Trouble with Java Streams and Collectors: How to Group By and Count with Complex Objects?
I've been researching this but I'm confused about Hey everyone, I'm running into an issue that's driving me crazy. I'm working on a Java application using Java 11 and I'm trying to group a list of custom objects by a specific field and then count the occurrences of each group. I've created a class `Product` with fields like `category` and `name`. However, when I use `Collectors.groupingBy` in conjunction with `Collectors.counting()`, I end up with a `Map<Object, Long>` that doesn't seem to reflect the expected counts accurately. Here’s a simplified version of my code: ```java import java.util.List; import java.util.Map; import java.util.stream.Collectors; class Product { String category; String name; public Product(String category, String name) { this.category = category; this.name = name; } public String getCategory() { return category; } } public class Main { public static void main(String[] args) { List<Product> products = List.of( new Product("Electronics", "TV"), new Product("Electronics", "Radio"), new Product("Furniture", "Chair"), new Product("Furniture", "Table"), new Product("Electronics", "TV") ); Map<String, Long> productCount = products.stream() .collect(Collectors.groupingBy(Product::getCategory, Collectors.counting())); System.out.println(productCount); } } ``` When I run this, I get the following output: ``` {Electronics=3, Furniture=2} ``` This seems correct at first glance, but I expected to get a count of 2 for `Electronics` because there are only 2 unique products listed. I suspect that it’s counting duplicates based on the object instances rather than the unique product name. I’ve tried using `distinct()` before collecting, but that results in a compilation error since I'm trying to group by category. If I group by both `category` and `name`, I still don’t get the desired count of unique products. What’s the best way to achieve a count of unique products per category in this situation? Any help or guidance on this would be greatly appreciated! Any help would be greatly appreciated! Any help would be greatly appreciated! For reference, this is a production service. I've been using Java for about a year now. What's the correct way to implement this?