Java 8, Lambda: Sorting within grouped Lists and merging all groups to a list

Question

Based on the following answer: https://stackoverflow.com/a/30202075/8760211

How to sort each group by stud_id and then return a List with all Students as result of the grouping by stud_location and then sorting by stud_id)?

It would be great to have this as extension to the existing Lambda Expression:

Map<String, List<Student>> studlistGrouped =
    studlist.stream().collect(Collectors.groupingBy(w -> w.stud_location));

I need the Grouping based on the order of the Elements in the origin List.

First group: "New York"
Second group: "California"
Third group: "Los Angeles"

1726, "John", "New York"
4321, "Max", "California"
2234, "Andrew", "Los Angeles"
5223, "Michael", "New York"
7765, "Sam", "California"
3442, "Mark", "New York"

The result would then look like the following:

List<Student> groupedAndSorted = ....

    1726, "John", "New York"
    3442, "Mark", "New York"
    5223, "Michael", "New York"
    4321, "Max", "California"
    7765, "Sam", "California"
    2234, "Andrew", "Los Angeles"

I have tried the following:

studlistGrouped.entrySet().stream().sorted(Comparator.compar‌​ing(Map.Entry::getVa‌​lue))

But this doesn't work.

Answer 1

not 100% clear whether you're expected a Map<String, List<Student>> or just a List<Student>, nevertheless here are both solutions:

imports:

import static java.util.stream.Collectors.*;
import java.util.*;
import java.util.function.Function;

retrieving a Map<String, List<Student>> where each List<Student> contains students sorted by their ids.

Map<String, List<Student>> resultSet = studlist.stream()
      .collect(groupingBy(Student::getLocation,
             mapping(Function.identity(),
                  collectingAndThen(toList(),
                      e -> e.stream().sorted(Comparator.comparingInt(Student::getId))
                                            .collect(toList())))));

on the other hand, if you want to retrieve just a list of Student objects sorted by a given property then it would be a waste of resources to perform a groupingBy, sorted, collect and somehow reduce the map values into a single list. Rather just sort the Student objects within the list providing a sort key i.e.

studlist.sort(Comparator.comparingInt(Student::getId));

or

studlist.sort(Comparator.comparing(Student::getLocation));

or depending on whether you want to sort by more than one property then you could do something like shmosel's answer.