👀 Views: 6688 💬 Answers: 1 📅 Created: 2025-06-12

java hashmap equals hashcode collections Java

I'm working on a project and hit a roadblock... I'm getting frustrated with I've looked through the documentation and I'm still confused about I've been banging my head against this for hours... I'm stuck on something that should probably be simple. I'm experiencing inconsistent behavior when using custom objects as keys in a `HashMap`. I defined a custom class `Product` that overrides `hashCode()` and `equals()`, but I'm noticing that the map sometimes fails to retrieve values using keys that I know should match. Here's the relevant code for my `Product` class: ```java public class Product { private String id; private String name; public Product(String id, String name) { this.id = id; this.name = name; } @Override public int hashCode() { return id.hashCode(); } @Override public boolean equals(Object obj) { if (this == obj) return true; if (obj == null || getClass() != obj.getClass()) return false; Product product = (Product) obj; return id.equals(product.id); } } ``` I've created a `HashMap<Product, Integer>` to store product quantities: ```java Map<Product, Integer> productMap = new HashMap<>(); Product product1 = new Product("123", "Laptop"); productMap.put(product1, 10); ``` The scenario arises when I try to retrieve the quantity using a new instance of `Product` with the same ID: ```java Product product2 = new Product("123", "Laptop"); Integer quantity = productMap.get(product2); ``` It returns `null`, even though `product1` and `product2` should be considered equal. I've verified that `hashCode()` and `equals()` are implemented correctly, but I suspect there's something I'm missing in how `HashMap` is working with my custom objects. I've also tried using `==` to compare references, and that returns `false` as expected since they are different instances. Is there something specific about the `HashMap` implementation or the way I'm using my `Product` class that could be causing this scenario? Any help would be appreciated! For context: I'm using Java on Linux. Is there a better approach? This is part of a larger CLI tool I'm building. Am I missing something obvious? This is happening in both development and production on CentOS. Thanks for your help in advance! I'm developing on Windows 11 with Java. Has anyone dealt with something similar?