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 g

Answer 1

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

imports:

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

retrieving a Map> where each List contains students sorted by their ids.

Map> 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.