for循环嵌套的实例，很容易理解和掌握哦

import java.util.Scanner;

public class Jisuan {
public static void main(String[] args) {
int classNum = 3;
int stuNum = 4;
double sum = 0;
double avg = 0;
Scanner input = new Scanner(System.in);
for (int i = 1; i <= 3; i++) {
sum=0;
System.out.println("请输入第" + i + "个班级的成绩：");
for (int j = 1; j <= 4; j++) {
System.out.print("请输入第" + j + "学生的成绩:");
int score = input.nextInt();
sum += score;
}
avg=sum/stuNum;
System.out.println("第"+i+"个班级的j个学生的平均分是："+avg);
}

}
}