多个参数的Java流averagingInt

Question

我有一堂课

static class Student {

    private String surname;
    private String firstName;
    private String secondName;
    private int yearOfBirth;
    private int course;
    private int groupNumber;
    private int mathGrade;
    private int engGrade;
    private int physicGrade;
    private int programmingGrade;
    private int chemistryGrade;

并且有一种方法可以将学生添加到课程的地图中

public Map<Integer, Double> averageInt(List<Student> students) {
            Map<Integer, Double> map2 = students.stream()
                    .collect(Collectors.groupingBy(Student::getCourse,
                            Collectors.averagingInt(Student::getEngGrade)));
            return map2;
        }

但是，我需要同时在一张地图中获得多个平均值。不仅engGrade，还有mathGrade、programmingGrade等等。我认为这种情况下的代码应该是格式，Map<Integer, List<Double>> 但我不知道该怎么做。请告诉我

例如，我现在显示 "Course = 1, average eng grade = ..."

而且我需要显示"Course = 1, average eng grade = ..., average math grade = ..."，即，以便在地图中有多个 Double 值？？

Answer 1

我建议使用这种方法

    public static Map<Integer, Double> averageInt(List<Student> students, ToIntFunction<? super Student> mapper) {
        Map<Integer, Double> map2 = students.stream()
                .collect(Collectors.groupingBy(Student::getCourse, Collectors.averagingInt(mapper)));
        return map2;
    }

并像这样使用它

Student.averageInt(students, Student::getMathGrade);
Student.averageInt(students, Student::getProgrammingGrade);

Answer 2

您可以使用 HashMap 的 HashMap。外部哈希映射键是课程编号，值是一个 EnumMap，其中包含您想要的特定课程平均值的枚举。

public enum Averages {
    MATHAVG, PROGRAMMINGAVG, etc..
}

从中检索就像

courseMap.get(courseNumber).get(Averages.MATHAVG);