如何从HashMap的内部是ArrayList的HashMap的等级计算平均值？ [重复]

Question

该问题与以下内容完全相同：

• 我确实将HashMap值转换为我无法转换为String类型的值；无法转换为double值以对其进行数学运算

我已经问过这个问题，但后来情况有所不同。 现在，我在HashMap中获得了一个ArrayList。 老实说，我不知道如何在HashMap中为每个学生计算平均值。 程序无法在size（）和lenght（）上进行操作

我尝试使用for-loop和for-each，在两种情况下都失败了。

我已经问过这个问题，但后来情况有所不同。 现在，我在HashMap中获得了一个ArrayList。 老实说，我不知道如何在HashMap中为每个学生计算平均值。 程序无法在size（）和lenght（）上进行操作

我尝试使用for-loop和for-each，在两种情况下都失败了。

import java.util.*;
import java.lang.*;
import java.io.*;

class Student{
    String firstName;
    String lastName;

    public Student(String firstName, String lastName){
        this.firstName = firstName;
        this.lastName = lastName;
}

    @Override
    public boolean equals(Object o){
    final Student e = (Student) o;
    return this.firstName.equals(e.firstName) && this.lastName.equals(e.lastName);
}
public String toString(){
    return firstName + " " + lastName;
}
}

class Grades {private List grades = new ArrayList <>（）;

/*public Grades(List<Double> grades){
    this.grades = grades;
}*/
@Override
public boolean equals(Object o){
    final Grades e = (Grades) o;
    return this.grades.equals(e.grades);
}
public void addGrade(double grade){
    grades.add(grade);
}
public String toString(){
    return grades.toString();
}

}类StudentsAndGradesAndAverages {

public static void main (String[] args) throws java.lang.Exception{
    Student schoolar1 = new Student("Marcin", "Kossakowski");
    Student schoolar2 = new Student("Maria", "Salomea");
    Student schoolar3 = new Student("Paulina", "Szymborska");
    Student schoolar4 = new Student("Mariusz", "Januszowski");

    Grades gradesList1 = new Grades();
    gradesList1.addGrade(4);
    gradesList1.addGrade(2);
    gradesList1.addGrade(3);
    gradesList1.addGrade(2);
    gradesList1.addGrade(4);
    gradesList1.addGrade(5);

    Grades gradesList2 = new Grades();
    gradesList2.addGrade(5);
    gradesList2.addGrade(4);
    gradesList2.addGrade(5);
    gradesList2.addGrade(5);
    gradesList2.addGrade(2);
    gradesList2.addGrade(3);

    Grades gradesList3 = new Grades();
    gradesList3.addGrade(5);
    gradesList3.addGrade(6);
    gradesList3.addGrade(2);
    gradesList3.addGrade(2);
    gradesList3.addGrade(6);
    gradesList3.addGrade(4);

    Grades gradesList4 = new Grades();
    gradesList4.addGrade(5);
    gradesList4.addGrade(2);
    gradesList4.addGrade(3);
    gradesList4.addGrade(5);
    gradesList4.addGrade(4);
    gradesList4.addGrade(5);

    HashMap<Student, Grades> studentAndGradesList = new HashMap<Student, Grades>();

    studentAndGradesList.put(schoolar1, gradesList1);
    studentAndGradesList.put(schoolar2, gradesList2);
    studentAndGradesList.put(schoolar3, gradesList3);
    studentAndGradesList.put(schoolar4, gradesList4);

    System.out.println("Our students grades are: \n ");    

    for(Map.Entry<Student, Grades> entry: studentAndGradesList.entrySet()){
        System.out.println(entry.getKey() + " grades are " + entry.getValue());
    }
    double sum=0;

}
}

我想计算平均分数，而不管分数是多少。