Java 11: Why does my HashMap return an incorrect size after concurrent updates?
I'm not sure how to approach I've been struggling with this for a few days now and could really use some help. I've been banging my head against this for hours. I'm encountering an issue where the size of my `HashMap` seems to be inconsistent after performing multiple concurrent updates. Specifically, I have a `HashMap` that is shared across multiple threads, and I'm using `Executors` to manage thread execution. Below is a simplified version of my code: ```java import java.util.HashMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.atomic.AtomicInteger; public class ConcurrentHashMapTest { private static final HashMap<Integer, String> map = new HashMap<>(); private static final AtomicInteger counter = new AtomicInteger(0); public static void main(String[] args) throws InterruptedException { ExecutorService executor = Executors.newFixedThreadPool(10); for (int i = 0; i < 100; i++) { final int key = i; executor.execute(() -> { map.put(key, "Value " + key); counter.incrementAndGet(); }); } executor.shutdown(); while (!executor.isTerminated()) { } System.out.println("Map Size: " + map.size()); System.out.println("Counter: " + counter.get()); } } ``` I expect both the `map.size()` and the `counter.get()` to return 100, but I often see the map size reported as less than that. This seems particularly pronounced when I run the code multiple times. I understand that `HashMap` is not thread-safe, but I'm not sure how to manage this correctly without converting the entire structure to a `ConcurrentHashMap`, which I believe might affect performance due to locking. I've tried wrapping the `put` operation in a synchronized block, but it didn't resolve the issue. Here's an example of that attempt: ```java synchronized (map) { map.put(key, "Value " + key); } ``` However, this made the application considerably slower without fully solving the inconsistency. I am looking for a solution that allows me to maintain performance while ensuring the integrity of the size count. Is there a better approach or design pattern I should be considering here? This is part of a larger API I'm building. Has anyone else encountered this? For context: I'm using Java on Ubuntu. Any help would be greatly appreciated! Could this be a known issue?