Map a list and map into a map

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@Getter 
static class Student {
    private Map<Status, Marks> statusAndMarks; 
}

class Status {
}

class Marks {
}

Map<Status, Student> map = new HashMap<>();
for (Student student : studentList) {
    for (Status status : student.getStatusAndMarks().keySet()) {
        map.put(status, student);
    }
}

How do I write the above written code in functional style in Java 8? I have written the following code but how do I write that the result of my code is an immutable map and I do not need to create a map before the lambda?

studentList
    .forEach(student -> {
        student
        .getStatusAndMarks()
        .keySet().stream()
        .map(key -> map.put(key, statusKeyImportantMessage));
        });
java-stream java-8 java
2 Answers

You can try something like this:

studentList.stream()
        .flatMap(student -> student.getStatusAndMarks()
                .keySet().stream().map(status -> Map.entry(status, student)))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

Map.entry is from Java 9. For lower Java it can be replaced with:

status -> new AbstractMap.SimpleEntry(status, student)
user_117337590

You were close enough if you would have used forEach for iterating the keyset again as:

Map<Status, Student> map = new HashMap<>();
studentList.forEach(student -> 
        student.getStatusAndMarks().keySet()
                .forEach(k -> map.put(k,student)));

A similar representation as stated in another answer for this would be 

Map<Status, Student> map = studentList.stream()
        .flatMap(s -> s.getStatusAndMarks().keySet()
                .stream()
                .map(status -> new AbstractMap.SimpleEntry<>(status, s)))
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> b));

Note, the merge function is to overwrite a new student with the same status found later in the iteration. On the other hand, what could be really handy information to query would be a list of students given a status. These can be grouped using a similar stream pipeline as following:

Map<Status, List<Student>> statusWiseStudent = studentList.stream()
        .flatMap(s -> s.getStatusAndMarks().keySet()
                .stream()
                .map(status -> new AbstractMap.SimpleEntry<>(status, s)))
        .collect(Collectors.groupingBy(Map.Entry::getKey,
                Collectors.mapping(Map.Entry::getValue, Collectors.toList())));
user_17461180
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