ConcurrentHashMapPractice.java

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class HelloWorld {
    static Map<String, AtomicLong> orders = new ConcurrentHashMap<>();
    
    static void processOrders() {
        for (String city : orders.keySet()) {
            for(int i=0; i < 50; i++) {
                Long oldOrder = orders.get(city);
                orders.put(city, oldOrder+1);
            }
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        orders.put("Bombay", 0l);
        orders.put("Beijing", 0l);
        orders.put("London", 0l);
        orders.put("NewYork", 0l);
        
        ExecutorService service = Executors.newFixedThreadPool(2);
        
        service.submit(HelloWorld::processOrders);
        service.submit(HelloWorld::processOrders);
        
        service.awaitTermination(1, TimeUnit.SECONDS);
        service.shutdown(); 
        
        System.out.println(orders);
    }
}