👀 Views: 5928 💬 Answers: 1 📅 Created: 2025-06-06

java concurrenthashmap equals-hashcode Java

Quick question that's been bugging me - I'm converting an old project and I'm encountering an issue when using `ConcurrentHashMap` in Java 8 with custom objects as keys. The keys are instances of a class named `User`, which has two fields: `id` and `name`. I have overridden both the `equals()` and `hashCode()` methods in the `User` class to ensure that two `User` instances with the same `id` are considered equal. However, I'm experiencing unexpected behavior when adding and retrieving values from the map. Here's the implementation of the `User` class: ```java public class User { private final String id; private final String name; public User(String id, String name) { this.id = id; this.name = name; } @Override public boolean equals(Object o) { if (this == o) return true; if (!(o instanceof User)) return false; User user = (User) o; return Objects.equals(id, user.id); } @Override public int hashCode() { return Objects.hash(id); } } ``` I created a `ConcurrentHashMap<User, String>` to store user information based on their `id`. However, when I add a new `User` instance with the same `id` as an existing one, it seems that the new entry is not replacing the old one as expected. Here’s how I’m using the map: ```java ConcurrentHashMap<User, String> userMap = new ConcurrentHashMap<>(); User user1 = new User("1", "Alice"); User user2 = new User("1", "Alice Updated"); userMap.put(user1, "Some info"); userMap.put(user2, "Some updated info"); System.out.println(userMap.get(user1)); // Expected: "Some updated info" ``` When I run this code, the output still shows "Some info" instead of "Some updated info". I suspect that the issue lies with my `equals()` and `hashCode()` implementations or how the `ConcurrentHashMap` handles these methods. I've also checked thread safety, and I'm confident it's not a concurrency issue, since I'm adding elements sequentially. Can anyone help me understand why this behavior is occurring? What am I missing in my implementation? Thanks for any help you can provide! This is part of a larger REST API I'm building. Any ideas how to fix this?