从流Api计算映射中的值之和

Question

我是个十足的新手，所以我请你不要说太多脏话。）

有一个集合（ArrayList）保存学生类的对象(

 sumOnSubjectInGroup = studentsInGroup.stream()
        .filter(e -> e.getRating().entrySet().iterator().next().getKey().equals(subject))
        .mapToInt(e -> e.getRating().entrySet().iterator().next().getValue())
        .reduce(0, (x, y) -> x + y);
 countMarkInGroup = (int) studentsInGroup.stream()
        .filter(e -> e.getRating().entrySet().iterator().next().getKey().equals(subject))
        .count();
 return (double) sumOnSubjectInGroup / countMarkInGroup;

其中SumonSubjectionGroup是所有学生科目分数的总和。CountMarkingGroup—此类标记的数量。studentsInGroup是包含所有学生对象的集合。getRating（）-返回一个HashMap

Answer 1

您可以使用收集器。Averaging计算平均值，无需按科目单独计算总和和计数。

import java.util.*;
import java.util.stream.*;

class Student {
    String name;
    Map<String, Integer> rating;
    
    public Student(String name, Map<String, Integer> grades) {
        this.name = name;
        this.rating = grades;
    }
    
    public String getName() { return name; }
    public Map<String, Integer> getRating() { return rating; }
}

public class Main {
    public static void main(String args[]) {
        List<Student> students = Arrays.asList(
            new Student("John", Map.of("Math", 90, "Physics", 94, "Chemistry", 93, "English", 92)),
            new Student("Jack", Map.of("Math", 73, "Physics", 76, "English", 89)),
            new Student("Jane", Map.of("Math", 87, "Chemistry", 91, "French", 94)),
            new Student("Joel", Map.of("Math", 81, "Physics", 79, "French", 88))
        );
        students
            .stream()
            .flatMap(s -> s.getRating().entrySet().stream())
            .collect(Collectors.groupingBy(
                Map.Entry::getKey, Collectors.averagingInt(Map.Entry::getValue)))
            .entrySet()
            .forEach(System.out::println);
    }
}

输出：

English=90.5
Chemistry=92.0
French=91.0
Math=82.75
Physics=83.0

Answer 2

您可以使用。平均（）来计算平均值，并使用包含密钥来检查地图中是否存在以筛选并获得分数。

Double average = studentsInGroup.stream()
            .filter(e -> e.getRating().containsKey(subject))
            .mapToInt(e -> e.getRating().get(subject))
            .average()
            .orElse(0.0);