Styjun

So I am trying to make a simple producer/consumer code in java. My producer seems to work just fine however it reaches a point where it produces 10 items and then just stops producing items. Then I have to wait for the consumer to remove the 10 items added to the array List and I am left with the consumer having nothing left to consume since the producer doesn't add to the list.

I initially thought that for some reason the code seems to be breaking out of the while loop in my producer class but it could never do that so I'm honestly lost. Here is my code

import java.util.Random;

public class Main 


 public static boolean eating = true;
 private static ArrayList<Integer> buffet = new ArrayList<Integer>(10);

 public static void main(String[] args) throws InterruptedException 

 //make our producer
 Thread producerThread = new Thread(new Runnable() 
 @Override
 public void run() 
 producer();
 System.out.println("BUFFET IS CLOSED, NO MORE FOOD TO MAKE");
 
 );

 //make our consumer
 Thread consumerThread = new Thread(new Runnable() 
 @Override
 public void run() 
 consumer();
 
 );

 producerThread.start();
 consumerThread.start();

 consumerThread.join();
 producerThread.join();


 

 private static void producer()

 Random randomCooking = new Random();

 while(true)
 if(buffet.size() < 10) 
 System.out.println("Producer added a dish");
 buffet.add(randomCooking.nextInt(5));
 try 
 Thread.sleep(1000);
 catch (InterruptedException e) 
 e.printStackTrace();
 
 
 




 

 private static void consumer() 

 Random randomEating = new Random();

 while (true) 
 try 
 Thread.sleep(500);

 if (randomEating.nextInt(5) == 0) 

 if(buffet.size() > 0) 

 Integer value = buffet.remove(buffet.size() - 1);

 switch (value) 
 case 0:
 System.out.println("Dish 0: Shrimp & Rice, eaten by consumer! Dishes left: " + buffet.size());
 break;
 case 1:
 System.out.println("Dish 1: Mashed Potatoes and Gravey, eaten by consumer! Dishes left: " + buffet.size());
 break;
 case 2:
 System.out.println("Dish 2: Spaghetti and Meatballs, eaten by consumer! Dishes left: " + buffet.size());
 break;
 case 3:
 System.out.println("Dish 3: Burger & Fries, eaten by consumer! Dishes left: " + buffet.size());
 break;
 case 4:
 System.out.println("Dish 4: Salad & Tofu, eaten by consumer! Dishes left: " + buffet.size());
 break;

 
 else
 System.out.println("Food is all finished");

 

 else
 System.out.println("consumer decided not to eat! Dishes left: " + buffet.size() );
 

 catch(InterruptedException e)
 e.printStackTrace();
 
 

 

 

After a while this is all I am left with

consumer decided not to eat! Dishes left: 0
consumer decided not to eat! Dishes left: 0
consumer decided not to eat! Dishes left: 0
consumer decided not to eat! Dishes left: 0
Food is all finished
consumer decided not to eat! Dishes left: 0
consumer decided not to eat! Dishes left: 0

ArrayList is not suited for concurrent access. It means if a thread modifies it and multiple threads access it concurrently, it must be synchronized externally. The simplest you can do - wrap it in synchronized counterpart and synchronize access to it:

List<Integer> buffet = Collections.synchronizedList(new ArrayList<>());
...
while (true) 
 synchronized (buffet) 
 // perform your operations
 

Failure to follow this advice may result in non-deterministic behavior - as explicitly mentioned in the documentation (and, basically, what you are observing).

However, typically, the most useful data-structure for producer-consumer problems is BlockingQueue. So you can use some of its implementations, e.g. ArrayBlockingQueue or LinkedBlockingQueue.