Question

I have to write this produce consumer application using multithreading. I wrote the following java code but havn;t been able to figure out where it is getting wrong. Also i want to know whether my class design is apt or if my coding style is appropriate.

Thanks in Advance!!!

EDIT

I have modified the produce consumer code: But it still has some problem.

import java.util.*;
import java.lang.Thread;

public class pc_example {

public static void main (String [] args) {
    Store store = new Store( 10 );
    produce p = new produce(store);
    consume c = new consume (store);
    p.start();
    c.start();
}

}

class Store {
public Queue<Integer> Q;
public int max_capacity;

Store( int max_capacity ) {
    Q = new LinkedList<Integer>();
    this.max_capacity = max_capacity;
}

}

class produce extends Thread {

private Store store;
private int element;

produce ( Store store ) {
    this.store = store;
    this.element = 0;
}



public void put() {
    synchronized (store) {
        if (store.Q.size() > store.max_capacity) {
            try {
                wait();
            } catch (InterruptedException e) {}
        }
        else {
            element ++;
            System.out.println( "Producer put: " + element );
            store.Q.add(element);
            notify();
        }               
    }           
}

}

class consume extends Thread {
private int cons;
private Store store;

consume (Store store) {
    this.store = store;
    this.cons = 0;
}

public void get() {
    synchronized (store) {
        if (store.Q.size() == 0) {
            try {
                wait();
            } catch (InterruptedException e) {}
        }
        else {
            int a = store.Q.remove();
            System.out.println( "Consumer put: " + a );
            cons++;

            if (store.Q.size() < store.max_capacity)
                notify();
        }
    }
}

}

Answer 1

For a complete example see the producer-consumer example in the java api for BlockingQueue.

---

There are several errors in the code. For the first the producer and the consumer are not using the same queue e.g. there are two instances of the queues. Secondly notify and wait methods are also operating on different objects.

Getting your example to work needs several things:

1. Only one queue
2. Thread safe handling of the queue
3. Handling notification and waiting on the same object

The producer code could be rewritten to:

public void produce() {
    int i = 0;
    while (i < 100) {
        synchronized(Q) {
            if (Q.size() < max_capacity) {
                Q.add(i);
                System.out.println("Produced Item" + i);
                i++;
                Q.notify();
            } else {
                try {
                    Q.wait();
                } catch (InterruptedException e) {
                    System.out.println("Exception");
                }
            }
        }
    }
}