Why is synchronized not working properly?

Question

Here\'s my code:

private int count = 0;

  public synchronized void increment() {
      count++;
  }

 public void doWork() throws InterruptedException {

          


        

Answer 1

The increment method can be run on the other thread after the increment method returns, but before count is retrieved for the concatenation

count+"  "+Thread.currentThread().getName()

You could e.g. fix this by modifying and retrieving count in one synchronized block:

public synchronized int incrementAndGet() {
    count++;
    return count; // read access synchronized
}

for (int i = 0; i < 5; i++) {
    System.out.println(incrementAndGet()+"  "+Thread.currentThread().getName());
}

Or use the class in the standard library specifically designed for this purpose:

private final AtomicInteger counter = new AtomicInteger(0);

public void doWork() throws InterruptedException {

    Thread t1 = new Thread(new Runnable() {
        public void run() {
            for (int i = 0; i < 5; i++) {
                System.out.println(counter.incrementAndGet() + "  " + Thread.currentThread().getName());
            }
        }
    });

    Thread t2 = new Thread(new Runnable() {
        public void run() {
            for (int i = 0; i < 5; i++) {
                System.out.println(counter.incrementAndGet() + "  " + Thread.currentThread().getName());
            }
        }
    });

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

Of course this does not necessarily lead the numbers 1 to 10 being printed in order, just that no number is retrieved more than once. The following output could happen:

2  Thread-0
3  Thread-0
4  Thread-0
1  Thread-1
5  Thread-0
6  Thread-1
7  Thread-0
8  Thread-1
9  Thread-1
10  Thread-1