CODEWITHSUNDEEP
javablockingqueue
Rohan
Rohan

Reputation: 13

Unexpected output in Consumer Producer error

I got a code which I am trying to run it which is giving unexpcted output.

Code:

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ProducerConsumerBlockingQueue {

    public static void main(String[] args) {
        Buffer buffer = new Buffer();

        ExecutorService executor = Executors.newFixedThreadPool(2);
        executor.execute(new ProdBlock(buffer));
        executor.execute(new ConsBlock(buffer));
        executor.shutdown();
    }

}
class ProdBlock implements Runnable{

    Buffer buffer;
    public ProdBlock(Buffer buffer) {
        this.buffer = buffer;
    }

    public void run() {
        for(int i = 1; i <= 10; i++){
            buffer.put(i);
        }

    }

}
class ConsBlock implements Runnable{

    Buffer buffer;
    ConsBlock(Buffer buffer){
        this.buffer = buffer;
    }
    public void run() {
        for(int i = 1; i <= 10; i++){
            buffer.get();
        }       
    }

}
class Buffer{
    int i;
    BlockingQueue<Integer> sharedObject = new ArrayBlockingQueue<Integer>(1);

    public void get(){
        try {
            System.out.println("Getting - " + sharedObject.take());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    public void put(int i){
        this.i = i;
        try {
            sharedObject.put(i);
            System.out.println("Putting - " + i);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Output:

Putting - 1
Putting - 2
Getting - 1
Putting - 3
Getting - 2
Getting - 3
Putting - 4
Putting - 5
Getting - 4
Getting - 5
Putting - 6
Getting - 6
Putting - 7
Putting - 8
Getting - 7
Getting - 8
Putting - 9
Putting - 10
Getting - 9
Getting - 10

Expectd Output:

Putting - 1
Getting - 1
Putting - 2
Getting - 2
Putting - 3
Getting - 3
Putting - 4
Getting - 4
Putting - 5
Getting - 5
Putting - 6
Getting - 6
Putting - 7
Getting - 7
Putting - 8
Getting - 8
Putting - 9
Getting - 9
Putting - 10
Getting - 10

As expecting, blocking queue should take only 1 value at a time, how it is storing multiple values?

Upvotes: 1

Views: 49

Answers (1)

JB Nizet
JB Nizet

Reputation: 692073

It's not storing multiple values. It's just that once the take() method has returned, the producer can add a value to the queue and print it before the other thread prints the taken value:

  1. producer thread: call put(1) - the queue has 1 element
  2. producer thread: print "Putting 1" - the queue has 1 element
  3. consumer thread: call take() - the queue is empty
  4. producer thread: call put(2) - the queue has 1 element
  5. producer thread: print "Putting 2" - the queue has 1 element
  6. consumer thread: concatenating the string "Getting - " with the integer 1 obtained at step 3, and printing the result - the queue has 1 element

Upvotes: 1

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