Question

This is an implementation of producer consumer pattern for a homework. What’s wrong with the below implementation. I have googled for various implementations, but I am not able to understand what went wrong in mine.

I have a shared queue

I synchronize the producer and consumer on the same lock

Implementation

Shared Queue:

 class SharedQueue{
    public static Queue<Integer>   queue  = new LinkedList<Integer>();
 }

Producer Thread :

//The producer thread
class Producer implements Runnable{
    public void run()
    {
        synchronized (SharedQueue.queue)
        {
            if(SharedQueue.queue.size() >=5)
            {
                try {
                    SharedQueue.queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            Random r  = new Random();

            int x = r.nextInt(10);
            System.out.println("Inside Producer" + x);

            SharedQueue.queue.offer(x);


            SharedQueue.queue.notify();

        }
    }
}

Consumer Thread:

class Consumer implements  Runnable{
    public void run()
    {
        synchronized (SharedQueue.queue)
        {
            if(SharedQueue.queue.size() == 0)
            {
                try {
                    SharedQueue.queue.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.
                }
            }

            int k = SharedQueue.queue.remove();

            System.out.println("Inside consumer" + k);
        }
    }
}

The Main program

public class ProducerConsumerTest {

    public static void main(String[] args)
    {
        
        Thread p = new Thread(new Producer());
        Thread q = new Thread(new Consumer());

        p.start();
        q.start();

    }
}

Answer 1

Try replacing:

if(SharedQueue.queue.size() >= 5)

with:

while(SharedQueue.queue.size() >= 5)

and this:

if(SharedQueue.queue.size() == 0)

with:

while(SharedQueue.queue.size() == 0)

Just to re-check the condition after calling notify().