loops - 如何只迭代一次java流

Question

我有资料 Map<String, List<Students>>其中键是主题名称。
我想遍历每个科目并添加与学生对应的其他科目的分数。
我试过以下

wing:

    data.entrySet().stream().forEach(subject -> sumScoresOfAStudent(subject, data));
    
    private void sumScoresOfAStudent(Entry<String, List<Student>> subject, Map<String, List<Student>> data) {
            String firstSubject = subject.getKey();
            List<Student> firstSubjectScores = subject.getValue();
            List<Student> otherSubjectScores = data.entrySet().stream()
                    .filter(subjectData -> !subjectData.getKey().equalsIgnoreCase(firstSubject))
                    .map(subjectData -> subjectData.getValue()).flatMap(subjectData -> subjectData.stream())
                    .collect(Collectors.toList());
            ...
        }

但是在这里我注意到如果我有 3 个主题作为我的 Key，它会为相同的数据循环 3 次。有没有其他更好的方法只迭代一次？
在此先感谢您的帮助。

Eg:
        Student s11 = new Student("A", 20);
        Student s21 = new Student("B", 10);
        data.put("Maths", students);
        Student s12 = new Student("A", 30);
        Student s22 = new Student("B", 40);
        data.put("Science", students);
        Student s13 = new Student("A", 45);
        Student s23 = new Student("B", 20);
        data.put("History", students);

Expected Output :
Student A - 95
Student B - 70

Answer 1

对不起，我完全误解了你想要什么。这假定 Student 类具有 setter/getter getScore和 getName .这只是需要 map values分别是 lists of students并将它们压平成一个大流并计算每个学生的分数总和。

Map<String, Integer> scores =  data.values().stream().flatMap(List::stream)
        .collect(Collectors.groupingBy(Student::getName,
                Collectors.summingInt(Student::getScore)));

scores.forEach((name,score) -> System.out.printf("Student %s - %s%n", 
              name,score));

打印

Student A - 95
Student B - 70