👀 Views: 63 💬 Answers: 1 📅 Created: 2025-07-03

java stream groupingby comparator Java

I'm maintaining legacy code that Quick question that's been bugging me - After trying multiple solutions online, I still can't figure this out... I'm working with an scenario while trying to group a list of objects using Java's Stream API and a custom comparator. I want to group a list of `Person` objects based on their last names and then sort these groups by the number of people in each group. However, the output is not what I expected. Instead of grouping correctly, I see some groups are empty or merged incorrectly. Here's the relevant code snippet: ```java import java.util.*; import java.util.stream.*; class Person { String firstName; String lastName; // Constructor, getters, and setters Person(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } } public class Main { public static void main(String[] args) { List<Person> people = Arrays.asList( new Person("John", "Doe"), new Person("Jane", "Doe"), new Person("Alice", "Smith"), new Person("Bob", "Smith") ); Map<String, List<Person>> grouped = people.stream() .collect(Collectors.groupingBy( Person::getLastName, TreeMap::new, // Custom comparator for sorting by last name Collectors.toList() )); // Sort groups based on size List<Map.Entry<String, List<Person>>> sortedGroups = grouped.entrySet().stream() .sorted((e1, e2) -> Integer.compare(e2.getValue().size(), e1.getValue().size())) .collect(Collectors.toList()); sortedGroups.forEach(entry -> System.out.println(entry.getKey() + " : " + entry.getValue().size())); } } ``` I was expecting the output to show `Doe : 2` and `Smith : 2`, but instead, I see the following output: ``` Doe : 0 Smith : 0 ``` I've tried debugging by printing the list before the grouping step and confirmed that the `Person` objects are correctly instantiated. I also checked if `getLastName()` is returning the expected values, which it is. I'm using Java 11, and I suspect that the scenario lies in how I'm using the `Collectors.groupingBy()` method with the custom comparator. Any ideas on what could be going wrong? I'm working on a web app that needs to handle this. Any ideas what could be causing this? Thanks in advance! My development environment is Windows. I'd really appreciate any guidance on this. Am I missing something obvious? This is for a REST API running on Linux.