Atomic Concurrent Map Updates

Atomic Concurrent Map Updates

Save as Main.java and run with a Java 17+ JDK: java Main.java.

import java.util.concurrent.ConcurrentHashMap;

public class Main {
  public static void main(String[] args) throws Exception {
    ConcurrentHashMap<String, Integer> counts = new ConcurrentHashMap<>();
    Thread a = new Thread(() -> { for (int i = 0; i < 1000; i++) counts.merge("hits", 1, Integer::sum); });
    Thread b = new Thread(() -> { for (int i = 0; i < 1000; i++) counts.merge("hits", 1, Integer::sum); });
    a.start(); b.start();
    a.join(); b.join();
    System.out.println(counts.get("hits"));
  }
}

1. Expected output: 2000
2. Both workers update the same key. Atomic merging prevents lost increments; joining ensures printing happens after both finish.