implementing Concurrent Modification in Java 17 When Using HashMap in Multi-threaded Environment
I'm performance testing and I'm wondering if anyone has experience with This might be a silly question, but I'm working with a `ConcurrentModificationException` while trying to update a `HashMap` in a multi-threaded environment using Java 17..... I understand that `HashMap` is not synchronized, but I assumed using `Collections.synchronizedMap()` would solve my scenario. However, the question continues. Here's a snippet of my code: ```java import java.util.Collections; import java.util.HashMap; import java.util.Map; public class ConcurrentHashMapExample { private static final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<>()); public static void main(String[] args) { Thread writer = new Thread(() -> { for (int i = 0; i < 1000; i++) { map.put(i, "Value " + i); } }); Thread reader = new Thread(() -> { for (int i = 0; i < 1000; i++) { System.out.println(map.get(i)); } }); writer.start(); reader.start(); try { writer.join(); reader.join(); } catch (InterruptedException e) { e.printStackTrace(); } } } ``` When I run the above code, I sometimes get the following behavior: ``` Exception in thread "Thread-1" java.util.ConcurrentModificationException at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1580) at java.base/java.util.HashMap$KeyIterator.next(HashMap.java:1605) at ConcurrentHashMapExample.lambda$main$1(ConcurrentHashMapExample.java:15) ``` I’ve tried synchronizing the `get` operation, but it doesn’t seem to work. I think the main scenario is that during the iteration by the reader thread, the writer thread is modifying the map, causing the exception. How can I safely read from and write to the `HashMap` concurrently without running into this exception? Should I be using a different data structure altogether, like `ConcurrentHashMap`? If so, how would that change my implementation? My development environment is Windows. Am I missing something obvious? I'm working in a Ubuntu 22.04 environment. What are your experiences with this? How would you solve this? I'm coming from a different tech stack and learning Java. What would be the recommended way to handle this? Any advice would be much appreciated. I've been using Java for about a year now.