转换List< Object>到Map< String，Integer>这样使用Java 8 Streams字符串就不会是重复的值

问题描述

我们有一个Student类，如下所示:

We have a Student class as follows:

class Student {
    private int marks;
    private String studentName;

    public int getMarks() {
        return marks;
    }

    public void setMarks(int marks) {
        this.marks = marks;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public Student(String studentName, int marks) {
        this.marks = marks;
        this.studentName = studentName;
    }
}

我们有一个学生名单，如下:

We have a LIST of Students as follows :

List<Student> studentList = new ArrayList<>();
studentList.add(new Student("abc", 30));
studentList.add(new Student("Abc", 32));
studentList.add(new Student("ABC", 35));
studentList.add(new Student("DEF", 40));

此列表需要转换为HashMap<String, Integer>，以便:

This List needs to be converted into a HashMap<String, Integer> such that:

1. 地图上没有重复的学生
2. 如果发现重复的学生姓名，则应在他的标记后加上以前的事件.

1. the map does not contain any duplicate Student
2. if a duplicate student name is found, his marks shall be added withthe previous occurrence.

因此输出应为:

{ABC = 97, DEF = 40}

---

我尝试如下解决此问题:

---

I have tried to solve this issue as follows:

Map<String, Integer> studentMap = studentList.stream()
        .collect(Collectors.toMap(
                student -> student.getStudentName().toLowerCase(),
                student -> student.getMarks(),
                (s1, s2) -> s1,
                LinkedHashMap::new));

但是merge函数不允许我连接标记，这会将输出返回为:

But the merge function does not allow me to concatenate the marks and this returns the output as:

{abc = 30, DEF = 40}

有人可以为此建议一个有效的解决方案吗?

Can someone suggest an efficient solution for this?

推荐答案

那是因为合并功能不正确，您应该改用:

That's because of an incorrect merge function, you should instead use:

Map<String, Integer> map = studentList.stream()
        .collect(Collectors.toMap(
                student -> student.getStudentName().toLowerCase(),
                Student::getMarks,
                (s1, s2) -> s1 + s2, // add values when merging
                LinkedHashMap::new));

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