Example Threadsafe BlockingQueue implementation in Java

BlockingQueue.java

public class BlockingQueue implements Queue {
 
    private java.util.Queue queue = new java.util.LinkedList();
 
    /**
     * Make a blocking Dequeue call so that we'll only return when the queue has
     * something on it, otherwise we'll wait until something is put on it.
     * 
     * @returns  This will return null if the thread wait() call is interrupted.
     */
    public synchronized Object dequeue() {
        Object msg = null;
        while (queue.isEmpty()) {
            try {
                wait();
            } catch (InterruptedException e) {
                // Error return the client a null item
                return msg;
            }
        }
        msg = queue.remove();
        return msg;
    }
 
    /**
     * Enqueue will add an object to this queue, and will notify any waiting
     * threads that there is an object available.
     */
    public synchronized void enqueue(Object o) {
        queue.add(o);
        // Wake up anyone waiting for something to be put on the queue.
        notifyAll();
    }
 
}

Consumer.java

public class Consumer implements Runnable {
    // This will be assigned in the constructor
    private Queue queue = null; 
 
    public void process(Object msg) {
        try {
            //process message non-trivially (IE: it takes awhile).
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
 
 
    public void run() {
        while(true) { 
            doStuff();
        }
    }
 
 
    public void doStuff() {
        Object msg = queue.dequeue();
        process(msg);
    }
 
}

Producer.java

public class Producer implements Runnable {
    // This will be assigned in the constructor
    private Queue queue = null;
 
    public void run() {
        // Binds to socket, reads messages in
        // packages message calls doSomething()
        // doSomething(Object msg);
    }
 
    public void doSomething(Object msg) {
        queue.enqueue(msg);
    }
}

I think you also need to consider the case that queue is full. In the real world, queue cannot has unlimited space.

@jeremyshi is correct, you need a limit attribute in the class of BlockingQueue

does while loop is required?? having just if condition should serve the purpose rite??

@vikranthpatoju is right. In enqueue method it is enough to do notify instead of notifyAll. The while(isEmpty) is not safe, since if there are 2 consumers simultaneously read the same element in the queue, you will have a race condition.

       public class BoundedList<T> {
            private final int capcity;
            private final Queue<T> list;
            private final Object lock = new Object();
        
            public BoundedList(int capcity) {
                this.capcity = capcity;
                this.list = new ArrayDeque<>(capcity);
            }
        
            T poll() throws InterruptedException {
                synchronized (lock) {
                    while (list.isEmpty()) {
                        lock.wait();
                    }
                    T e = list.poll();
                    lock.notify();
                    return e;
                }
            }
        
            void add(T e) throws InterruptedException {
                synchronized (lock) {
                    while (list.size() == capcity) {
                        lock.wait();
                    }
                    list.add(e);
                    lock.notify();
                }
            }
        
        
        }

will this work

@shankyty No, unfortunately that won't work, because it can result in the lost wakeup problem. Check out this stack overflow article: https://stackoverflow.com/a/3186336/7602403

One solution is to replace your calls to lock.notify() with lock.notifyAll()