Accessing to a non-thread-safe collection by multiple threads leads to problems

gistfile1.txt

package com.foo;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;

public class UnsynchronizedCollectionAccess {
    public static void main(String[] args) throws InterruptedException {
        final CountDownLatch latch = new CountDownLatch(1);
        final List<Integer> list = new ArrayList<Integer>();

        Runnable modification = new Runnable() {
            @Override
            public void run() {
                try {
                    latch.await();
                    for (int i = 0; i < 1_000_000; i++) {
                        list.add(i);
                    }
                }
                catch (InterruptedException e) {
                    // stop running
                }
            }
        };

        Thread t1 = new Thread(modification);
        Thread t2 = new Thread(modification);

        t1.start();
        t2.start();

        latch.countDown();

        t1.join();
        t2.join();

        if (list.size() != 2_000_000) {
            System.out.println("something's wrong. Lack of synchronization maybe?");
        }
    }
}