UnsafeCounter.java

package app;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class UnsafeCounter {
    private int nextValue = 0;

    public int getNextValue() {
        return nextValue++;
    }

    public static void main(String[] args) throws Exception {
        final List<Integer> results = Collections.synchronizedList(new ArrayList<Integer>());
        final UnsafeCounter c = new UnsafeCounter();

        Runnable runnable = new Runnable() {
            public void run() {
                for (int i = 0; i < 3000; i++) {
                    results.add(c.getNextValue());
                }
            }
        };

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

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

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

        int total = 0;
        for (int i : results) {
            total += i;
        }
        System.out.println(total);
    }
}