Why consumer is not consumin data in this program

Question

I'm new to java and I'm trying to implement simple producer consumer problem. Below is the code that i've written to test it. I have 3 classes, Main class, producer class and consumer class. Now the problem is my producer is producing the data but my consumer is not consuming it. Could anybody please explain me why this is happening. Thanks in advance.

public class ProducerConsumerWithQueue {
    /**
     * @param args
     */
    public static void main(String[] args) {

        ArrayList<String > queue = new ArrayList<String>();         
        Producer producer = new Producer( queue);           
        Consumer consumer = new Consumer( queue);

        consumer.start();
        producer.start();   
    }
}


public class Producer extends Thread{
    ArrayList<String> queue;
        public Producer(ArrayList<String> queue) {
        this.queue = queue;
    }

    public void run(){
        System.out.println("Producer Started");
        System.out.println("Producer size "+queue.size());
        for(int i=0;i<50;i++){
            try {
                    synchronized (this) {               
                    if(queue.size()>10){
                        System.out.println("Producer Waiting");
                        wait();
                    }else{
                        System.out.println("producing "+i);
                        queue.add("This is "+i);
                        notifyAll();
                    }
                }
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } 
        }
    }

}

public class Consumer extends Thread{

    ArrayList<String> queue;

    public Consumer(ArrayList<String> queue) {
        this.queue = queue;
    }

    public void run(){
        System.out.println("Consumer started");
        System.out.println("Consumer size "+queue.size());
            try {
                synchronized (this) {               
                    for(int i=0; i>10; i++){
                        if(queue.isEmpty()){
                            System.out.println("Consumer waiting()");
                            wait();

                        }else{
                            System.out.println("Consuming Data "+queue.remove(i));
                            notifyAll();
                        }
                    }
                }
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

    }

}

Following is printed then program is waiting Consumer started Consumer size 0 Producer Started Producer size 0 producing 0 producing 1 producing 2 producing 3 producing 4 producing 5 producing 6 producing 7 producing 8 producing 9 producing 10 Producer Waiting — user2261968, Jul 18 '13 at 07:09
@Bohemian No I'm not using ArrayBlockingQueue, I want to do it using wait and notify. With ArrayBlockingQueue everything works fine. — user2261968, Jul 18 '13 at 07:11
Uh, why don't you want to use a `BlockingQueue` since this is exactly what they are for, and are provided with the JDK? — fge, Jul 18 '13 at 07:12
Also, you synchronize on `this` and not the list. This cannot work. — fge, Jul 18 '13 at 07:14
If you insist on avoiding the standard library, then you should fix the locking in addition of the consumer loop. You need to synchronize on the same object, the queue for example. — kiheru, Jul 18 '13 at 07:14

score 5 · Answer 1 · answered Jul 18 '13 at 07:10

5

Your consumer will never run as the for loop does not even run once

for(int i=0; i>10; i++){

Check the i>10 constraint. Probably you want to try i<10

answered Jul 18 '13 at 07:10

Matthias

3,582
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Did stare at that code for quite some time. Did not see that one ;-) – André Stannek Jul 18 '13 at 07:12

score 0 · Answer 2 · answered Jul 18 '13 at 07:09

what happens when i gets over 10 for the consumer? Assuming you did the synchronization right, the producer is producing 50 elements and the consumer is only going to look at ten of those. also when you do queue.remove(i) how do you know there is an element at that slot? what if

producer runs 1 iteration and inserts element at 0 consumer runs 1 iteration, consumer's i will now be 1 producer runs 1 iteration and inserts element at 0 consumer runs 1 iteration on awake, but cannot consume anything so it waits again.

Might want to reconsider the solution you have :-)

http://www.tutorialspoint.com/javaexamples/thread_procon.htm

pouyan · Answer 3 · 2013-07-18T07:26:41.540

this is because in your producer you have this :



if(queue.size()>10){
           System.out.println("Producer Waiting");
           wait();
       }

as your produser start first (your consumer waited ) when your producer rich this line it will wait too , and your consumer and producer will be in infinit wait !!! (and this is because you use producer in a 50 loop and so when the produces item reach to 11 the if condition will be true and your producer will wait )

Why consumer is not consumin data in this program

3 Answers3