public class WhySynchronized {

    private final static int TESTS = 1000;

    /*
     * Run this program several times and look at the result.
     */

    public static void main(String[] args) {
        int[] values = new int[3];
        for ( int i = 0; i < TESTS; ++i ) {
            // data container
            TheThread.Data d = new TheThread.Data();
            // create threads
            Thread t1 = new TheThread(d);
            Thread t2 = new TheThread(d);
            // start threads
            t1.start(); t2.start();
            try {
                // wait for threads to complete
                t1.join();
                t2.join();
            } catch (InterruptedException ignored) { }
            // count values
            values[d.a]++;
        }
        System.out.println(
                "After adding 1 twice, repeating "+TESTS+" times, the result was:\n"+
                " * 1: "+values[1]+" times\n"+
                " * 2: "+values[2]+" times");
    }

}

class TheThread
        extends Thread {

    private final Data _d;

    public TheThread(Data d) {
        _d = d;
    }

    public void run() {
        _d.a++;
    }

    /**
     * Inner data class.
     */
    public static class Data {
        public int a = 0;
    }

}