Constructing list from multi level map in Java 8

One method use Java 8 Stream and lambda function:

String format = "In class %s %s with roll number %d";
List<String> result = new ArrayList<>();
outerMap.entrySet().stream()
    .forEach(v -> {
        String className = v.getKey();
        v.getValue().entrySet().stream()
            .forEach(stringStudentEntry -> result.add(String.format(format,className,stringStudentEntry.getKey(),stringStudentEntry.getValue().getRollNumber())));
    });

You can simply use Map.forEach for this operation as:

List<String> messages = new ArrayList<>();
outerMap.forEach((cls, students) ->
        students.forEach((name, student) -> 
                messages.add(convertToMessage(cls, name, student.getRollNumber()))));

where convertToMessage is a util as :

// this could be made cleaner using 'format'
String convertToMessage(String cls, String studentName, String rollNumber) {
    return "In class" + cls + "--> " + studentName + " with roll number: " + rollNumber;
}

You may do it like so,

List<String> formattedOutput = outerMap
    .entrySet().stream()
    .flatMap(e -> e.getValue().entrySet().stream().map(se -> "In class " + e.getKey()
        + " " + se.getKey() + " with roll number " + se.getValue().getRollNumber()))
    .collect(Collectors.toList());

You have to use the flatMap operator instead of map operator.