Java 17: implementing Stream API and Collectors.groupingBy() returning unexpected null values
I've encountered a strange issue with I've looked through the documentation and I'm still confused about I'm using Java 17 and running into a question while trying to group a list of objects using the Stream API with the `Collectors.groupingBy()` collector. My data model consists of a `User` class with fields `id` (Integer), `name` (String), and `age` (Integer). I want to group users by their age and create a map where the key is the age and the value is a list of users of that age. However, I am experiencing null values in the resulting map, and I need to seem to figure out why. Here is the code I'm working with: ```java import java.util.*; import java.util.stream.Collectors; class User { private Integer id; private String name; private Integer age; public User(Integer id, String name, Integer age) { this.id = id; this.name = name; this.age = age; } public Integer getId() { return id; } public String getName() { return name; } public Integer getAge() { return age; } } public class Main { public static void main(String[] args) { List<User> users = Arrays.asList( new User(1, "Alice", 30), new User(2, "Bob", null), new User(3, "Charlie", 25), new User(4, "David", 30) ); Map<Integer, List<User>> groupedUsers = users.stream() .collect(Collectors.groupingBy(User::getAge)); System.out.println(groupedUsers); } } ``` The expected output should be a map containing two keys (30 and 25), with the corresponding lists of users. However, I am seeing a null key in the output, like this: ``` {25=[User@1], 30=[User@2, User@4], null=[User@3]} ``` I tried using `Collectors.groupingBy(user -> user.getAge() != null ? user.getAge() : -1)` to replace null ages with a placeholder, but that only introduces more complexity and doesn't give me a clean map of ages. What’s the best way to handle this situation while ensuring I only group by valid age values? Should I filter out users with null ages before grouping them, or is there a more elegant approach? Any insights would be greatly appreciated! Am I missing something obvious? This is my first time working with Java 3.11. Hoping someone can shed some light on this.