Why does not the size of ArrayBlockingQueue object update?

Question

I'm following an example at Chapter 23.6 Proceducer/Consumer Relationship: ArrayBlockingQueue ("Java - How to Program 10th).

I tried to run example but I don't understand why "buffer.size()" is not updated at the beginning.

I did a little modification in Consumer class. I set 300 millisecs for Thread.sleep(Generator.nextInt(300)). Here are my results.

Producer writes 1   Buffer cells occupied: 0
Consumer reads  1   Buffer cells occupied: 0
Producer writes 2   Buffer cells occupied: 1
Consumer reads  2   Buffer cells occupied: 0
Producer writes 3   Buffer cells occupied: 1
Consumer reads  3   Buffer cells occupied: 0
Producer writes 4   Buffer cells occupied: 1
Consumer reads  4   Buffer cells occupied: 0
Producer writes 5   Buffer cells occupied: 1
Consumer reads  5   Buffer cells occupied: 0
Producer writes 6   Buffer cells occupied: 1
Consumer reads  6   Buffer cells occupied: 0
Producer writes 7   Buffer cells occupied: 1
Consumer reads  7   Buffer cells occupied: 0
Producer writes 8   Buffer cells occupied: 1
Consumer reads  8   Buffer cells occupied: 0
Producer writes 9   Buffer cells occupied: 1
Consumer reads  9   Buffer cells occupied: 0
Producer writes10   Buffer cells occupied: 1
Producer done producing
Terminating producer
Consumer reads 10   Buffer cells occupied: 0

Consumer reads values totaling 55
Terminating Consumer

This is code.

public class BlockingBufferTest {
public static void main(String[] args) throws InterruptedException {
    ExecutorService executorService = Executors.newCachedThreadPool();

    BlockingBuffer sharedLocation = new BlockingBuffer();

    executorService.execute(new Producer(sharedLocation));
    executorService.execute(new Consumer(sharedLocation));

    executorService.shutdown();
    executorService.awaitTermination(1, TimeUnit.MINUTES);

} // end main method
} // end class

public class BlockingBuffer implements Buffer {

private final ArrayBlockingQueue<Integer> buffer; // shared buffer

public BlockingBuffer() {
    buffer = new ArrayBlockingQueue<>(1);
}
@Override
public void blockingPut(int value) throws InterruptedException {
    buffer.put(value); // place value in buffer
    System.out.printf("%s%2d\t%s%d%n", "Producer writes", value, "Buffer cells occupied: ", buffer.size());
}

@Override
public int blockingGet() throws InterruptedException {
    int readValue = buffer.take(); // remove value from buffer
    System.out.printf("%s %2d\t%s%d%n", "Consumer reads", readValue, "Buffer cells occupied: ", buffer.size());
    return readValue;
}
}

public class Consumer implements Runnable {

private static final SecureRandom generator = new SecureRandom();
private final Buffer sharedLocation; // reference to shared object

public Consumer(Buffer sharedLocation) {
    this.sharedLocation = sharedLocation;
} // end constructor

// stores values 1 to 10 in sharedLocation
@Override
public void run() {
    int sum  = 0;
    for(int count=1; count<=10; count++) {
        try {
            Thread.sleep(generator.nextInt(300));// random sleep
            sum +=sharedLocation.blockingGet();

        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    } // end for

    System.out.printf("%n%s %d%n%s%n","Consumer reads values totaling", sum, "Terminating Consumer");
} // end run method
}

public class Producer implements Runnable {

private static final SecureRandom generator = new SecureRandom();
private final Buffer sharedLocation; // reference to shared object

public Producer(Buffer sharedLocation) {
    this.sharedLocation = sharedLocation;
} // end constructor

// stores values 1 to 10 in sharedLocation
@Override
public void run() {
    int sum  = 0;
    for(int count=1; count<=10; count++) {
        try {
            Thread.sleep(generator.nextInt(3000));// random sleep
            sharedLocation.blockingPut(count);
            sum += count;

        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    } // end for

    System.out.printf("Producer done producing%nTerminating producer%n");
} // end run method
}

Answer 1

This looks like an artifact of thread ordering / preemptive multitasking.

Basically each of the cycles of producer/consumer is not atomical. They can preempt each other in midst of a cycle.

Here's how for example the ordering of the operations might work:

Producer thread: put value              (size=1)
Consumer thread: read value             (size=0)
Producer thread: x1 = buffer.size() --> x1 = 0
Producer thread: print x1 --> you observe 'Producer writes 1   Buffer cells occupied: 0'
Consumer thread: x2 = buffer.size() --> x2 = 0 
Consumer thread: print x --> you observe 'Consumer reads  1   Buffer cells occupied: 0'

P.S. It's actually a little bit trickier than that and there is no linear ordering of all operations but only some groups, but for this explanation it's ok