Transform a List<Object> to a Map 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() {
        r         


        

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

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.

Answer 3

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}

Answer 4

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)));