc.num].age;
            cc.num ++;
        }
        double avg = (double)sum/ (double)cc.num;

        System.out.println("班级学生"+cc.num+"人，平均年龄"+ String.format("%.1f",avg) +".");
    }
}

class Student{
    String name;
    String id;
    int age;
    String sex;
}

class Cla{
    Student[] stu = new Student[100];
    int num;
}

才学Java几天，不知道怎么做了

package com.azang.java;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util .List;
import java.util.Scanner;

public class ByteCodeInterview {
    public static void main(String[] args) {
        List<Student> studentList = new ArrayList<>();
        Scanner sc = new Scanner(System.in);
        while (sc.hasNextLine()) {
            String str = sc.nextLine();
            if (str.isEmpty()) {
                break;
            } else {
                String[] split = str.split(" ");
                Student student = new Student(split[0], split[1], Integer.parseInt(split[2]), split[3]);
                studentList.add(student);
            }
        }
        sc.close();
        BigDecimal avg = BigDecimal.ZERO;
        for (Student student : studentList) {
            // 四舍五入除法保留两位小数
            BigDecimal divide = BigDecimal.valueOf(student.age).divide(BigDecimal.valueOf(studentList.size()), 2, BigDecimal.ROUND_HALF_UP);
            avg = avg.add(divide);
        }
        System.out.println("班级学生" + studentList.size() + "人，平均年龄" + String.format("%.1f", avg) + ".");
    }
}

class Student {
    String name;
    String id;
    int age;
    String sex;

    public Student() {
    }

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

class Cla {
    Student[] stu = new Student[100];
    int num;
}

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