App.java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class App {
    // Integer wrapper object to be passed by reference.
    Integer sum = 0;

    public static void main(String[] g){
        System.out.println("The total sum is " + new App().sum);
    }

    private App() {
        // Creating a thousand threads is easier with a ThreadPool
        ExecutorService executor = Executors.newFixedThreadPool(1000);
        // You need only one instance of the AddSums class.
        // Synchronization will only work on a per instance basis.
        final AddSums addSums = new AddSums();

        for (int i = 0; i < 1000; i++) {
            // Add the tasks to be performed.
            executor.submit(addSums);
            //System.out.println("Thread " + i +"'s sum: " + sum);
        }

        executor.shutdown();
    }

    class AddSums implements Runnable{
        @Override
        public synchronized void run() {
            sum++;
        }
    }
}