👀 Views: 83 💬 Answers: 1 📅 Created: 2025-06-08

java stream collectors sorting Java

I'm not sure how to approach I'm having trouble with I'm dealing with I'm working on a personal project and I'm encountering an unexpected behavior when using the `Collectors.groupingBy()` method in Java 11..... I have a list of `Person` objects and I want to group them by their age, but I also want to sort the groups by name within each age group. However, the output doesn't reflect the expected sorting of names, and I'm not sure why. Here's the relevant 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)); groupedByAge.forEach((age, group) -> { group.sort(Comparator.comparing(Person::getName)); System.out.println("Age: " + age + " -> " + group); }); } } ``` When I run this code, I see the correct groups by age, but the names within each group are not sorted when printed. The output looks like this: ``` Age: 30 -> [Person@1b6d3586, Person@4554617c] Age: 25 -> [Person@1d44bc19, Person@6d06d69c] ``` The output shows the default `toString()` representation of the `Person` objects rather than the sorted names. I tried overriding the `toString()` method in the `Person` class as follows: ```java @Override public String toString() { return name; } ``` ``` But it still doesn't reflect the sorted order. I'm not sure if I'm missing something in how the sorting or grouping works with streams in Java 11. Any suggestions on how to resolve this issue or a better way to achieve the desired output? For context: I'm using Java on macOS. Am I missing something obvious? Am I missing something obvious? I'm developing on Ubuntu 20.04 with Java. Am I approaching this the right way? I'm using Java 3.9 in this project. What would be the recommended way to handle this? Thanks, I really appreciate it!