Transform a List<Object> to a Map<String, Integer> such that the String is not a duplicate value using Java 8 Streams

Question

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;
    }
}

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));

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

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

So the output should be:

{ABC = 97, DEF = 40}

---

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));

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

{abc = 30, DEF = 40}

Can someone suggest an efficient solution for this?

Answer 1

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));

Answer 2

An alternative solution is to use groupingBy with summingInt:

Map<String, Integer> studentMap = studentList.stream()
        .collect(Collectors.groupingBy(
                s -> s.getStudentName().toLowerCase(),
                Collectors.summingInt(Student::getMarks)));