Question

I have the followings:

public class ProducerConsumer {

  private BlockingQueue<Integer> q;
  private Random rnd;
  private boolean run;

  public ProducerConsumer(){
    rnd = new Random();
    q = new ArrayBlockingQueue<>(10);
    run = true;
  }

  // high leve - with ArrayBlockingQueue

  public void QProducer() throws InterruptedException{       
    int i;
    while(run){
        i  = rnd.nextInt(100);

        q.put(i);
        System.out.println(i +" Added. Size is: "+ q.size());
    }
  } 

  public void  QConsumer() throws InterruptedException{
    int i;
    while(run){
      Thread.sleep(100);
      if (rnd.nextInt(10) == 0) {

          i = q.take();
          System.out.println(i + " Taken. Size is: "+ q.size());
      }
    }
  }

public void changeRun(){
    run = false;
 }    
}

---

 public static void main(String[] args) throws InterruptedException {

    // Producer Consumer

    final ProducerConsumer pc = new ProducerConsumer();

    Thread t1 = new Thread(new Runnable() {

        @Override
        public void run() {
            try {
                pc.QProducer();
            } catch (InterruptedException ex) {
                Logger.getLogger(Threading.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    });

    Thread t2 = new Thread(new Runnable() {

        @Override
        public void run() {
            try {
                pc.QConsumer();
            } catch (InterruptedException ex) {
                Logger.getLogger(Threading.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    });

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

    Scanner scn = new Scanner(System.in);

    scn.nextLine();
    pc.changeRun();

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

}

Ouptput:

20 Added. Size is: 1
8 Added. Size is: 2
71 Added. Size is: 3
72 Added. Size is: 4
61 Added. Size is: 5
97 Added. Size is: 6
6 Added. Size is: 7
64 Added. Size is: 8
58 Added. Size is: 9
27 Added. Size is: 10
20 Taken. Size is: 10 *
93 Added. Size is: 10
8 Taken. Size is: 9
95 Added. Size is: 10
71 Taken. Size is: 10 *
70 Added. Size is: 10
72 Taken. Size is: 10 *
85 Added. Size is: 10
61 Taken. Size is: 9
43 Added. Size is: 10
64 Added. Size is: 10 **
... 

I wonder how can there be numbers that are taken, and yet the size remains the same ( * ),
and howcome values are added after Queue is full ( ** ).
AFAIU, BlockingQueue is sychronized, and waits for values to to be added if queue is empty, and to be removed if full.
Thank you in advance.

Answer 1

The put() and the println/size() after it, and the take() and the println/size() after it, can be interleaved because of the multithreading. So the value returned by size() may be out-of-date by the time it is printed. E.g.

1a. Add (size = 10)
  1b. Print size 10
2a. Take (size = 9)
3a. Add (size = 10)
  2b. Print size 10
  3b. Print size 10