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:

the map does not contain any duplicate Student
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?

Community · Accepted Answer · 2020-12-28 04:22:02Z

7

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));

edited Dec 28, 2020 at 4:22

CommunityBot

11 silver badge

answered Jan 20, 2019 at 7:24

Naman

32.7k32 gold badges240 silver badges385 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2020-12-27 22:25:46Z

5

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)));

edited Dec 27, 2020 at 22:25

CommunityBot

11 silver badge

answered Jan 20, 2019 at 9:33

Eran

395k57 gold badges726 silver badges793 bronze badges

Comments

fps · Accepted Answer · 2019-01-21 03:15:28Z

3

Your merge function is incorrect. It could be either (s1, s2) -> s1 + s2 or just Integer::sum if you'd rather use a method reference.

Another way to do it is without streams:

Map<String, Integer> studentMap = new LinkedHashMap<>();
studentList.forEach(s -> studentMap.merge(
                         s.getStudentName().toLowerCase(),
                         s.getMarks(),
                         Integer::sum));

This iterates the list of students and uses the Map.merge method to group them by name, summing their marks.

answered Jan 21, 2019 at 3:15

fps

34.5k8 gold badges62 silver badges116 bronze badges

Comments

user14838237 · Accepted Answer · 2020-12-27 19:03:27Z

0

You can use Collectors.groupingBy(classifier,mapFactory,downstream) method for this purpose:

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

Map<String, Integer> map = list.stream()
        .collect(Collectors.groupingBy(
                Student::getStudentName,
                () -> new TreeMap<>(String.CASE_INSENSITIVE_ORDER),
                Collectors.summingInt(Student::getMarks)));

System.out.println(map); // {abc=97, DEF=40}

answered Dec 27, 2020 at 19:03

user14838237

Collectives™ on Stack Overflow

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

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related