public void main() {
        System.out.println("Starting ...");
        long start = System.currentTimeMillis();
        Thread t1 = new Thread(new Runnable() {
            public void run() {
                process();
            }
        });

        Thread t2 = new Thread(new Runnable() {
            public void run() {
                process();
            }
        });

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

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException ignored) {}

        long end = System.currentTimeMillis();

        System.out.println("Time taken: " + (end - start));
        System.out.println("List1: " + list1.size() + "; List2: " + list2.size());
    }