Java 8 Stream API Produces Unexpected Results with GroupingBy Collector and Custom Comparator
I'm experimenting with I've been banging my head against this for hours... I need some guidance on I'm working through a tutorial and Quick question that's been bugging me - I'm working with an scenario when using the `Collectors.groupingBy` method from the Stream API in Java 8. I have a list of `Person` objects that I want to group by age, but I also want to sort the groups by the name in descending order. However, the output is not what I expected. Here's a simplified version of my code: ```java import java.util.*; import java.util.stream.*; class Person { String name; int age; Person(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public int getAge() { return age; } } public class Main { public static void main(String[] args) { List<Person> people = Arrays.asList( new Person("Alice", 30), new Person("Bob", 25), new Person("Charlie", 30), new Person("David", 25) ); Map<Integer, List<Person>> groupedByAge = people.stream() .collect(Collectors.groupingBy( Person::getAge, Collectors.mapping(Person::getName, Collectors.toList()) )); System.out.println(groupedByAge); } } ``` When I run this code, the output is grouped by age, but the names within each group are not sorted: ``` {25=[Bob, David], 30=[Alice, Charlie]} ``` What I really want is for the names to be sorted in descending order within each age group, like this: ``` {25=[David, Bob], 30=[Charlie, Alice]} ``` I tried using a custom comparator with the `Collectors.toList()` method, but it seems I want to directly apply it within the `mapping` collector. Here’s what I attempted: ```java Map<Integer, List<Person>> groupedByAge = people.stream() .collect(Collectors.groupingBy( Person::getAge, Collectors.mapping(Person::getName, Collectors.toList()) .sorted(Comparator.reverseOrder()) ) )); ``` This code snippet doesn’t compile due to an behavior regarding the sorting not being applicable. Can anyone suggest how I can achieve the desired result of grouping by age and sorting the names within each group? I would appreciate any guidance or examples! For context: I'm using Java on macOS. Is there a better approach? This is my first time working with Java LTS. I'm open to any suggestions. This is happening in both development and production on Windows 11. I'd be grateful for any help. I've been using Java for about a year now. Could someone point me to the right documentation?