Question

I am new to Java and I’m attending a Concurrent Programming course. I am desperately trying to get a minimal working example that can help to demonstrate concepts I have learnt like using ‘synchronized’ keyword and sharing an object across threads.
Have been searching, but could not get a basic framework. Java programmers, kindly help.

Answer 1

Here is a very shot example of sharing an array between two threads. Usually you will see all zeros, but sometimes things get screwy and you see other numbers.

final int[] arr = new int[100];
Thread one = new Thread() {
    public void run() {
        // synchronized (arr) {
            for (int i = 0; i < arr.length * 100000; i++) {
                arr[i % arr.length]--;
            }
        // }
    }
};
Thread two = new Thread() {
    public void run() {
        // synchronized (arr) {
            for (int i = 0; i < arr.length * 100000; i++) {
                arr[i % arr.length]++;
            }
        //}
    }
};
one.start();
two.start();
one.join();
two.join();
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}

But, if you synchronize on arr around the looping you will always see all 0s in the print out. If you uncomment the synchronized block, the code will run without error.